Structural Redesign Gravity System

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1 Redesign Gravity System Design Considerations A composite floor system was used for this design to maximize the efficiency of the material being used. This type of system requires less material and provides more strength than its noncomposite counterpart. Smaller members are needed when the structural slab provides the necessary composite action for the framing system. These smaller members and less overall material present a distinct cost advantage for the composite system when used properly. Design Loads Floor Loads The following is a summary of the floor loads acting on the structure. The building is divided into many areas each with a different purpose and therefore different loading conditions. The Wisconsin Administrative Code was referred to and used as the guideline for many of the live load calculations. Dead load values have been given allowances or calculated using standard values. Self weight of the system has been accounted for in the RAM design. First Floor Laboratory Live Load: Dead Load 150 PSF (floor load) 50 PSF (mech below) 200 PSF 15 PSF Typical Laboratory Floor Live Load Dead Load 125 PSF (floor load) 20 PSF (misc partition) 5 PSF (ceiling/misc/mech) 150 PSF 15 PSF Typical Classroom Floor Live Load Dead Load 80 PSF (floor load) 20 PSF (misc partition) 5 PSF (ceiling/misc/mech) 105 PSF 15 PSF Jonathan Hill 17

2 Administration Floor Live Loads Dead Load 80 PSF (floor load) 20 PSF (misc partition) 5 PSF (ceiling/misc/mech) 105 PSF 150 PSF (library) 20 PSF (misc partition) 5 PSF (ceiling/misc/mech) 175 PSF 170 PSF (bookstore/vault) 20 PSF (misc partition) 5 PSF (ceiling/misc/mech) 195 PSF 125 PSF (main corridor) 20 PSF (misc partition) 5 PSF (ceiling/misc/mech) 150 PSF 15 PSF Bar Joist w/ Flat Roof Snow Load Dead Load 30 PSF with applicable drift 4 PSF (bar joists) 6 PSF (4 rigid insulation) 3 PSF (metal roof deck) 12 PSF (roofing & ballast) 5 PSF (ceiling/misc/mech) 30 PSF Curved Roof over Laboratories Snow Load 30 PSF with applicable drift Dead Load 8 PSF (beams & girders) 3 PSF (metal roof deck) 8 PSF (roofing & insulation) 6 PSF (ceiling/misc/mech) 25 PSF Jonathan Hill 18

3 Wall Loads All wall loads are taken as industry and company standards and are reflect the self-weight of the material and or systems. Interior Partitions 20 PSF (min) Exterior CMU w/ Brick Veneer 4 Brick 50 PSF 8 CMU 50 PSF 100 PSF Curtainwall / Metal Panel System 20 PSF Design The design process began with the investigation of the existing system. To limit the impact on the architectural and special features of the building minimal changes were done to locations and dimensions of structural members. All foundations and footings locations remain unchanged as to not affect the site layout and overall building footprint. Size Restrictions Finished column sizes were designed to remain within the original square concrete column dimensions. This produces W14, W18, and W21 shapes in place of 16x16, 20x20, and 24x24 columns respectively. Smaller members were considered but not used because larger shapes provide a higher moment of inertia, and hence are better in bending and help to control deflection. Depth restrictions on beams and girders were implemented to control the finished ceiling height. In the original concrete design an exposed structure approach was used and therefore did not require ceiling finishes; however, with the new steel design an exposed structure may not be acceptable in some areas of the building. To compensate for this, size restrictions were placed on the beams in the design program to allow for an equal floor depth as the original design, which was 25 and taken from the depth of the largest concrete framing member. This number was used as a maximum allowable depth. The actual designs will yield a much shallower system. Using a steel system requires fireproofing of members. A slab thickness was chosen (according to code standards) so the structural deck need not be fireproofed. All exposed members must be sprayed to abide by the UL regulations. Jonathan Hill 19

4 Floor Model Using RAM Systems a grid was set up representing the original structural grid and columns laid out according to their positions for the concrete design. Typical 30 x 32 bays were constructed using steel beams and completed using beams spaced at 10-0 on center. Several spacing and framing types were considered before deciding on the final layout. These included steel joists spaced closely together instead of beams, as well as framing the beams in different directions. The heavy floor loads and typical slab openings called for strong members spanning the north south direction. Having the beams span the long direction and the girders span the short resolves perceptible vibration issues that could arise in the school and office atmosphere. To simplify design and construction only one type of floor deck was used despite the varying floor loads throughout the building. Designing for a maximum superimposed live load of 200psf found in the laboratory a 1.5 VLR (21 gage) metal deck was picked. Having a capacity of 216 psf in a span of 10-0 proves to be an acceptable design choice. The deck is topped with 4.5 inches of concrete combining for a total system thickness of 6 inches. The slab is reinforced with 6x6-W2.1xW2.1 welded wire fabric. As the beams span N/S the deck is laid out to span in the E/W direction. All columns, beams, and deck layouts were entered into RAM for analysis. Material properties and codes were set in accordance with the AISC LRFD 3 rd Edition as well as any local design standards that could affect the use of steel in this system. A rendering of the building frame is shown below in Figure 6. Complete floor layouts can be seen in Appendix A. Figure 6: 3D Building Frame Jonathan Hill 20

5 Roof Model Due to the complexity of the vaulted roof structure the curved roof could not be modeled in the RAM analysis program. The original roof system was not redesigned, as the original was a steel system. However, to provide an accurate design of the rest of the building the roof loads must be applied to the lower members and carried throughout the structure. The flat roof system which occurs over half of the building was modeled successfully in RAM keeping all framing layouts and member sizes equal to those found in the original design. The elaborate vaulted roof described earlier in this report had to be analyzed in a separate program and the results inserted into the RAM model. To do this, the 3D analysis program RISA was used. A model of several bays of the curved system was constructed and member sizes were selected to reflect the original system. Snow, dead, and wind loads were applied to the structure and the resulting forces analyzed. The roof system is held up by a series of columns supported by either a girder or standard column from the main building system below. These supports were modeled in RISA and their reactions transferred as point loads into the RAM model effectively modeling the roof loads on the rest of the structural system. The roof was considered a separate system and when tied in to the rest of the building s gravity system it could resist applied lateral forces in both directions. A printout of the results from this analysis is located in Appendix A. Load Layout Once the framing system and slabs have been laid out, loads were created and applied to corresponding areas in the RAM model. These loads consist of all those found in the Design Loads section above. Complete loading diagrams for each floor can be found in Appendix A. Results The gravity system design proved successful and acceptable. The loads placed on specific members acting as the vaulted roof provide an accurate representation of the design loads expected from the actual system. The framing of the floors worked out well in the north-south direction. The location of the slab openings were framed consistently with the rest of the floor. Even though the design loads changed throughout the building, the beam spacing could remain constant with a slight increase in member size. The system thickness was kept within the limitations of the original system (maximum of 25 as taken from concrete beam sizes). Even though vibration was not considered a design issue it was checked against regular office standards. Results fell in the acceptable half of slightly perceptible and therefore no further investigation was needed. Jonathan Hill 21

6 Redesign Lateral System Design Considerations Bradley Tech is made up of 4 distinct levels, each different than the previous causing a need for a lateral system that steps through the building with the floors. Due to the complexity of the buildings façade, mainly the large amount of glazing present in many areas of the building, the majority of the lateral system was limited to interior bays. The presence of large moment frames or braced members along the exterior of the building may induce deflection issues with the building envelope as well as an architectural change to the exterior appearance. The buildings plan geometry is antisymmetrical and thus requires separate investigations of the lateral forces in each direction and the use of different resisting systems to compensate for load irregularity. Design Loads Wind Loads The wind loads on this building were calculated using ASCE 7-02 methods. A 90mph worst case wind was applied to the building with an exposure category of B. Since the building is below 60 feet in height a low-rise approach could have been taken. However, due to the minimal effect on the wind pressures, the standard analysis was done. Shear calculations for windward and leeward sides of the building have been calculated from the known pressures and heights. Diagrams summarizing the shear forces acting on the building for each direction of wind appear in the figures below. Detailed load calculations are available, please see Appendix B. Figure 7: East / West Wind Shear Forces Jonathan Hill 22

7 Figure 8: North / South Wind Shear Forces Seismic Loads The seismic loads on this building were calculated using ASCE 7-02 methods. The building was assumed to have an occupancy category of III and seismic category of II. Based on the code an importance factor of 1.25 was incorporated into the load calculations. In the equivalent frame analysis approach assumptions were taken and include the analysis of a rigid building and the presence of ordinary moment frames in each direction. To adhere to modern code, the seismic shear forces have been calculated and shown in Figure 9. For calculations, please refer to Appendix B. Figure 9: Seismic Shear in Both Directions Jonathan Hill 23

8 Load Cases Load combinations taken from ASCE 7-02 were used in the analysis of the lateral system of Bradley Tech. There are multiple combinations mentioned in the code, however the combinations analyzed were those produced by the RAM analysis program. These combinations include: dead load, live load, snow load, east-west wind load, north-south wind load, and seismic loads. A total of 66 load combinations were analyzed and the worst case scenario design for. In the case of the acting lateral forces, the wind shear and seismic shear were compared to find the worst case scenario. In both directions shear forces due to wind loads controlled. Design The lateral system of Bradley Tech could have been designed in many different ways. Several of these systems were examined before a final design approach was decided on including: masonry shear walls, braced frames, and moment frames. After an extensive drift analysis using RAM Frame a system combining both braced frames and moment frames was found to be the most efficient and effective use of the members. Initial Design Concepts The initial concepts for the lateral system design tried to eliminate resizing gravity members by providing enough moment frames in both directions to resist the applied lateral forces. It was found that the most effective building design is not one that allows one condition to govern (either gravity or lateral). The best design comes when a combination of lateral and gravity forces control the design. This provides maximum use of strength and material. Detailed comparisons of a fully resistive moment frame system, fully braced frame systems, and a combination of the two were done to determine the best design approach. All three systems were acceptable designs that yielded adequate drift control and stress values. Due to interference with architectural aspects of the building, however, a fully braced system could not be effectively used. This resulted in the use of a system containing both moment frames and braced frames. Jonathan Hill 24

9 Lateral Model Using RAM Frame a lateral model of Bradley Tech was developed. Systematically placed lateral frames throughout the building were assigned member sizes similar to those needed for the already designed gravity system. Each direction was analyzed separately due to the complex nature of the building. The lateral model created in RAM is shown in Figure 10. Figure 10: RAM Lateral Model Loads Lateral loads were calculated using RAM Frame s internal methods, which follow IBC 2000 standards. Each load case was entered in the model and then compared with a manual calculation done to ensure the results. After all load cases were entered, RAM generated a series of load combinations to act on the system. Jonathan Hill 25

10 Results Once accurate loads were applied to the initial framing system a drift analysis was done to control building and story drift. A drift criteria of L/400 was maintained throughout the design Lateral Frames The following figures are the final designed lateral frames. Member sizes are detailed in each. Figure 11: E/W Braced Moment Frame 1 Figure 12: E/W Braced Moment Frame 2 Jonathan Hill 26

11 Figure 13: E/W Braced Frames 3 Figure 14: E/W Braced Frames 4 Figure 15: N/S Moment Frame 5 Jonathan Hill 27

12 Figure 16: N/S Moment Frame 6 Figure 17: N/S Moment Frame 7 Figure 18: N/S Moment Frame 7 Jonathan Hill 28

13 Drift Design In the north/south direction four moment frames were designed to resist the lateral forces and maintain an acceptable drift ratio. Braced frames were considered but due to the open nature of the interior floor plan effective locations for the frames could not be found. The moment frames were able to provide enough stiffness to maintain an acceptable deflection without having to increase the member sizes much higher than the gravity members. The east/west direction was more complicated due to the complexity of the building profile. A series of moment frames was an acceptable design for this direction; however braced frames were able to be placed near the exterior of the building without affecting the interior face or exterior façade. This direction consists of 4 isolated braced frames and 2 frames made up of a moment frame with braced members located on the exterior bays. Within the moment frame, member fixities were able to be released effectively eliminating unnecessary moment connections on the lower sections of the building and only keeping part the moment frame to transfer shear from the higher roof system. Below are the stress values in each of the frame members after they were design to control drift. This was a preliminary design approach to determine initial member sizes. Anything that yielded a value of 0.85 or greater from the steel interaction equation warranted further design. The first round of results is shown below in Figure 19. Figure 19: Member Stress in Drift Design Jonathan Hill 29

14 Stress Design After the lateral frames were designed for drift the members were analyzed for stress. This analysis uses the LRFD s interaction equation to check for stress values exceeding code standards. This was done using RAM and Figure 20 shown below is the second output depicting the final design stresses in the updated members. Each member that was considered overstressed in the drift design approach was resized and re-evaluated for stress. It was thought that the members should be in similar stress states and have values near 0.85 from the interaction equation. The similar color of the members shows this result. Figure 20: Member Stress in Stress Design Jonathan Hill 30

15 Redesign Checks Connections Gravity Members Using RAM Beam the connections of the gravity members could be checked against typical design values. This was done to make certain the simple double angle connections used for gravity members would be sufficient for the design forces. A company standard RAM connection file was obtained from HGA and imported into the program. A table showing the assuming connection strengths is given below. It must be stated that these values are quite conservative and a detailed analysis could yield a strength increase of 2kips or more for each reaction. Using these values a connection check was done in RAM Beam resulting in several warnings. Upon investigation and reanalysis the questionable connections were deemed acceptable. Please see Appendix C for a print out of the connection check. Reaction Shape (kips) W W10 18 W W W W18 54 W21 63 W W W W Table 21: User Defined Allowable Connection Values Jonathan Hill 31

16 Lateral Members Using RAM Frame lateral member connections were checked against code standards described in the LRFD 3 rd Edition. Below is an image showing the types of connections needed to remain in the acceptable stress range. None of the connections require more strength than what a typical moment connection provides. Thus, the moment connections detailed are sufficient to transfer the design loads. Figure 22: Lateral Connection Analysis Foundations The foundations were redesigned for the new steel system using RAM Foundation. Typical spread footing dimensions were specified along with reinforcing types to match the original design. RAM was able to design for uplift and all foundations and resultant loads were compared with the originals to verify design acceptability. Foundation dimensions and reinforcement details can be found in Appendix C. Jonathan Hill 32

17 Redesign Final Design Summary Summary The complete structural redesign of Bradley Tech was a success. The extensive roof and canopies present in the original design were ignored in the redesigned. They original design of those systems will be used in the redesign. The gravity system was effectively designed for the original design loads and members were kept within established size restrictions. The framing layout fits the floor plan very well. This is mainly due to the size of the bays, the direction in which the beams span, and how well they frame the large number of slab openings needed in the building. The lateral loads were calculated using ASCE 7-02 design guidelines and the lateral system designed to resist these loads. A system incorporating a series of moment frames and braced frames was developed to provide an efficient solution to lateral forces. These frames were designed based on story and building drift criteria to prevent unacceptable deflections in critical areas of the building. The members analyzed for drift were then evaluated for stress using the LRFD s interaction equation. Failed members were increased and reevaluated. Once all lateral members were deemed acceptable by stress criteria their resulting drift was investigated and found not to be critical. After all members were sized the connections of both the lateral and gravity system were analyzed. Using design values provided by Hammel, Green and Abrahamson as well as RAM calculated values all connections passed inspection. To complete the redesign the foundations were designed and compared to the original system. The new system is acceptable for all gravity, lateral and uplift forces acting below grade. To complete the structural redesign a cost comparison of the existing system and the new system was done. This helps to evaluate cost of material, labor and equipment used during construction. Along with the cost analysis, structural schedules were developed to compare construction time of the individual systems. As a final investigation of the redesigned system, a new façade system will be looked into. Problems arising from such a system will be presented as well as effective design and construction solutions to typical problems. Jonathan Hill 33