ADAPT-PT 10 Training Manual Version 0.0
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Table of Contents
Page 1. Initia Initiall Consid Considera eratio tions ns a. Pour Sequence and Stressing Conditions b. Typical Span vs. Thickness c. Co Cover Requirements d. Loading 2. General Settings 3. Design Code 4. Design Setting 5. Span Geometry 6. Support Geometry and Stiffness 7. Support Boundary Conditions 8. Loads 9. Material-Concrete 10. Material-Reinforcement 11. Material-Post-Tensioning 12. Base Non-Prestressed Reinforcement 13. Criteria-Allowable Stresses 14. Criteria-Recommended Post-Tensioning Values 15. Criteria-Calculation Options 16. Criteria-Tendon Profile 17. Criteria-Cover/CGS 18. Criteria-Minimum Bar Extension 19. Criteria-Load Combinations 20. P/T Recycling 21. Report
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This manual was created to introduce new users to the basics of designing post-tensioned slabs, beams, and girders using the ADAPTPT software from Adapt structural concrete software. This manual should be used in conjunction with the CWI Design Guidelines for the structural design of Parking Facilities. This manual is the sole property and for the sole use of Carl Walker, Incorporated and its employees. Any unapproved use or distribution of this manual is subject to legal action. For P/T garages, CWI typically uses ADAPT for three main purposes: •
Design of P/T Slabs
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Design of P/T Beams/Girders
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Analysis of deflections
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As with all engineering software, the output you receive from software should be viewed with some skepticism. An improperly created file is useless to the user. It may be a cliché, but it is a good reminder: garbage in/garbage out. P/T garages will have a slope to allow for drainage. This means that the T/beam elevations will vary along the length of the beam. CWI will not model a sloping beam in ADAPT. The design will be done for straight beams, unless there is an actual floor offset.
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1.0 Initial Considerations A. Pour Sequence and Stressing Conditions The initial step in the P/T design of a structure should always be the development of a suggested pour sequence for construction. The suggested pour sequence should take into account many factors including: pour sizes, contractor flexibility, site limitations, and project phasing. The pour sequence should be well thought out so that the contractor can most efficiently work his way from the ground up and eventually off the site. Ideally, the pour size should be around 10,000 square feet. A reasonable upper limit for a pour is 15,000 square feet, while a lower limit of 5,000 square feet should be used. Smaller pours can always be combined, but large pours should be avoided as they will add cost due to additional manpower and possible reworking of the pour sequence. If possible, the pours should be arranged so that the contractor has flexibility in choosing his next pour. The engineer should understand that once a pour has been made, the stressing of the tendons may not occur for several days (see spec section 03210, paragraph 3.4D for time limit on stressing). If another pour can be made during this time frame, the construction schedule can be shortened. A pour strip could be added to allow more contractor flexibility, but the added cost of the pour strip should be weighed against the potential benefit. Site limitations will sometimes dictate the pour sequence. Be especially aware of existing structures that may restrict where the stressing of tendons can occur. Other site limitations may involve the locations where cranes and concrete pumping trucks will operate. Many projects will involve special phasing issues that must be addressed in the suggested pour sequence. Early occupancy of any part of the final parking structure is an issue that should be discussed prior to the development of the pour sequence. If early occupancy is required, then the engineer must pay attention to the need for operational stair/elevator towers and an operational ramping system. Other potential phasing considerations include: roadways and railways, construction of adjacent buildings, and demolition of existing buildings. The engineer should also understand that the contractor may propose to change the pour sequence at a later time. A disclaimer should be included stating that changes to the pour sequence will require additional services by the engineer. Refer to existing projects for examples of the wording. 4
B. Typical Span vs. Thickness Most P/T slabs will fall within the range of 5 to 7 inches thick; primarily because slabs less than 5 inches may not have adequate tendon profile, and slabs greater than 7 inches are usually to heavy. For moderate exposure conditions, a depth/span ratio of L/45 is used, and for severe exposure conditions a depth/span ratio of L/42 is used. Typical slab spans would be as follows:
Span 18' 20' 22' 24' 26'
Exposure Condition Southern Northern (L/45) (L/42) T" (actual) T" (actual) 5" (4.8") 5.5" (5.14") 5.5" (5.33") 6" (5.71") 6" (5.87") 6.5" (6.29") 6.5" (6.4") 7" (6.86") 7" (6.93") 7.5" (7.43")
Post Tensioned beams have several variables that must be considered in determining its size. As a general rule, most p/t beams will be between 12-16” wide and 24-36” deep. For instances where the design snow load is large or if there is landscape loading the engineer should determine the depth and width of the beam from a ADAPT run to ensure the floor to floor heights are adequate. To simplify the process, a table was developed below which lists beam depths for various exposure conditions, slab spans, and beam spans. In developing the table, the following assumptions were made: Southern Exposure Allowable tensile stress = 9(f’c)^1/2 Northern Exposure Allowable tensile stress = 6(f’c)^1/2 Short Span Slab L = 18’-0” Long Span Slab L – 26’-0” Beam Width 14”-16” Span 56' 58' 60' 62'
Exposure Condition Southern Northern (L/45) (L/42) 28-30 30-32 30-32 32-34 32-34 34-36 34-36 36-38
As a general rule, girders will be 6 to 8 inches wider than the beams to accommodate the increased number of p/t anchorages. C. Cover Requirements 5
Providing adequate cover for durability reasons is discussed thoroughly in chapter 4.4 of the design guidelines. The engineer should establish the proper cover values to ensure adequate durability prior to performing any design calculations. Another cover requirement that must be addressed prior to design is the fire-resistance rating. The required fire-resistance rating generally has the units of hours and is dictated by the governing building code. The following discussion is based on the IBC. The engineer should use the greater cover of either durability or fire-resistance in the design. 1. Establish whether you have an open or enclosed parking garage. If the structure does not meet the requirements for an open parking garage then is it defined as an enclosed parking garage. For open parking garages, proceed to step 2. For enclosed parking garages proceed to step 3. 2. Determine the type of construction by using Table 406.3.5. Since garages are generally constructed from noncombustible materials, the type of construction will fall into one of five possible categories: IA, IB, IIA, IIB, or IV. If the area of a tier exceeds 50,000 square feet or the structure exceeds 8 tiers then the engineer must proceed up the table from type IIB until both the tier area and the number of tiers for the structure does not exceed the tabulated values. 3. Determine the type of construction by using Table 503. The occupancy group is S-2 per section 311.3. Since garages are generally constructed from non-combustible materials, the type of construction will fall into one of the five categories: I, II, III, IV, or V. The engineer will need to find the category that allows the area of each tier and number of tiers for the garage. Also the overall height should not be exceeded for the type of construction. 4. Determine the fire-resistance rating in hours. Enter table 601 with the construction type established in either step 2 or 3 and read the applicable fire-resistance rating for floor construction. 5. Determine the required cover for structural elements. Enter Table 720.1(1) with the fire-resistance rating determined in step 4. The amount of concrete cover is tabulated for different floor members and the different fireresistance rating values. It is important for the engineer to note the difference between restrained and unrestrained members. A restrained member is any interior span of a continuous slab, beam or girder. End spans are generally
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considered unrestrained and are therefore subject to more stringent cover requirements. D. Loading The final consideration before proceeding into the actual design of P/T members is the loading that will be placed on the members during their service life. The engineer should review the applicable building code provisions on live load reduction and snow load. The IBC does not allow any reduction in live load for floor members in parking garages. Be careful when examining older projects that may have been designed for live load reduction or a different live load than is specified today. Snow loading should be calculated using the building code procedure and added to the parking live load for any member that is exposed to the sky. There fore the top tier will require separate ADAPT runs from typical tiers. Refer to allowable stresses for members with snow loading vs. members without snow loading.
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General Settings
When a user opens ADAPT-PT they will select a new file. The first thing a user will need to do is enter the general settings of the current design file. This is the location where you can name your file (i.e. Grid 5 A-B). The user can select what type of member they would like to design (i.e. slab, beam). The user is given two options for the geometry input, conventional and segmental. A conventional geometry input is used when the beam will have a continuous width, depth, and effective flange. If the beam will have a different width, depth, or effective flange width along its span the user will need to select the segmental geometry input. The CWI standard is to consider effective flange width in bending (for beams and girders).
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Figure 1 General Settings
3.0
Design Code
This screen is pretty self explanatory. The user will need to select the design code that will be used for their project.
Figure 2 Design Code
4.0
Design Setting
The execution mode should be set to interactive. This will allow the designer the option to modify the final effective force and tendon profile during the execution process. The CWI standard is to reduce the moments to the face of support. Be aware that special circumstances may arise when this is not the case. The CWI standard is to not re-distribute moments during the design process. The CWI standard is to NOT increase the moment of inertia over the support.
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Figure 3 Design Settings
5.0
Span Geometry
This is the screen where the user inputs all of the span properties. The user will need to define the effective flange width (for T-beams) based on the current ACI-318. The left and right multipliers are used to dictate the amount of the effective flange on each side of the beam stem. For an L-Beam, the multipliers would be 1 and 0. The user will input the effective flange width for bf. The next screen will allow the user to check the effective flange width based on ADAPT-PT’s ACI-318 effective flange width. Adapt does not center to center spacing of beams for effective flange width. That means if Adapt shows a higher effective flange width the user input may be ok. The effective flange width should be based on the user input and not Adapt’s be. We do not use the flange width for bf because ADAPT will use this value in calculating the centroid of the beam, which is incorrect.
Figure 4 Span Geometry
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Figure 5 Effective Flange Widths
6.0
Support Geometry and Stiffness
This screen will allow the user to input the size and stiffness of the supports conditions. For a slab the user the user will select the option of lower wall. The wall height shall be set to zero. The dimension B should be set to the width of the design slab strip, while D should be set to the beam width. For a beam supported by a girder the user would select the option to use no columns. This allows the beam to be designed for the reduced moment at the face of the girder, since the user will input the width of the girder as the D, while the B is the beam width. If the beam is supported by columns the user will select the correct support selection of either both columns or lower column. The user can input a percentage of stiffness to use for the columns. It is typical for CWI to assume 70% when designing the columns, so the user would input 70 for the stiffness to accurately modal the designed condition. When inputting the length for the lower columns on the first supported level the designer should not forget to include the additional distance to the top of footing.
Figure 6 Support Geometry and Stiffness
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7.0
Support Boundary Conditions
At this screen the user will then select the boundary conditions for the supports. CWI typically checks the SW box. The user must ensure that the dimensions for the supports are correct on the previous screen. The user then can select whether the support is pinned or fixed. Typically CWI assumes a pinned connection at the top of footing. The CWI standard is to not consider full fixity at the ends of the slab/beam. This will allow the program to determine the moments based on the relative stiffness of the frame elements. Only in certain situations, such as a stiff shear wall as a support, would the designer select this option.
Figure 7 Support Boundary Conditions
8.0
Loads
This is the screen where the user will input the loads applied to the beam/slab. CWI standard is to skip the live load with a skip factor of 1, but the user should verify this with the applicable building code. For concrete structures the unit weight is set to 150 pcf. For slabs the user can either use the include self weight option or not. If the user chooses not to include self weight they must ensure they include it as a Dead load. For beams the user will not include the self weight option. This is because the program will calculate the weight based on the effective flange and this will not include the entire loading from the slab. The user then can input loads as SW, SDL, LL, and X.
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Figure 8 Loads
9.0
Material-Concrete
The CWI standard is to use cylinder information. The user will input the correct concrete strengths and modulus of elasticity for both the beams/slab and columns. The creep coefficient is set to 2, which is the CWI standard. The CWI standard is to set the f’ci to 3000 psi for beams and 2500 psi for slabs.
Figure 9 Concrete
10.0 Material-Reinforcement
The user will specify the type of reinforcement to be used for the project. The standard is fy=60 ksi, E= 29000 ksi. For beams and girders the top and bottom bars should be selected as #11. This allows the user to select any size bar for the mild reinforcement without re-calculating the effective depth of the section. For slabs this should be set to #5 bars, as CWI typically tries to limit the size of bars in slabs to #5. The preferred stirrup size can be inputted at this screen. When selecting a stirrup size the user must ensure the bends can be made to fit in the beam size you have chosen (typically CWI 12
uses #4 stirrups). It is typical for CWI to use 4 legs in beams greater than or equal to 36” wide.
Figure 10 Reinforcement
11.0 Material-Post-Tensioning
CWI uses unbonded post tensioning in C.I.P. garages. The typical inputs are as follows: area=.153 in^2, fpu= 270 ksi, and fse= 175 ksi. There maybe some occasions when different post-tensioning would be used.
Figure 11 Post-Tensioning
12.0 Base Non-Prestressed Reinforcement
The CWI standard is to select no. 13.0 Criteria-Allowable Stresses
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This is the screen where the user will input the allowable stresses for an element. For slabs the tension stresses are as follows: initial = 3 ksi, final= 6 ksi. If the slab is to support plaza loads (hot-applied waterproofing must be used) the allowable stress can be increase from 6 ksi to 9 ksi. For beams the following stresses are to be used: initial= 3 ksi, sustained=total= 7.5 ksi (for unreduced live load). For beams that have Plaza loading (hot-applied waterproofing must be used) the total and sustained stress can be 10 ksi. Compression stresses shall be 0.6f’ci for initial stresses, 0.45f’c for sustained stresses, and 0.6f’c for total stresses.
Figure 12 Allowable stresses
14.0 Criteria-Recommended Post-Tensioning Values
The next screen allows the user to input the recommended post tensioning values. For slabs the minimum pre-compression value shall be 150 psi for Zone 1 and 180 psi for Zone 2 & 3. The maximum precompression value shall be 500 psi for all zones. The minimum and maximum percentage of dead load is to be set at 25% and 85%
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respectively. These values can be altered, but deflections must be met to ensure proper drainage and deflection criteria are met. For beams/girders the minimum pre-compression shall be 150 psi for Zone 1 and 200 psi for Zone 2 &3. The maximum pre-compression shall be set to 500 psi for beams and 999 psi for girders. These maximums can be increased but the user must ensure that the end view detail can be constructed (this is typical even if the maximum pre-compression is not exceeded.). The minimum and maximum percentage of dead load is to be set at 25% and 85% respectively. These values can be altered, but deflections must be met to ensure proper drainage and deflection criteria are met.
Figure 13 Recommended Post-Tensioning Values
15.0 Criteria-Calculation Options
The CWI standard is to select the force option method. This allows the user to select the final effective force. CWI assumes that a tendon will have a final effective force of 27 kips (1/2” diameter tendon). The force the user will input will be a multiple of 27. 16.0 Criteria-Tendon Profile
The CWI standard is to use a reverse parabola tendon profile. The inflection points will be at the 1/12 th points (i.e. X1/L=X3/L=1/12=.083, X2/L=0.5). The default extension of a terminated tendon as a fraction of the span should be set to 0.2. This matches the CWI standard of extending add tendons to L/5 of the adjacent span. The shape of the tendon extension should be set to Downward Parabola; Anchor at the centroid. If the user would like to specify a different tendon profile they must be careful to properly show it on the drawings and make sure the P/T supplier has properly accounted for the different profile in the shop drawing process. CWI will typically try to avoid using
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alternative profiles to limit the possibilities of mistakes during construction.
Figure 14 Tendon Profile
17.0 Criteria-Cover/CGS
This is the screen where the user will input the minimum clear cover to the bottom and top mild reinforcement. They will also input the minimum distance to the CGS from the top and bottom fibers. It is important to include the effect of bundling when inputting the minimum distance to the CGS. For example a garage in zone 3 will have a minimum clear cover of 3”. If we assume there will be bundled tendons the minimum CGS could be 3”+1/2”=3 ½”, CWI will use a CGS=4”. CWI will typically round this number to the nearest whole number for beams and girders. The clear covers and CGS will come from the applicable codes.
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Figure 15 Cover/CGS
18.0 Criteria-Minimum Bar Extension
This screen is not used by CWI. The user can use the default settings. The user will ensure from the report the cutoff lengths and extensions of the bars. For beams the typical details will most likely control the cutoff and extensions. For slabs the user will determine the location that steel is no longer needed and from this determine the length of the reinforcement bars. The user must also ensure that the minimum lengths have also been met. 19.0 Criteria-Load Combinations
The user will input the applicable load combinations . The user should remember to include the dead load only case. CWI will use the load X for snow loading and the load combinations should reflect this. For each load case the user must set the factor for HYP=1 (posttensioning). For the initial loading condition the user will set the PT force to 1.25. Initial load factors should be determined by the amount of each load case that will be present at the time of stressing (LL=0). The load combinations and strength reduction factors will be determined by the applicable code. The user will enter lateral load combinations and moments for all applicable beams/girders. The user should determine the sign convention in ADAPT and enter the lateral moments correctly. Also the user must remember to select whether the lateral moments are reversible.
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Figure 16 Load Combinations
20.0 PT Recycling
This is the screen where the user will determine the final effective force and tendon profile required for the loading condition. The user should seek to optimize the beams by minimizing the amount of posttensioning required and increasing the tendon drape. The user should also use a PT force that is a multiple of 27. The user must have the ordinate at the CGC at locations of stressing. This includes construction joints and beam ends. Add tendons can be included in two ways. The first is to use a single tendon path. This allows the user to specify different PT forces for different spans. The next option is to include the multiple tendon paths. This will require the user to input information into multiple Tendons (i.e Tendon A, B, C). The user needs to make sure the same profile is kept for each set of tendons in the same spans. The user needs to make sure that are criteria are met. This is shown by the green ok near the upper left corner. The PT force max and balanced dead load are allowed to be NG. However the user must ensure that the end anchorages can be constructed and the deflection limits are met. For initial stresses the compression must be ok before the user is to move on to the report. The tension stresses can be slightly higher than allowed. The user will just require additional mild steel in these locations, which is shown in the report. There are multiple tabs on this screen. To see the actual stresses select the extreme fiber stresses tab. Here you can determine how close to the allowable stress you are. When the designer has a beam and girder intersection where both sets of tendons would like a high ordinate over the column the designer should be cautious on selecting the ordinates for the girder. The designer needs to ensure that the beam tendons can pass over the girder tendons.
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Figure 17 PT Recycling
21.0 Report
At this point the user will have defined the final effective force and tendon profile for each beam/slab span. In the report sections 1-4 allow the user to check and verify their inputs. For beams supported by girders the user will need to use sections 5.2 and 5.4 to determine the point loads applied to each girder. When selecting the live load point load the user will select the maximum value. The user will also need to use section 7.6 to determine the point loads applied to the girder from the tendons. Section 10 will not be used by the engineer. This section provides the mild steel required, however the user will determine the amount of mild steel at a later point from the report. The next section is section 12. This is the section that ADAPT reports on the shear design. It is important for the user to remember the size and number of legs they selected in the input process. This screen will inform the user on the maximum spacing at 1/20 th points along the beam. IF you are designing a slab there should be no required shear reinforcing. The user must make sure that if the program informs the user of required shear reinforcing that it is true for slabs. The program will provide required shear spacing but indicate it would be only applicable to beams. The CWI standard is to provide #4 shear reinforcement at: 3@2”, 8@6”, BAL@24” for interior spans (T-Beams)
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and 3@2”, 8@6”, BAL@12” for exterior spans (L-Beams). The next section provides the deflections from each load and the final long term deflection. CWI tries to limit the long term deflection to L/400 to L/500. This allows the drainage to still properly work along the length of the beam. If the deflection is greater than L/500 the designer should ensure that proper drainage can still be achieved. The next section (section 21) allows the user to determine the height of the tendon profile along the length of the span. This is used to determine the distance from supports that the construction joints are placed for slabs. It also allows the user to determine the CGS of the tendons at construction joints. The CGS of the tendons needs to be near the CGC at construction joints to allow all of the anchorages to fit properly. The CGS can differ slightly from the CGC but the designer will need to perform a calculation (CWI has a MathCAD file to perform this) to determine the initial stresses at the construction joint. The designer will then need to use Section 29 Detailed Rebar to design the mild steel of the beams/slabs. This section will provide the designer with the calculated required rebar and the minimum required rebar along the span length. The user then can select the size and number of bars to achieve this required rebar. For slabs the user will determine the length from the support (plus development) that a bar is required and verify this is not shorter than the minimum required length. For a beam/girder the user will verify that the required rebar will match the typical beam details for the locations of rebar. The CWI standard is to provide a minimum of 2#4 bars at the top of the beam in the center of the span and 3#6 bars at the bottom of the beam at the support. If four legs of stirrups are used there will need to be 4#4 at the top and 4#6 used. Since this is a P/T garage the user should try to limit the amount of reinforcing bars to allow the P/T tendons more access to fit inside the beam/girder cage.
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Figure 18 Report Generator
References:
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1. “Analysis and Design of Post-Tensioned Buildings Beams, Slabs, and Single Story Frames.” ADAPT Corporation. Redwood City, CA. 2004. 2. “Building Code Requirements for Structural Concrete and Commentary.” American Concrete Institute. Farmington Hill, MI. 2002. 3. “International Building Code.” International Code Council. Country Club Hills, IL. 2006. 4. “Post-Tensioned Design Guidelines” Carl Walker, Inc. Kalamazoo, MI. 5. ADAPT-PT ‘Help’ Menu.
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