FAQ
midas Civil ▶ Post-Processing
Why are the pre-stress losses given by midas Civil Civil not matchi matching ng with w ith t he manua manuall calculations? The probable reasons are: 1. Improper definition of tendon property property or tendon tendon profile 2. Int Inte erna rnal
cons consid ide eratio ration n
of
the the
soft softwa ware re for for
pre pre-stre stress ss
calculation 3. Consideration Consideration of transformed concrete section 4. Consideration Consideration of time depende dependent nt properties of concrete 5. Sequence Sequence of pre-stressing pre-stressing considered 6. Prestressing forc e applied at at one end or both ends 1. Improper definition o f tendon prop erty, tendon tendon pr ofile:
The common mistakes in this dialogue dialogue box are: type. - 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 selection - Make sure to input coordinates coordinates based on reference reference axis selected, selected, whether whether based on element local axis or on global axis system or along a specified curve.
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midas Civil ▶ Post-Processing
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 tangents as shown
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
Node
Element
tendon profile assigned to it, 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 wouldn’t be
at its most accurate value.
Entered Tendon Profile Internal tendon profile
4. Consideration of transformed concrete section:
The manual calculations are in general, done considering the gross concrete section properties. The variation in section property 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 consid er transform ed 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
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midas Civil ▶ Post-Processing
Under Constr uctio n Stage analysis control the
effects
of
Time Dependent Effect Control
time
dependent behavior like, creep,
shrinkage,
internal creep calculation steps,
etc.
could
controlled.
be The
implications of common options
are
explained
besides the image below.
To consider creep/shrinkage effects To
co nsi der
ei th er
cr eep ,
or
shrinkage or both If user want’s to use their own creep coefficients instead of auto calculation, activate ‘Only user’s creep coefficient’ . Th e
u s er
d ef i ned
c oef fi ci en ts
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
ap pl y
c om pr es si ve
s tr en gt h
variation effects in tendon pretension and consider the elastic shortening.
Elastic shortening when manually calculated is usually
based on constant initial force.
Where in with the software, elastic shortening could be calculated with the f orce variation as well, which might vary the software results as compared to manual calculations. 6. Sequence of pre-stressing consid ered:
The age of concrete during when the tendons are pre-stressed, the sequence 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.
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midas Civil ▶ Post-Processing
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 prestress 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 applied 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 selected and so on. With proper inputs and assumptions,
the
software
results
and
manual
calculations would greatly be comparable.
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