2016 FAQ Midas Civil

FAQ SOURCE BOOK

2016 Edition

Frequently Asked Questions about midas Civil

Technical Materials

Pre-Processing / 23 1.

Various display options

2.

How to provide support at base of a curved bridge deck?

3.

How to define moving load to curved bridge? 2 point method is not working. Error is displayed that no element is on the defined lane.

4.

How to generate uniform tapering irrespective of the number of segments?

5.

How to copy tendon profile of one girder to all other girders?

6.

Warnings displayed during analysis that certain load cases have not converged. How to converge all the load cases?

7.

How to define more modes of vibration? Only a single mode is shown on performing response spectrum analysis.

8.

Why are certain supports/links, though defined in the model are not applied while performing analysis?

9.

How to model bearings? How to rotate bearings in case of curved bridge?

10. How to create a composite PSC / RCC section with an unsymmetrical slab? 11. How to import PSC cable profile from AutoCAD drawing? 12. How to use the material properties defined in one model file in any other model file, without defining it again? 13. How to reduce the section capacity to model a cracked section approximately? 14. How to import a section from an AutoCAD *.Dxf file? 15. What are Wood Armer moments? How to view in midas Civil? 16. How is the varying longitudinal stiffness of ballast/concrete bed considered for rail track analysis? 17. How to make a taper from composite T section to composite I 18. Plate thickness is not visible in the model while viewing solid view, why? 19. What kind of boundary conditions & elements are available in eigenvalue, response spectrum, and time history analysis? 20. How to input surface spring coefficient for underground structure? 21. How to input temperature gradient load for a general or PSC section? 22. How to get concurrent reactions due to moving load? 23. What does the error ‘PSC/Composite type of beam section temperature cannot be applied to section of the element’, imply?

Analysis / 9 1.

What is the difference between rigid link and rigid type elastic link?

2.

What is the difference between lane element and cross beam method for vehicular load distribution? When should each be used?

3.

How is a truss element and a cable element considered in midas Civil?

4.

Why is the pre-stress elastic deformation loss sometimes positive?

5.

Why is there a kink whi le elements activated in differ ent construction stage are conn ected, when graphically viewing the results?

6.

How does the sof tware consider beam property changes with pre-stressing tendon?

7.

What is the basis of P- Δ analysis in midas Civil? Secondary moment from P-Δ analysis matches with classical method calculation using actual stress rather failure stress. Why?

8.

The deformations of maste r and slave nodes of a rigid lin k are not ex actly same. Why?

9.

Cable element is automatically transformed to equivalent truss element for linear analysis. What does this message imply?

Post-processing / 12 1.

Why is the m odel showing reactions at all the nodes , though support has not be en provided?

2.

How to verify for the uplift due to moving load? And how to obtain the corresponding vehicle positioning?

3.

How to obtain vehicle position causing Max./Min. Force or mo ment on an element?

4.

Why is dead load results of last construction stage not matching with dead load results in post cs?

5.

How to obtain cross sectional stresses for line element?

6.

Why are the pre-stress losses given by midas Civil not matching with the manual calculations?

7.

How to view the result table for construction stages?

8.

Why are the stresses not being displayed for moving load cases in the results?

9.

Displacement of the structure looks unrealistic. How can that be changed?

10. How to view the ultimate moment capacity of PSC girder along with the design moment? 11. How to view results of a particular load case separately in construction stages? 12. How to formulate load combinations for construction stage results?

FAQ

midas Civil ▶ Pre-Processing

Various display options

1. Thickness of the plate element

2. To distinguish plate elements from beam elements

3. To show the outline of the Inactive elements

4

www.MidasUser.com


4. Display only to certain parts If you enable the Display by Group option in the Display, only the information of the selected group is displayed. In the figure below on left, the entire " live load surcharge" is displayed, while in the figure further below only a part of the upper slab loads as defined in a group is displayed.

Display by Group options can be applied to loads as well as all display information that can be expressed in midas Civil . 5. Different colored elements depending on thickness

5

www.MidasUser.com

FAQ


How to provide support at base of a curved bridge deck? In cases where a superstructure is modelled without substructure, the bottom nodes of the superstructure will have to depict the behavior of bearing.

For curved bridges, the rotation and translation of bearings have to be in accordance with the curvature (tangential or perpendicular to the curve). This behavior can be simulated by defining node local axis for the support nodes which will declare the direction of translation and rotation of bearings. To define Node local axis

1. Boundary  Node Local Axis. 2.

Local Axis could be defined in four ways. - Angle : Angle of rotation about global X, Y and Z axis needs to be specified to rotate the axis for specified node. This method is the simplest way o f assigning node local axis. - 3 Points - Vectors - Line Vector.

For details on other methods refer ‘Online help manual’ (Press F1 on working window) Node Local Axis could be viewed only for those nodes where the axis have been changed and not at all nodes.

y y

z

x

x

z

To view the assigned node local axis, navigate to 1. View



Display Options.

2. Under Node tab, check the Node Local Axis option. Supports defined at nodes with node local axis will have restraints along the defined axis of the node and not the global axis directions. The reactions as well as the displacements could be obtained along these local axis as well as global axis.

6

www.MidasUser.com

FAQ


How to define moving load for curved bridge? 2 point method is not working. Error is displayed that no element is on the defined lane. For a curved bridge, selection by ‘Picking’ or ‘Number’ option should be used to define lane. From Main Menu select Load > Moving Load > Moving Load Code > Traffic Line Lanes 1. Selection by Picking Click elements one by one in sequence to define the lane.

2. Selection by number

Enter element numbers in the text box and click on add button. Make sure that the element numbers are in sequence. Alternatively elements could be renumbered in an increasing order along the direction of vehicle movement.

7

www.MidasUser.com

FAQ


How to generate uniform tapering irrespective of the number of segments? While an element is assigned with a tapered section and is divided into number of segments, tapering happens with each segment of the element. From Main Menu select Properties > Section > Tapered Group To get uniform tapering irrespective of the number of segments, Tapered Section Group should be assigned.

3

4

5

2 1

5 4 3 1

2

For the members, grouped in Tapered Section Group, the sectional properties of the non-prismatic section are automatically calculated such that the group section varies uniformly irrespectiv e of the number of segments in the group. The auto-calculated sectional information can be converted into the model data on clicking ‘Convert to Tapered Section’. NOTE: Tapered sections defined by Value Type cannot be assigned as a Tapered Section Group

8

www.MidasUser.com

FAQ


How to copy tendon profile of one girder to all other girders? Where several girders have identical tendon profiles, then the profile defined for one girder could be copied to other girders if certain criteria are met. From Main Menu select Load > Temp/Prestress >Tendon profile Select the required Tendon profile and click ‘Move/Copy’. The tendons could be copied by 3 methods.

1. Element Increment Copies single or multiple tendons. For example

to Copy/Move the tendon profile defined in elements 1, 3, 5, 7 (G1), the following conditions should as referred in the image below should be satisfied.

• Increment of each corresponding element

should be identical. For example, Tendon Profile entered in G1 can be copied to G2 because the increment is identical. On the other hand, it can’t be G3 because the increment is different for each element.

• The number of elements to copy should be same. For example, the Tendon Profil e entered in G1 can be copied to G4 which has the same number of elements. On the other hand, it can’t be copied to G5 since the numb er of elemen ts is differ ent. As

shown in image above, length of elementdoesn’t matter while copying tendon profile.

9

www.MidasUser.com


2. Equal Distance Copies single or multipl e tendons. To copy by this option, the coordi nates for tendo n profile insertion has to be entered and also the following conditions are to be satisfied.

•

A node should exist at the distance specified, to copy the tendon profile, from the initial insertion point of the srcinal element. This node will define the new Insertion Point. An error message is displayed on absence of this node.

•

If there is more than one element attached to the initial element, then the Tendon Profile will be assigned to the attached element that is most linear.

• If Straight/Curve type of tendon profil e is to be copied, then the srcinal lengt h and

number of elements should be equal to the length and number of elements at the new location. NOTE: Clicking on ‘Current assigned element’ check box copies the tendon profile to the same element. This option is useful when generating tendons with similar profile in same element (say, box girder webs). Checking this option off, copies tendons to other elements provided other element exists at the distance specified.

3. New Assigned Elements Copies only singl e tendon. elements at the same time.

This opti ons enabl es copyi ng singl e tendon to multiple To copy the tendon pro file, the new assi gned elements and

inserting poin t of tendon needs to be provi ded.

Inserting poin t should be among the

selected elements.

10

www.MidasUser.com

FAQ


Warnings displayed during analysis that certain load cases have not converged. How to converge all the load cases? These type of warnings during analysis, can be avoided by altering the default convergence criteria from Main Control Data as shown below. From Main Menu select Analysis > Main Control Data

The number of iterations for load case convergence can be increased or convergence tolerance can be altered if load cases do not converge .

11

www.MidasUser.com

FAQ


How to define more modes of vibration? Only a single mode is show n on performing response spectrum analysis. Under the Eigenvalue Analysis Control, the number of frequencies has to be set accordingly.

From Main Menu select Analysis > Analysis Control > Eigenvalue In general, the number of modes to be considered for free vibration analysis are determined by modal mass participation. Most of the literature concedes that at least 90% of modal mass participation should be considered for an acceptable analysis.

The default number of frequencies for eigenvalue analysis is set to 1. Usually, it is more than 1 which could be altered with suitable number of frequencies in the dialogue box highlighted above . Lanczos method Adopted for relatively simpler structure to study the lower modes. The Lanczos method may miss some Eigen pairs in the required ones. However for practical eigenvalue

analysis method, the exact dynamic response has to be obtained which requires the missed eigenvalues to be included. ‘Sturm Sequence Check’ should be selected to check the same. Subspace Iteration method When performing Eigenvalue analysis for a finite element system of a large scale (large matrix system), Subspace Iteration method is effectively used. Ritz Vector For a model with large degrees of freedom (Say, for model with pile spring supports), Ritz vector method may be more appropriate. Unlike the natural eigenvalue modes, Ritz vectors are load dependent and produce more reliable results in dynamic analyses with relatively fewer modes. The Ritz Vectors are generated reflecting the spatial distribution or the characteristics of the dynamic loading.

12

www.MidasUser.com

FAQ


Why are certain supports/links, though defined in the model are not applied while performing analysis? This predominantly happens due to issues related to construction stage analysis. This might happen when , 1.

Nodes defined with boundary conditions are not assigned to Structure Group

2.

Structure Group incorporating the boundary nodes are not activated

3.

Support/link left out from being adding to the Boundary Group

4. The Boundary Group with the support nodes itself is not being activated The easiest way to detect these problems, is to display all the supports and boundary from View > Display ( 1 & 2 ) and visually inspect the construction stages for all the supports/elements that should be active in the respective stages ( 3 ) .

1 3

2

To rectify this issue, ensure that all the nodes and elements which are to be activated at specific stage are added to the corresponding structure group. 111111

For example, refer the image to the right where dark blue line & dots represents the element and nodes respectively. The rigid link is in light blue, and the brown line with a rectangle is an elastic link, below which the supports are assigned.

13

www.MidasUser.com


To ensure proper analysis: 1. Assign all the nodes (inclusive of nodes connecting support and links ) and elements to the structure group which is to be activated. 2.

Now this Structure Group has to be added in the “Activation” window of the Construction Stage Dialogue box, indicated as

A

in

image below.

A

3.

Assign all the supports, support springs, elastic links, general links, rigid links, etc. to

4.

This Boundary Grou p is also to be act ivated in the

respective Boundary Group which is to be activated in construction stage. Construction Stage Dialogue box, indicates as

B

“Activation” window of the

in image below.

B

Not assigning and activating these supports appropriately during the Construction Stages, would affect the stability of the structure leading to Warning or Error Messages on performing Analysis. Implementing the above solution should resolve the problem.

14

www.MidasUser.com

FAQ


How to model bearings? How to rotate bearings in case of curved bridge? Elastic Links are used to model bearings connecting the bridge substructure and superstructure.

From Main Menu select Boundary > Link > Elastic Link Enter the Displacement stiffness (SDx, SDy, SDz) and Rotational stiffness (RDx, RDy, RDz) for the elastic link to simulate the bridge bearings. The image below on right, shows the elastic link local axis direction, positive x axis being in the connecting direction of the two nodes. It is important to note that the displacement and rotational stiffness are assigned along elastic link local axis direction. In case the exact stiffness values are

not known, arbitrary stiffness values

y

z

could be assigned to reach a better approximation

of

result.

x

For

example, neoprene bearings have high axial stiffness (SDx) and hence could be high,

111111

say 10 7 kN/m and

lower shear stiffness (SDy and SDz),

say 100

kN/m. For guided bearings (like POT-PTFE bearing), high values for SDy and/or SDz could be assigned to simulate high lateral bearing stiffness.

Bearings for Curved Bridges In case of curved bridges, beta angle should be assigned to these elastic

links

to change

bearing local axis orientation, either

the

aligned

tangentially to the curve of in line with the fixed support, as depicted in the image below. Beta angles could be assigned while creating the elastic link or later from the tables.

Ta nge nti a l to c ur v e

15

I n l i ne w i th F i x S upp or t

www.MidasUser.com

FAQ


How to create a composite PSC / RCC Section with an unsymmetrical slab? Section Property Calculator (SPC) tool can be used to generate a PSC or RCC section with unsymmetrical slab at top.

From Main Menu select Tools > Generator > Section Property Calculator

The steps to generate the section: 1.

Generate an AutoCAD .DXF drawing file of required

2.

Launch the SPC tool from Tool s > Section Property

section (no polylines) Calculator. Set units same as in the AutoCAD .d xf file drawing 3.

Import the .dxf file from File > Import > AutoCAD DXF

4.

Check the impor ted drawi ng for disco ntinuities when

5.

Define Slab and Girder Material from Model > Material

6.

Model > Section > Composite Section > Generate , to

asked.

generate the Composite Section, and fill in the required details like the numb er of parts. If the section has only slab and girder, then the number of parts would be 2.

Select & Click Apply

16

www.MidasUser.com


7. Calculate

the

sectional

properties

of

the

composite

section

by clicking

on

Property > Calculate Composite Property . Enter the mesh size, click on any part of the composite section and click apply.

8. To generate the section file which could be imported in midas Civil, click on Model > Section > Composite Section > Export . Enter the file name by clicking on 3 dots in the file name option. Click on file name, any part of composite section and click apply.

9.

In midas Civil, click on Section Property

Add and import the generated SEC file as

shown in Section Data Dialogue box above .

Other methods in which AutoCAD drawings could be used for easier modelling with midas Civil is demonstrated in https://www.youtube.com/watch?v=aDAQbBWEnd4

17

www.MidasUser.com

FAQ


How to import PSC cable profile from AutoCAD drawing? The Tendon Profile Generator tool could be used to import the tendon profile which has already been drawn i n AutoCAD.

From Main Menu select Tools > Generator > Tendon Profile Generator AutoCAD drawing format files need to be converted into DXF files

1

2

3 4 5 7

11

9

13

6 8 10 12 14

15

16 17

The steps are detailed in the page to follow :

18

www.MidasUser.com


1.

Import the .DXF file which contains the tendon profile.

2.

Select the layer with the tendon profile.

3.

Enter tendon name.

4.

Enter appropriate tendon property. The ‘tendon property’ should be predefined in the model file.

5.

Enter the element numbers to which the tendon profile has to be assigned.

6.

Click on locations shown by step no. 6 to 9 in sequence. This is to select the starting

point of the tendon and the x-z plane coordinates for the tendon 10. Click on locations shown by step no. 10 to 13 in sequence. This is to select the starting point of the tendon and the x-y plane coordinates for the tendon 14. Enter the element number at which the tendon has to be inserted. 15. Enter the required offset distance for the profile insertion if necessary. 16. Click ‘Add’ 17. Then OK to create the tendon coordinates on text format. 18. Copy the content 19. Open MCT Command Shell from Tools > Command Shell > MCT Command Shell 20. Paste the contents in the ‘MCT command shell’ dialogue box 21. Click ‘Run’. The profile should now be inserted in the model.

18

20

21

Other methods in which AutoCAD drawings could be used for easier modelling with midas Civil is demonstrated in https://www.youtube.com/watch?v=aDAQbBWEnd4

19

www.MidasUser.com

FAQ


How to use the material properties defined in one model file in any other model file, without defining it again? For importing material properties from an existing midas model, follow the instructions as shown below. From Main Menu select Properties > Material Properties The same procedure can be followed for importing section properties from an existing midas model, as well .

1

1. Click on ‘import’ and select the midas model

file

from

which

material

2

properties needs to be imported. 2. The materials in the selected list will

be added in the active model on clicking ‘OK’. 3. On

selecting

‘Keep

ID’

existing

properties in current model having same IDs as the one being copied, will be replaced

3

If New ID option is selected, the imported mate rial prop erties will be added with the specified ID or in case

4

the current model is having an element property with same ID then the Id of imported property will be incremented by 1.

20

www.MidasUser.com

FAQ


How to reduce the section capacity to model a cracked section approximately? Section stiffness could be reduced or manipulated using the Section Manager under Properties tab. From Main Menu select Properties > Section Manager > Stiffness… To model a section as cracked and to arbitrarily reduce its load carrying capacity, its

stiffness could be reduced, by applying Stiffness reduction factors. Select the section whose capacity has to be modified from the

‘Section Manager’ window

and input the appropriate reduction factors.

As boxed out in the image above, suitable modification factor could be applied for changing the cross sectional area, shear area, torsion moment of inertia or weight for any section at desired location and in desired direction.

For example, say to reduce the stiffness of the section on cracking, the moment of inertia about local y direction coul d be reduc ed by a factor.

This factor coul d be assigned to

particular boundary groups as well in case construction stage or boundary change assignment needs to be pe rformed. The scale factors for I and J end are kept same by default. However, these can be different in case of a tapered section.

21

www.MidasUser.com

FAQ


How to import a section from an AutoCAD *.dxf file? For importing a section from an AutoCAD .dxf file, section property calculator is used. Follow the instructions given below: From Main Menu selectTools > Generator > Sectional Property Calculator 1. Keep the units same as AutoCAD .dxf file. 2. The tolerance is relevant to the units used, if two nodes are spaced within the mentioned

tolerance then they will be merged and 1

considered as one node. 3. Angle step is used to create line divisions of a curve, lesser the angle more will be the line divisions for the same curve.

2

4. Click ‘OK’ 5. Go to File > Import > AutoCAD .dxf > Browse

3

for the relevant .dxf file 6. Click OK > Click Yes 7. Go to Model > Section > Generate

4

Select the whole section 8. For generating Line type section (generally used for generating

should

be

built-up steel sections), width of the line

specified

prior

to

step

No.

7

8

from

‘Model > Curve > Change Width > Select the line for width definition > Check the Width option & specify width> Apply’ For generating Plane type section (generally used for generat

9

ing Pre-stressed Concrete sections), defining line width is not required. The section is considered by the area bounded by the lines and will be meshed.

10 10 9.

Name the Section

11

10. Check on ‘Calculate Properties Now’

11

11. After clicking Apply, meshed section will be shown in the model window 22

www.MidasUser.com


12. Go to Model > Section > Export 13. Click on ‘ MIDAS Section File’ 14. Click and define the file path and section name in 14

the format *.sec as defined here as “Section1.sec”. 15. Click on the section and section turns red 16. Click Apply 17. From Main Menu select Properties > Section 13

Properties > Add 18. So the *.sec file can be imported as a general section or a PSC-Value type section

16

15 Click on the section and section turns red

18

18

23

www.MidasUser.com

FAQ


What are Wood Armer moments? How to view in midas Civil? Wood Armer method allows moment triads from plates (Mx, My, Mxy) to be transformed into simple bending moments in two directions (Wood Armer moments) for reinforcement design of plate elements. This is important because the twisting moment Mxy can be significant. At any point in the slab, the normal moments in a direction, resulting due to design moments Mx, My, and Mxy, must not exceed the ultimate normal resisting moment in that direction. The ultimate normal resisting moment is typically provided by ultimate resisting moments Mux and related to the reinforcement in the x- and

α-

Muα

directions. Mx, My and Mxy are bending and

twisting moments, usually obtained from a finite element analy sis program.

α

is angle of

transverse steel, measured clockwise, from the Mx axis. For those plates where, Wood Armer moments are to be found the reinforcement direction

needs to be defined. Create plate elements either with Element > Plate or by using the

Node/Element > Create

Mesh option. Next is to define Doma in. Invoke the

‘Define Domain’ dialogue box from Node/Element Tab. To assign the plates to a particular domain, follow the steps below:

1

Enter the domain Select the element type

2

Select the required plates in the model or directly enter the

3

element numbers Click Add

4

Now, to add sub domain, one could either click on the Sub-Domain button form

‘Define Domain’

dialogue box or click on ‘Define sub domain’ from Node/Element ribbon menu. The sub-domain is where details like the angle of reinforcement to the global axis, angle between the reinforcements spanning in either direction and reference axis definition are specified.

24

www.MidasUser.com


Also if different parts of the same domain have different details, then these could be provided using the subdomain. On applying the necessary loads and boundary conditions, the Wood Armer moments could be checked graphically as well as in form of table. For an irregular hypothetical slab as shown below, the Wood Armer moment result in the direction of

reinforcement (i.e., the design moment) will be more than those in local X and Y directions of the plate.

Along the red line shown in the image above, the Mxx value obtained for its dead load was 62kNm, while the Wood Armer moment, considering the Mxx, Myy and Mxy, which would all be acting along the direction of reinforcement, i.e., at 20 o angle to the global X

direction is 74.2kNm. This is the moment for which the slab needs to be designed. The Wood Armer moments could be obtained at top and bottom of plates along both the reinforcement directions. This option could be accessed by clicking on the ‘Wood Armer Moment’ button as highlighted in the image above.

The results could be viewed in tabular format from

Results > Results

Tables > Plate > Force (Unit Length) … Once the table inte rface opens, click on Plate Force (UL:W-A Moment as in image below and select the required load cases as well as output positions.

25

www.MidasUser.com

FAQ


How is the varying longitudinal stiffness of ballast/concrete bed considered for rail track analysis? For Rail Track Analysis, Multi linear elastic links are used to simulate the bilinear curve of longitudinal stiffness for loaded and unloaded conditions of ballast/concrete bed connecting rail and superstructure

The multi-linear link data is available under “MEInk” Tab, in the table of Elastic link data.

26

www.MidasUser.com

FAQ


How to make a taper from composite T section to composite I? The number of points to define the section in I-end and J-end of the taper section to be generated has to be same. Since a Composite T (say end I of taper section) will have lesser defining points (

) as

compared to Composite I section (end J ) , the tapered element could not be generated. However an as an alterna tive, we could define T section with so many points as tha t of I section and generate the tapering. Under Properties > Section Properti es , click Add > Composite tab > Composite I option from the drop down menu, enter the data as per the guide diagram shown below and generate the required I section. To generate a composite T section, using the composite I girder option, enter the BL1 and BR1 values slightly lesser (say, 0.5mm lesser) than the BL4 and BR4 values. By doing so, the additional points required would have been entered, still

retaining

the

sectional

properties of a T section. By this way the tapering of section could be accomplished. Once the two sections are generated, the tapered section can now be made using these two section properties.

27

www.MidasUser.com


To generate such tapered section, follow the steps below in sequence: Click on Properties

Section Properties

Add

1 2

3

4

5

6

7 8

1.

Click on Tapered

2. Select Composite

PSC-I

3. Enter Name 4.

Enter basic data like slab width, thickness, etc and import the “Size-I” and “Size-J” sections. These would be the sections that are generate d using Composi te I girder option.

5. Enter material data for equivalent section property calculations 6.

Select they type of variation (Linear, Parabolic or Cubic)

7.

Modify the offset as required

8. Click OK

28

www.MidasUser.com

FAQ


Why plate thickness is not visible in the model while viewing solid view? In case the plate thickness is not being displayed after switching off the hidden view, the plate thickness option is not activated.

For viewing the plate thickness, please follow the instructions given below:

1 2 3 4

5

6

Check “Plate Thickness Option”

7

29

www.MidasUser.com

FAQ


What kind of boundary conditions & elements are available in response spectrum, and time history analysis? Elements and boundary conditions used when performing an Eigen analysis, response spectrum analysis, or a time history analysis can be summarized as follows.

1. Eigen Value Analysis / Response Spectrum Analysis 1)

Element: Tension only / Hook / Cable, Compression only / Gap elements

2)

Boundary conditions: General Link such as Tension only / Compression only

are replaced with Truss Elastic Link are replaced with General Type of Elastic Link

2. Linear time history analysis 1) Element: Same as Eigen analysis / response spectrum analysis 2) Boundary conditions: Force Type and Tension only / Compression only Elastic Link of General Link are replaced with General Type of Elastic Link

30

www.MidasUser.com


3. Non-linear time history analysis 1) Element: Same as Eigen analysis / response spectrum analysis 2) Boundary conditions: Tension only / Compression only Elastic Link is replaced with General Type of Elastic Link Note relationships with eigenvalue analysis and time history analysis can be summarized as follows: - If you choose Time history analysis method as Modal (mode superposition method), you must first perform the eigenvalue analysis. - If the Damping Method for Time History Anal ysis is Modal or Strain Energy Proportional, you must previously perform the eigenvalues analysis.

31

www.MidasUser.com

FAQ


How to i nput surface spring coefficient for underground structure? Surface Spring Supports function of midas Civil is a feature to enter the Soil boundary conditions that is automatically calculated based on the area section shared. You can automatically enter a number of boundary conditions representing the ground at once.

Ground structure are those which rests on the ground. The structure may sink depending on the siz e and shape of the

ground sup port

and the load app lied. Therefore, the

boundary conditions of the soil must be in the form of spring having a stiffness rather than full constraints (Supports).

Midas Civil has boundary conditions, in form of spring stiffness, namely Elastic Link and Point Spring Suppo rts each with six degree s of freedom. Point Spring Suppor t is a boundary condition to be entered at a node, and Elastic Link is a boundary condition for entering between two nodes. Therefore, Point Spring Supports and Elastic Link has to consider the allotment area for each node to give the spring coefficients in the boundary condition. Ground Spring Coefficient = Section Area × Ground Reaction Force Coefficient In the figure below, on left, shows a three-dimensional shape with three nodes, and on the right is the plan of the figure, the bottom of the tank.

For the boundary conditions to represent the soil type at the bottom surface of the water tank, you must enter a value at node 17 to input the spring coefficient corresponding to the area 1m2. Similarly at node 20 and 5, spring coefficient corresponding to the area 0.5m and

0.25m 2

2

respectively, has to be entered.

If the geotechnical boundary conditions are to be entered only for a small number of nodes,

you can enter as above, taking into account the contribution of each sectional area. However, if the nodes you need to enter the geotechnical boundary conditions are multiple in number, it can become a very cumbersome task. 32

www.MidasUser.com


Surface Spring Supports function of midas Civil is a feature to enter the soil boundary conditions which automatically calculates spring coefficient based on the shared sectional area. By this way you can automatically enter a number of boundary conditions of the ground at once. Surface Spring Supports feature is available both for the line-element model and plate / three-dimensional elements model. For Plate elements / three-dimensional element mode,

the shared area of the selected nodes are automatically considered in the spring coefficient calculation, and for the line element it is calculated by considering the Shared length

×

Width entered.

Therefore, the user to enter the geotechnical boundary conditions in the model, must be using the Surface Spring Suppo rts. And provide width of the ground reaction force coefficient for line elements.

33

www.MidasUser.com

FAQ


How to input temperature gradient load for a general or PSC section? For assigning temperature gradient loads, please follow the procedure given below. From Main Menu selectLoad > Temp./Prestress > Beam Section Temp. For the Sample Positive Temperature Differences shown below, following input is required for a section of depth 3000mm: Enter H1 as 0 mm H2 as 150 mm T1 as 17.8 [C] T2 as 4 [C] Click on Add

Change Parameters: Enter H1 as 150 mm H2 as 400 mm T1 as 4 [C] T2 as 0 [C] Click on Add

4. Select Relevant Load case 5. Select relevant section type. ( For PSC

Change Parameters: Enter H1 as 2850 mm H2 as 3000 mm T1 as 0 [C] T2 as 2.1 [C] Click on Add

“Value type-

sections”

select

h1

3

Input Initial temperature, Elasticity

2

Sample Positive Temperature Differences Input

General option ) 6.

1

and

h2

thermal

coefficient over here or can be taken as defined in the material properties for individual elements.

h3

h1 = 0.15m h2 = 0.25m h3 = 0.15m

4

Select reference point depending on the input heights from top or bottom of section 7. For value type or general sections, average section width between height H1 & H2 should be entered manually, for PSC/Composite type sections width will be taken automatically as per

5

section definition 8. After entering the above data, Select the relevant elements and click “Apply”

34

www.MidasUser.com

FAQ


How to get concurrent reactions due to moving load? To get concurrent reactions at support node (i.e. maximum or minimum force at one node and corresponding forces at the other nodes), concurrent reaction groups has to be defined.

Create a Str ucture Group for concurrent reactions and group all support nodes in this group

After definition of live loads, define the concurrent reaction group using the work flow as shown. From Main Menu select

Load > Moving Load > Moving Load Code > Concurrent

Reaction Group

1 2

3

For Viewing Concurrent reactions after analysis, go to Results > Result Tables > Concurrent (Max/Min Reactions)

35

www.MidasUser.com

FAQ


What does the error ‘PSC/Composite type of beam section temperature cannot be applied to section of the element’, imply? This error happens, say, when a temperature gradient is assigned with PSC/Composite option for a simple beam element.

Usually, for a bridge superstructure, when the superstructure sections are defined using the in-built PSC/ Composite section templates,

assigning the

temperature gradient across the section becomes easier with the auto matically cons idered

section

width and the effective area for temperature gradient

load computation. However, this option is not applicable for certain type

of sections. Say for example, in a superstructure with PSC section, the diaphragm is solid, which is defined by PSC Value type sectio n, a section type other than from the PSC templates. So, for these sections, the temperature gradient load can’t be applied using the PSC/Composite option. In stead, the General option has

to

be

selected

for

temperature

gradient

application, and the width has to be manually entered.

If the user, by mistake applies the PSC/Composite option for such sections, then the user is warned of the said error message in the message window.

36

www.MidasUser.com

FAQ

midas Civil ▶ Analysis

What is the difference between rigid link and rigid type elastic link ? When defining a elastic link, under certain conditions Rigid type Elastic link may be defined. How is it different from defining the link as Rigid Link

Rigid link constraints geometric, relative

movement of a structure, where degrees of freedom of subordinated nodes called Slave Nodes

are

constraint

by

a

particular

reference node called Master Node. The relative movements of the master node and slave nodes are such as if they are interconnected by a three dimensional rigid

RIGID LINK

body.

In

this

case,

relative

nodal

displacements are kept constant. While an Elastic link connects two nodes to act as an element, where the user defines its stiffness. Rigid Type of Elastic Link and Rigid Link are similar in that both are used to simulate rigid behavior.

ELASTIC LINK

However,

the

user

must

be

cautious in using them because their internal processes are different in the program 1. Rigid type Elastic Link v s Rigid Link 0 . 0 0 1 -

111111

Rigid Type of Elastic Link

Rigid Link

In case of Rigid Type Elastic Link , the element stiffness is automatically calculated based on the working model , assigning a large stiffne ss value of magnitud e 10 5~108 times the

stiffness of neighboring elements. Such exceptionally large stiffness may cause a numerical error because of the relatively large stiffness of the link element. Therefore, when the model contains an element, which has large stiffness to replicate a rigid action, it is recommended that Rigid Link be used rather than Rigid Type Elastic Link. Rigid Link geometrically constrains the relative movements between the Master and Slave Nodes without being affected by large stiffness of other members.

37

www.MidasUser.com


2. Rigid Link + Support Slave Node

1

OK

Master Node

Slave Node

Improper 2

Master Node

Master Node

Improper

3

Slave Node

Rigid link should never be assigned with a boundary condition. Looking at Model 2, the support boundary condition is applied to the Master Node of Rigid Link, which implies the support condition is also applied to the Slave Node. This should be avoided. While in Model 3, the support condition is applied to the Slave node. Slave node is constrained by the Master node boundary condition and hence the Slave Node will be

ignored. Model 1 (Rigid Link



Elastic Link



Support), is the right way to define a boundary

condition.

3. Elastic Link + Support Support

Support

Support

Support

Elastic Link has not been assigned boundary conditions. In such a case, the links will be considered as beam elements having the equivalent stiffness. In order to correct this, the ends of the elastic links must be assigned proper boundary conditions or Point Spring Support.

38

www.MidasUser.com


4. Computational Time for Analysis The figure below shows a process in which a total of 72 (6x12) d.o.f are compressed to 54 d.o.f with in the plan e of the diaph ragm, depic ting the behavior of a cross girde r in a bridge deck. UZRXRY UZRXRY UZRXRY UZRXRY UZRXRY UZRXRY UZRXRY UZRXRY UZRXRY

Slave nodes

Master node

UZRXRY

UZRXRY UZRXRY

Ui : Displacement d.o.f in i direction at the corresponding node

Ri : Rotational d.o.f in i direction at the corresponding node 110001 : Ux Uy Rz d.o.f

If a degree-of-freedom of a particular slave node is constrained to a master node for the relevant degree-of-freedom, by using "Rigid Link" relationship, all the attributes (nodal load or nodal mass) including the stiffness component of the slave node are converted into an equivalent component of the master node. Giving such geometric constr aints reduces the number of degrees of freedom which can significantly reduce the computational time for analysis. For instance, consider the cable stayed bridge in the figure above where ‘Rigid Link’ action is used to depict cross girder diaphragm action. If this link is analyzed with the diaphragms modeled as Rigid type Elastic link,

the number of d.o.f will increa se substantially. Each

node represents 3 additional degrees of freedom. Hence in a model with large number of nodes in an analysis can result in excessive program execution time, or it may even surpass the program capacity. It is therefore recommended that the number of degrees of freedom be minimized as long as the accuracy of the results is not compromised.

39

www.MidasUser.com

FAQ


What is the difference between lane element and cross beam method for vehicular load distribution? When should each be used? The difference between lane element and cross beam element for vehicular load distribution is in considering the transverse rigidity of the system and the kind of model generated (line, plate or grillage model) In case, the structure is modelled as a line element and is assigned whole transverse cross-sectional property, lane element for vehicular load distribution option is used. For grillage models, live load distribution occurs as per the rigidity of transverse members (slab/diaphragm), hence cross beam method for vehicular load distribution is used.

In cross beam method, a cross-beam structure group (transverse elements group) has to be defined and selected for transve rse distribution of vehicular load as shown.

A structure group consisting of all transverse elements

40

www.MidasUser.com

FAQ


How is a truss element and a cable element considered in midas Civil? Truss element can resist both tension and compression, while a cable element can resist only tension.

Main differences in these elements are as tabulated below:

F e a tu r e

TrussElement

CableElement

Sag is predominant

No Sag Sag Effect

Truss element, is linear in general and has constant stiffness.

Superimposition possible Load Combinations

Cable elements are inherently non linear and the stiffn ess changes with the load applied. Hence consideration of sag becomes important. Non linear behaviour, no superimposition

Linear combinations of load cases can be made to compare truss force results.

When non linear behaviour of cable is considered, superimpos ition of load cases are ruled out and combined effect of loads has to be considered.

General Usage

Used for both cable bridges as well as for modelling struts and ties of general bridges.

It’s effective in case of cable bridges i.e. suspension and cable-stayed bridges, where in large deformation effects can not be neglected.

Usage in Cable Bridges

For preliminary design of the cable bridges we go for modelling of cables as equivalent truss elements. The model is checked if the stif fness of the truss is sufficient to resist the initial Dead load.

More detailed analysis may require cable elements be modelled. In this case geometric non linear analysis has to be carried and an elastic catenary behaviour of the cable is considered.

41

www.MidasUser.com

FAQ


Why is the pre-stress elastic deformation loss sometimes positive? Relief in Elastic Deformation of member is reflected as positive

Prestress elastic deformation losses includes elastic shortening losses, which are caused by subsequent loadings (self weight, live loads, creep, shrinkage, etc.) after the prestressing force is applied.

With the passage of time, tendons undergo relaxation which results in reduction of prestress forces and relief of deformation of prestressed member

which

underwent

shortening under prestress.

This relief in elastic deformation of member is reflected as a positive elastic deformation loss as shown above in the tendon loss results table.

If the prestress load/stress application is done in subsequent stages or as per the actual stressing sequence, then the elastic deformation loss would be negative as sequential stressing results in axial deformation and prestress loss.

42

www.MidasUser.com

FAQ


Why is there a Link while elements activated in different construction stage are connected, when graphically viewing the results? At joint between two construction stages, sudden change in force causes the Link. In general for line ar analysis, the principa l of superimpos ition is

applicable to get the

combined effects of load combinations. So is followed to combine results for different construction stages. From Main Menu select Analysis > Analysis Control > Construction Stage To view the output for current stage, activate the option to ‘Save output for current stage’ in the ‘Construction Stage Control Data’ dialogue box

Once that is done, perform the analysis and the results for summation of outputs as well as current stage output could be checked as shown below From Main Menu select Results > Deformations > Derformed Shape Such comparison is made for CS1 ( Construction Stage 1) and CS2 for a box girde r bridge.

The displacements of

different stages are compared.

1 CS 1 – Final Displacement of node 1 (-2.06) mm

43

www.MidasUser.com


2 1 1 CS 2 – Current Stage Displacement of node 1

CS 2 – Final Displacement of node 1

(+1.06) mm

(-2.06 + 1.06 = -1.00) mm

Since the node 2 is activated only in CS2, it has no initial displacement, and hence a kink appears as shown in the image above.

44

www.MidasUser.com

FAQ


How does the software consider beam property changes with pre-stressing tendon? The option for beam property variation can be found in Construction Stage Analysis Control Data

Analysis

Beam Section Property

Changes. If Change with tendon option is checked, the software considers section property as detailed below: -

For unbo nded tendons : The duct are a is deducted from the concr ete gros s area reducing the section property.

-

For bonded tendons : The pre-stressing steel property is converted into an equivalent concrete property as per the modular ratio and added with the concrete cross-section property as shown below. -

Initial Location of CG Shifted Location of CG

P – Pre-stressing Axial Force Ast – Area of pre-stressing steel Ac- Total Area of concrete section

With Just beam property, Stress at top fibre generated by Prestress = P/Ac + P.e1/(I/Y1) With modified beam property, Stress at top fibre generated by Prestress = P/(Ac – Ast + m.Ast) + P.e2/(I-modified/Y 2)

45

www.MidasUser.com

FAQ


what is the basis of P- ∆ analysis in midas Civil? secondary moment from P-∆ analysis matches with classical method calculation using actual stress rather failure stress. why? P-∆ analysis in midas Civil is a real-time displacement analysis, wherein as per the actual load application deflection is calculated and the stiffness matrix is modified accordingly.

The P-Delta analysis option in midas Civil is a type of Geometric nonlinearity, which accounts for secondary structural behavior when axial and transverse loads are simultaneously applied to beam or wall elements.

The flow chart for P- Δ analysis in midas Civil is given below Linear static analysis is performed first for a given loading condition and then a new geometric stiffness matrix is formulated based on the member forces or stresses obtained from the first analysis. The geometric stiffness matrix is thus

Input Analysis Model

Input Analysis Model Formulate Stiffness Matrix

Perform Initial Linear Static Analysis

repeatedly modified and used to perform subsequent static analyses until the given convergence conditions are satisfied.

Formulate Geometric Stiffness Matrix

Formulate Modified Stiffness Matrix

Perform Linear Static Analysis

Virtually all design codes such as ACI 318 and AISC-LRFD specify that the P-Delta effect be included in structural analyses to account for more realistic member forces.

Check for Convergence NO YE S

Static Analysis

Dynamic Analysis

Produce Analysis

Eigenvalue

Results

Analysis

The classical nominal curvature method is used for the calculation of curvature (1/r) using design strain, for calculating secondary moments theoretically considering the design yield strain and factoring it by load correction factors. The additional moment obtained in Midas are dir ectly based on the actual

deflected shape of the member due to the applied loading. Thus comparing classical method results calculated using actual stresses will yield results comparable with P- Δ analysis results.

46

www.MidasUser.com

FAQ


The deformations of master and slave nodes of a rigid link are not exactly same. Why? The rotations of slave node must be the same as master node. But, the translational displacements of slave node are not necessarily the same as master node because the rotation of master node will affect the translational displacements of slave node. Rigid Body Connection constrains the relative movements of the maste r node and slave nodes as if they are interconnected by a three dimensional rigid body. In this case, relative nodal displacements are kept constant, and the geometric relationships for the displacements are expressed by the following equations: UXs = UXm + R Ym ΔZ - RZ m ΔY UYs = UYm + R Zm ΔX - RX m ΔZ UZs = UZ m + R Xm ΔY - RY m ΔX RXs = RXm RYs = RYm RZs = RZm

where, ΔX = Xm - Xs, ΔY = Ym - Ys, ΔZ = Zm - Zs

The subscripts, m and s, in the above equations represent a m aster node and slave node s respectively. UX, UY and UZ are displacements in the Global Coordinate System (GCS) X, Y and Z directions respectively, and RX, RY and RZ are rotations about the GCS X, Y and Z-axes respectively. Xm, Ym and Zm represent the coordinates of the master node, a nd Xs, Ys and Zs represent the coordinates of a slave node. This feature may be applied t o certain members whose stiffnesses are substantially larger than the remaining structura l members such that their deformations can be ignored. It can be also used in the case of a stiffened plate to interconnect its plate and stiffener. Sample calculation of displacements of a slave node calculated from the displacements of master node for a sample load is given below: Node Master

DX (mm)

DY (mm)

DZ (mm)

-1.3725

-0.02404

-1.4286

Slave

X1

Y1

Z1

Distance between slave and master nodes:

RX ([rad]) RY ([rad]) RZ ([rad])

-0.00022 -0.00022

0.05501 0.05501

ΔX (mm)

-0.00049 -0.00049

ΔY (mm)

-210

0

ΔZ (mm) 550

X1 = -1.3725 + (0.05501) * (550) - (-0.00049) * (0) = 28.885 mm Y1 = -0.02404+ (-0.00049) * (-210) - (-0.00022) * (550) = 0.20145 mm Z1 = -1.4286 + (-0.00022) * (0) - (0.05501) * (-210) = 10.124 mm

47

www.MidasUser.com

FAQ


Cable element is automatically transformed to equivalent truss element for linear analysis. What does this message imply? A cable element is automatically transformed into an equivalent truss element in the cases of linear analysis and is considered as an elastic catenary cable element only in case of geometric nonlinear analysis. Cable is a three-dimensional line element, which is capable of transmitting only

axial tension force.

Cables are inherently non-linear and the cable element stiffness varies with internal tension forces.

This change in stiffness could not be captured with linear analysis and hence a geometric nonlinear analysis is often required. To over come this, midas Civil adopts the concept of Equivalent Truss Elements for linear analysis, which incorporates stiffness change due to sag effects of cable as well. The stiffness of an equivalent truss element is composed of the usual elastic stiffness and the stiffness resulting from the sag, which depends on the magnitude of the tension force. The following expressions calculate the stiffness:

 =

 

Truss Element

Kcomb

=

  +  



where,

=

 

   + 

 =

12 3   3

E : modulus of elasticity A : cross-sectional area

Cable as Equivalent Truss Element

L : length w : weight per unit length

In cable bridges, when we try to estimate cable pretension using the ‘Unknown Load Factor’ function, we

T : tension force

formulate a linear model with cables modelled as equivalent truss elements.

48

www.MidasUser.com

FAQ

midas Civil ▶ Post-Processing

Why is the model showing reactions at all the nodes, though support has not been provided? There could be two possible reasons for reactions being shown at unsupported nodes.

1.

Structure Type

In ‘Structure type’ option, X-Z Plane or Y-Z Plane or X-Y plane option is selected and loads are applied in the third plane. For example X-Y plane is selected and loads are applied along Z plane then all the nodes will be experiencing reactions in Z direction as the structure is restrained in ‘Z’ direction. So for a multi dimensional load application ‘3-D’ Structure type must be selected. From Main Menu select Structure > Structure Type

49

www.MidasUser.com


2. No Boundary Change assignment for Settlement Load Cases Settlement Loads have been defined in the model, but boundary change assignment is not done for the same. So for all the static-load cases in post-construction, settlement will be

considered first and then static analysis will be carried out. Definition of boundary change assignment can be done as shown below.

Go to Analysis  ‘Boundary Change Assignment’

3

1

2

4

5 6

1.

Select all support related data

2.

Select all permanent support boundary groups.

3.

Give a name for boundary group combination and click on Add/Replace.

4.

Select defined boundary group combination for Settlement Analysis

5.

Check the option of constraining DOF associated with settlement.

6.

Click OK to complete boundary change assignment

50

www.MidasUser.com

FAQ


How to verify for the uplift due to moving load and how to obtain the corresponding vehicle positioning? Activate the maximum displacement for the generated moving load cases, to view the uplift. To know the corresponding vehicle position, Moving Load Tracer option can be used. Go to Results



Deformations  ‘Deformed Shape…’ 1.

For the movi ng load com binations (max case ) check the deformed shape in the direction of gravity (DZ) at the support locations.

2.

Positive value of DZ implies there is an uplift at that support.

Alternatively, same could be checked by verifying the Reactions

at

supports

for

moving load combination (min case). To

track

vehicle

position

causing the maximum uplift at the

particular

node,

Invoke

moving load tracer from Results



Moving Load



Moving Trace

1.

In the mo ving load tracer menu, select the appropriate

2.

Enter the Key node, node with the maximum uplift.

moving load case (max)

Follow the same steps for reactions as well, except that only support nodes could be selected as Key node. NOTE: It is recommended to check the uplift results for moving load combinations along with permanent load. Clicking on ‘Write Min/Max Load to File’ option, generates a .mct file. Running the .mct file using, Tools



MCT command shell , the said vehicle position is added as a static load

case to existing static load cases.

51

www.MidasUser.com

FAQ


How to obtain vehicle position causing Max./Min. force or moment on an element? The moving load tracer option is used. Go to Results  Moving Tracer  ‘Beam Forces/Moments…’ or ‘Plate Forces/Moments…’ Midas Civil directly provid es the enve lop of

maximum hoggi ng and sagg ing bend ing

moments for each eleme nt. However, at times it becomes necess ary to investigate the vehicle position causing the worst bending moment. Depending assignment,

upon

the

Beam

Line

Lane

or

Forces/Moments

Surface or

Lane Plate

Forces/Moments respectively should be selected to view the results 1. Select MV max load case for max hogging moment. 2.

Click on the key element to input the element number

3. Location on the key element i.e ( i, ¼, … implying whether at element’s ith end or quarter length of the 4.

element etc. Required Force/Moment components.

5. The val ue is displayed in a dedicated box besides ‘Maximum Value’ Similarly, to view the maximum sagging moment, select the MV min Load case.

An image of the vehicle position could be saved for the purpose of Dynamic Report Generation and could later be added to the Analysis Report.

Clicking on ‘Write Min/Max Load to File ’ option, generates a .mct file. Running the .mct file using,

Tools



MCT command shell , the

said vehicle position is added as a static load case to existing static load cases.

52

www.MidasUser.com


Load Case .mct file to be used in MCT Command Shell

Paste the contents of the .mct file in this window and click on Run

MCT Command Shell window

Since this would be added as a new load case to the existing load case, model has to be

re-analysis. The converted static loadcould be verified in the treemenu.

53

www.MidasUser.com

FAQ


Why is dead load results of last construction stage not matching with dead load results in post CS? 1. Loads considered in Construction Stage and Post CS are different 2. Consideration of Locked in stresses 1.

Loads considered in Construction Stage and Post CS are different

All the static loads (like element, line, nodal, etc. ) when activated in construction stage are by default clubbed under, CS: Dead Load. Which implies, then results due to all these static loads, self weight of structure, crash barrier load, wearing course load, etc. would be displayed under CS: Dead Load case of last Construction Stage and not just the self weight. On the contrary, in Post CS stage, the static load cases (say, ST: Dead Load) would represent a user defined static load case, which might just include the self weight and hence the difference. But how to view the results of certain load cases separately within the Construction stages? Add these load cases under ‘Load Cases to be distinguished from Dead Load for C.S output’ box in the Construction Stage Analysis Control dialogue box as shown below.

Go to Analysis ‘Construction Stage Analysis Control’

54

www.MidasUser.com


2.

Consideration of locked in stresses

Construction stage analysis would also incorporate the time dependent effects of creep, shrinkage as well as compressive strength. The dead load calculations for elements activated under construction stage, are based on the age of the concrete on the day when the element is activated . However, in Post CS, the stren gth of concrete taken for the particular element is that at the end of last construction stage. Let us consider the simply supported beam loaded with 20kN/m. The maximum deflection for the same could be calculated and compared, based on the age at which the beam is loaded. Elastic Modulus of concrete is directly proportional to its compressive strength. 20 kN/m

20 m

The equation to calculate maximum deflection of simply supported beam with UDL is : dmax

=

.

Also the maximum deflection is inversely proportional to Elastic Modulus of concrete. Hence, later the age of loading, greater is the Elastic Modulus, and, lesser the deflection. Now say, this bea m has 2 construction stages of 3 days and 1000 day s. So, defining the static Dead Load case under the type ‘Construction Stage Load (CS)’ and activating it when age of concrete is 3 days, Elastic Modulus of concrete at 3 days (E3) would be considered for deflection calculation. However, checking the same in Post CS, Elastic Modulus corresponding to the age, at the end of 2nd stage, i.e., 1003 is considered. To

eliminate

issues,

it

recommended,

such is the

loads which are to be activated

under

construction stage be defined

as

‘Construction

Type Stage

Load (CS)’ as indicated in the image to the right.

The result of Loads under type ‘Construction Stage Load (CS)’ could only be viewed in Post CS on creating load combinations (even for single load case). The result however includes time dependent effects, which is otherwise omitted for Static Load Cases (except Prestress).

55

www.MidasUser.com

FAQ


How to obtain cross sectional stresses for line element? Stress Points have to be defined. Go to Properties  Section Manager  ‘Stress Points…’ Stress points could be defined from, Properties



Section Manager  Stress Points

Such additional stress points are displayed in blue as in image below.

Stress points could be generated at any position for the sections of types DB, PSC / PSC Value, Tapered Section, Composite Section, General and Composite General Section. In case of Composite Section, Stress Points can be defined only for Part2. The results for these additional stress points could be checked using the beam detail analysis option., which could be exported to excel. The option can be accessed from path Results  Beam Element  ‘Beam Detail Analysis…’

56

www.MidasUser.com

FAQ


Why are the pre-stress losses given by midas Civil not matching with the manual calculations? The probable reasons are: 1. Improper definition of tendon property or tendon profile 2. I nternal consideration of the software for pre-stress calculation 3. Consideration of transformed concrete section 4. Consideration of time dependent properties of concrete 5. Sequence of pre-stressing considered 6. Prestressing force applied at one end or both ends 1. Improper definition of tendon property, tendon profile: The common mistakes in this dialogue box are: - Assigning inappropriate tendon material, improper tendon type. - Duct diameters to be of adequate size for tendon area - Consideration of relaxation, entries for friction as well as wobble coefficients and the slip. - Also, confirm the type of bonding of tendon with the duct. - Improper tendon property, - Incorrect number of assigned elements, curve type and reference axis selection - Make sure to input coordin ates based on referen ce axis selecte d, whether base d on element local axis or on global axis system or along a specified curve.

57

www.MidasUser.com


Curve type, Spline is selected, then the tendon curve would pass smooth connecting the entered coordinates i.e P1, P2, P3 and P4 as on Spline curve

the figure to left. No abrupt change in angles at these locations. Curve type, Round is selected, radius R has to be entered. And this curve would have lines P1-P2 and P2-P3 as tan gents as sho wn in figure to the right, where P1,P2 and P3 are the coordinates entered. Unless R is 0, the tendon won’t pass through P2.

Round curve

3. Internal considerations of software for Prestress calculation: Internally midas Civil, divides the elements with tendon profile assigned to it,

Element

Node

into 4 parts and

the effects of Prestressing is calculated at a total

Internal Tendon

of 5 points ( inclusive of i and j ends). So, when

Profile

these divisions are not ending at locations where the tendon profile is having a major change in profile equation (Say, at the curvature of a parabolic profile), then loss calculations Entered Tendon Internal tendon profileProfile

wouldn’t be at its most accurate value. 4. Consideration of transformed concrete section:

The manual calculations are in general, done considering the gross concrete section properties. The variation in section propert y due to the tendon is neglected. Where as midas Civil accounts for the change in property due to the presence of tendons and considers an equivalent transformed section. However if this change in section property is to be neglected, the same could be done under Analysis



Analysis Control



Construction stage  Beam Section Property Changes as shown below:

Not to consider transformed section

To consider transformed section

5. Consideration of time dependent properties of concrete: To consider the time dependent effects such as variation in concrete compressive strength, creep and shrinkage, appropriate parameters would have to be defined, linked with base material and assigned to appropriate elements (Say, only longitudinal and not transverse). Also the time dependent effects are to be considered in the construction stage analysis control. For defining and linking these parameters with base material, options as highlighted on the image is to be used. 58

www.MidasUser.com


Under Construction Stage analysis control the

effects

of



Time Dependent Effect Control

time

dependent behavior like, creep,

shrinkage,

internal creep calculation steps,

etc.

could

be

controlled.

The

implications of common options

are

explained

besides the image below.

To consider creep/shrinkage effects To

c ons id er

ei ther

c re ep,

or

shrinkage or both If user want’s to use their own creep coefficients instead of auto calculation, activate ‘Only user’screep coefficient’ . The

us er

defined

coef fici ents

elements are to entered under Loads Construction stage



C.S. loads

for  

Creep coefficient for construction stage… To apply effect of creep and shrinkage to tendons in model To

a pply

c ompre s si ve

s trength

variation effects in tendon pretension . and consider the elastic shortening

Elastic shortening when manually calcu lated is usually

based on constan t initial force.

Where in with the software, elastic shortening could be calculated with the force variation as well, which might vary the software results as compared to manual calculations. 6. Sequence of pre-stressing considered: The age of concrete during when the tendon s are pre-stressed, the seque nce of prestressing etc. should be carefully assigned in the construction stage. For example, stressing 4 cables at the same instance yields different result as compared to stressing of each cable with a small time gap.

59

www.MidasUser.com


To pre-stress the tendons in sequence, ‘ Additional steps’ are to be defined in the Compose Construction Stage dialogue box. The tendons can be stressed or the load group for each tendon could be activates at these defined additional step time duration. 1

2

As highlighted in the image above, under step ( 1 ) define the additional steps for time duration when a particular tendon would be stressed. Under step ( 2 ), the tendon’s pre-

stress load group is to be activated at the specified time step under the Load tab. Doing so would yield different result as compared to having all the 5 tendons stressed at the same instance. 7.

Pre-stressing force app lied at one end or both

ends:

Jacking force could be applied at both ends or only one end.

Providing inappropriate choice would lead to

different pre-stressing force application as compared to what is intended and the results could vary from manual calculations. This option is highlighted on the image to the right.

Besides the above mentioned reasons, there can be other manual input errors, like incorrect force value, mistakenly entering ‘Stress’ values when the ‘Force’ option is sele cted and so on. With prop er inputs and assumptions,

the

software

results

and

manual

calculations would greatly be comparable.

60

www.MidasUser.com

FAQ


How to view the result table for construction stages? For viewing construction stage results, go to results and select any construction stage and then go to Result Tables

Beam

Forces as

shown below.

1 3

2

Select any construction stage

4

Select the relevant stage

Select the load-case to be displayed in tabular format

61

www.MidasUser.com

FAQ


Why are the stresses not being displayed for moving load cases in the results? The stress calculation option is not checked in the moving load analysis control data. Please follow the instructions given below for changing the same. Go to Analysis



‘Moving Load Analysis Control Data’

62

www.MidasUser.com

FAQ


Displacement of the structure looks unrealistic. How can that be changed? Usually, the deformations shown are not the real deformations, but scaled up deformations.

The real deformations could be viewed by activating

‘Real Displacement’

under

Deformation Detail dialogue box.

Auto-Scale deformed shape

Deformed shape with Real Displacement option

Generally, the scaled up deformation makes it easier to verify or check for any abnormality in the structure behaviour on applying load. If however one is interested in

real

displacement or relative displacement, choose the corresponding options as indicated in the image above. This feature can also be used with other Forces & Moment components.

63

www.MidasUser.com

FAQ


How to view the ultimate moment capacity of PSC girder along with the design moment? To view the sufficiency of the Design, pictorial depiction to compare Ultimate Moment/Shear capacity of the girder with the Design Force / Moment is possible. After performing the PSC design of the sections, under PSC tab



PSC Result Diagram,

the section capacity can be viewed in form of values or its capacity scaled up.

Capacity Design Force/ Moment

As depicted in the image above, the required components and options can be chosen to view the desired results. This is a very qu ick way of verifying if the section is over safe or failing at certain locations.

64

www.MidasUser.com

FAQ


How to view results of a particular load case separately in construction stages? For all the loads applied in construction stage s, the resul ts are clubbed under a single loads case i.e. “Dead load” except for the pre-stressing loads which are displayed under heads “Tendon Primary and Tendon Secondary”.

If a particular load case results needs to be separated from

“dead load case”, then it

needs to be segregated as shown below.

Go to Analysis ‘Construction Stage’

1

2 Load Case Name

3 Select Maximum of 3 Load-cases can

relevant

be separated from “Dead Load

cases

CS”

load

4

Maximum of 15 Load-cases can be se le cted un der one

Load

Case head.

65

www.MidasUser.com

FAQ


How to formulate load combinations for construction stage results? By default, the results of last construction stage will be available in Load combinations in post-construction mode after completion of analysis.

2 1

3

4

Thes e load ca se s give s the results for the last construction stage

fr om

whic h

the

load

combinations could be made

If one wants to extract results for construction stages other than last construction stage, then post-construction mode should be changed to that particular construction stage and then goto Results  Results Tables  Beam



Force…

2 3

1

66

www.MidasUser.com


location for beam results

Input beam numbers

Check the relevant load cases

Check relevant Construction stage/Step

Construction Stage-wise results are obtained for all selected load cases as shown below.

67

www.MidasUser.com

2016 FAQ Midas Civil

Recommend Documents