CEE 285BEHAVIOR OF STRUCTURAL SYSTEMS FOR BUILDINGS
DESIGN PROJECT
Professor H. Krawinkler
StanfordUniversity
Submitted: March 22, 2006
Team Members:
Jimmy Chan
Asphica Chhabra
Jennifer Moore
Jana Tetikova
Nick Wann
CEE 285BEHAVIOR OF STRUCTURAL SYSTEMS FOR BUILDINGS
DESIGN PROJECT
Professor H. Krawinkler
StanfordUniversity
Team Members:
Jimmy Chan
Asphica Chhabra
Jennifer Moore
Jana Tetikova
Nick Wann
Table of Contents
PART ONE: SYSTEM ASSESSMENT
1.0 Introduction
1.1 Project Proposal
1.2 Individual Roles
2.0 Load Determination
2.1 Gravity
2.2 Lateral
3.0 Structural Design
3.1 Gravity System
3.2 Perimeter Moment Frames
3.3 Shear Wall Design
3.4 Connections
3.5 Foundation
4.0 ETABS Modeling - Analysis and Discussion
4.1 Model Discussion
4.2. Shear Wall-Frame Interaction
4.3 ETABS Model and Frame- Shear Wall Interaction Comparison
5.0 Conclusions
PART TWO: APPENDIX - DESIGN CALCULATIONS
Appendix A – Load Determination......
Appendix B – Gravity System Design......
Appendix C – SMRF Design......
Appendix D – Shear Wall Design......
Appendix E – Connection Details and Calculations......
Appendix F – Analysis Results (ETABS and Interaction)......
1
BD Inc. Project: Palo AltoOfficeTower
PART ONE: SYSTEM ASSESSMENT
1.0 Introduction
1.1 Project Proposal
To build a 10 story office building in Palo Alto according to 1997 UBC specifications keeping the following constraints in mind:
Site Constraints:
- Seismic Loads: the building is located at 7 km from the San Andreas fault.
- Soil profile SD
Architectural Constraints:
- Clear Story height should be at least 8.5 ft.
- 80 ft x 140 ft floor plan
Other Considerations:
- Insure elastic behavior of structure under strong motion earthquake
- Consider foundation system
1.2 Individual Roles
Individual roles were given to each team member:
- Owner: Jennifer Moore
- Architect: Nick Wann
- Structural: Jimmy Chan
- Mechanical: Jana Tetikova
- Contractor: Ashpica Chhabra
The responsibilities of each are outlined below. Each person performed research in his/her own area in order to guide the building system design.
1.2.1 Owner
The owner wanted to have flexibility in the use of functional spaces that can support the unknown future demands on the structure as wells as to entice sales of spaces. Specific areas were chosen and designed for heavier loads in order to meet this flexibility requirement.To increase demand, the owner also requested specific physical characteristics such as an atrium on the first floor and a restaurant. Commercial space on the first floor was also set as a hard constraint in order to rent to retailers. Minimizations of costs were also important to the owner, who desired to have a cost efficient building system.
1.2.2 Architect
The architect responded to the owner’s vision of the building through an innovative and practical extension of the atrium to improve the overall space. Instead of having the atrium at the first floor level, he reversed the sequence and added a large opening running through the building from the 6th to 10th floor. This large open space leads to a reduction in the functional space of the building, however it allows ample natural light to enter the building, creating a livelier atmosphere and increasing the productivity of its occupants.The ceilings at the first floor were increased to 15 ft in order to increase the grandeur and aesthetic appeal of the commercial area. The architect opted against a basement. The lack of basement and commercial use of the first floor required that mechanical systems be placed on the second floor, increasing the 2nd floor story height to 15 ft.
The architect designed two continuous shear wall cores, one on each side of the opening. He has also provided for a restaurant on the fifth floor level, whichprovides for more retail space in the building. This floor was chosen because its central location would be more accessible to the building occupants, which would hopefully increase use. Also, the restaurant’s location on the 5th floor would allow diners to look up through the opening, improving the quality of the lunchtime experience. Additionally, people at the top floors would be able to look down at the decorated restaurant.
1.2.3 Structural Engineer
The mechanical and architectural requirements posed as the primary structural challenges for the structural engineer. Owners concerns were addressed through the architect and not the owner herself.
One of the most important decisions that the structural engineers made was the type of lateral load resisting system. The structural engineers decided on a dual system consisting of concrete shears walls and steel special moment resisting frames (SMRF) in both the EW and NS directions. Ductile shear walls provide excellent resistance to high lateral loads that are probable in highly seismic regions. To achieve this ductility, however, special attention had to be paid to the detailing of the walls’ reinforcement. Additionally, the special moment resisting frames (SMRF) act as a “backup” system providing necessary redundancy to the system.
Special attention also had to be paid to key areas for the heavy loads imposed by the mechanical system components. These areas were strategically placed in locations approved by the architect, so as not to interfere with the flow of the building, yet provide efficient service throughout. One of the most notable structural challenges in the building has to do with the large open core running down the center of the building. This architectural detail provided many structural challenges, beginning with the diaphragm that was assumed to be rigid in this building design. With a plan discontinuity such as this, the engineers would have to analyze the diaphragm further to validate the rigid diaphragm assumption. Many other structural decisions had to be made throughout the design process including the use of composite beams, shored construction, and fireproofing around the stairways.
1.2.4 MEP
The structural engineers collaborated with the mechanical engineers to come up with a scheme for the ductwork, which will primarily run along the interior corridor deck that surrounds the opening. On the 1st through 5thfloor, ductwork will run under the floor beams which are not very deep. The mechanical engineer specified that two chillers and cooling towers are required for the building. Chillers and other Origination systems will be housed in the two mechanical rooms on the second floor next to the cores. Coolers at the roof are also located next to the cores. Four elevators are located in the building. The shear core is housed around the stairway, allowing for most of the vertical pipes to also run along the core. The transformer and generator which account for heavy concentrated loads will be housed outside the building and hence do not affect the structural decisions. Typical MEP features and loads can be found in Table 2-2.
1.2.5 Contractor
The primary role of the contractor was to promote efficiency of the structural design. This affected decisions on member sizing, steel member and shear wall connections, and concrete work. The more similar the connections and member sizes, the more cost efficient the design. Also, connections and members that are readily available in the market are more desirable. Labor was also a concern especially related to the installation of the doubler plates which was avoided by increasing the interior column sizes. The contractor participated in the design process.
2.0 Load Determination
Gravity loads were computed based on MEP load requirements, typical dead loads, and live loads based on varying functional uses. Wind and seismic loads were determined to compute total lateral load effects.
2.1 Gravity
Table 2-1. Dead Load & Live Loads
Loads / ksfConcret+deck+misc. / 0.065
Partitions / 0.02
DL / 0.085
Exterior Cladding / 0.02
Roofing system / 0.05
Self Weights / klf
Floor Beams / 0.05
Girders / 0.1
Columns / 0.2
Live Loads / ksf
Offices / 0.05
Corridors, exits / 0.1
File Rooms / 0.15
Roof / 0.02
The chillers, which may weigh up to 10,000 lbs, were placed on the second floor. The cooling towers are in general placed on the roof for they require a continuous flow of air and are quite noisy. Since at the time of conceptual design no decision was made as to where exactly on the roof cooling towers would be placed, four areas of about 150 square feet where designed to support loads up to 300 psf (Ref.Roof Load Key Sheet).
In addition to chillers and cooling towers, another important consideration is the chilled water loop and condenser loop which will produce a reaction of about 80,000 lb. at the base of the building.
Other geometric constraints arise from providing the building with plumbing, storm, and electrical system. Table 2-2 summarizes the MEP loads and considerations.
Table 2-2. MEP Loads and Considerations
Category /Related Constraints
/ Vertical Load1. Elevator system
Elevators and dumbwaiters (DL and LL) / accessibility and fireproofing / 2 x10000 lb
2. HVAC System
i) Origination System / - / -
Chillers / area of 10 ft. x 20 ft.
4(+) thick raised concrete pad
12 - 15 ft. ceiling height / 300 psf
Cooling towers / area of 15 ft. x 20
ft. height of 15 ft. - 20 ft.
raised above deck / 300 psf
Condenser loop (2 loops needed) / - / 80000 lb
Chilled water loop (2 loops needed) / - / 80000 lb
Masonry wall enclosures and
increased slab thickness for pumps and
compressors / - / 100 psf - 130 psf
ii) Distribution System
Ductwork / - / 5 psf
3. Electrical System
Transformers / concrete encasing 2 ft. x 6 ft. / 300 psf
Emergency Generator / 80000 lb
4. Plumbing System
Tanks and boilers / - / -
5. Fire Protection System
Distribution lines and sprinkler heads / - / -
A summary of the gravity loads along with the architectural renderings of the typical floor plans are included herein.
2.2 Lateral
Once the gravity loads arecomputed and finalized the lateral loads can be determined. The lateral loads are applied in addition to the gravity loads and typically control the size of the members. In our case, the lateral loads are resisted by a shear wall and moment resisting frame system. Wind loads can be very high in some regions such as near the shoreline of a major body of water, such as the Pacific Ocean or theGulf of Mexico. However, the seismic forces imposed on our building were much greater than the wind forces, and therefore controlled the design. Other forms of lateral load, such as blast loading or impact loading are not relevant for the design of an office building and therefore were not considered in this preliminary design.
2.2.1 Wind Loads
The loads imposed on the building were calculated using the UBC formula 20-1. A design wind speed of 90 mph and an exposure category B were used in the formulation of the lateral wind loads. Using the following equation as well as Table 16-G of the UBC, containing values for Ce, the wind pressure at each story and at each mid-story was interpolated:
p = CeΣCqqsI where ΣCq = 0.8 + 0.5 = 1.3
Then, as shown in Figure 2-1, the values of the wind pressure, p, are averaged at each interval and this value is then used as the design wind pressure over the entire half-story. The design wind load was then represented as a line load over the width of the floor by multiplying the wind pressure of the half-story above and below each floor by their respective half-story heights and summing.
Figure 2-1: Distribution of the Wind Pressures over the Height of the Building.
This line load, W in k/ft, was then multiplied by the width of the building to calculate the total force imposed on each floor by the wind. These story forces were then summed cumulatively down the building to arrive at the story shear force. Each story shear force was then multiplied by the story height and again summed cumulatively down the building to determine the overturning moment imposed by the wind loading. The calculations are summarized in Appendix A. As expected, the NS wind produces higher base shear forces and overturning moments of 428 kips and 31,142 kip-ft, respectively. This is nearly twice the loads imposed by an EW windproducing a base shear of 245 kips and an overturning moment 17,950 kip-ft. However, while these lateral load effects are notably large due to the close proximity to the PacificCoast, they were ultimately neglected in place of the even larger seismic loads.
2.2.2 Seismic Loads
The seismic loads imposed on structures in the Palo Alto area are expected to be significant. The seismic loads were calculated according to the UBC (1997). As prescribed by the code, the total base shear is calculated according to design parameters, such as proximity to an active fault, seismic zone, soil profile, type of lateral system, period and the effective seismic weight of the building. The seismic weight was determined in Appendix A using many preliminary assumptions for material and mechanical weights. These assumptions were later verified as conservative averages. The elastic fundamental period of vibration of the structure was determined using code Method A (equation 30-8):
T = Ct(hn)3/4,
where Ct = 0.035 for steel moment-resisting frame was used. Then, the base shear was calculated using equations UBC (1997) 30-4 through 30-7:
Once the total base shear was determined, the forces were distributed to each floor. Since the natural fundamental period was determined to be 1.3 sec > 0.7 sec, thewhiplash force, Ft was determined according to:
Ft = 0.07TV< 0.25V,
This force was applied to the roof of the building to account for the wave reflection which causes a higher inertia force on the top floor. The rest of the base shear was then distributed to the individual floors based on their seismic weight and height. As was the case with the wind loading, the seismic shear story forces were summed cumulatively down the building to determine the individual story shears and the base shear at the ground level. The story shear was then multiplied by the story height and cumulatively summed once more to determine the overturning moment. The results of these calculations can be observed in Figure 2-2.
Figure 2-2:Distribution of the Seismic Forces over the Height of the Building.
As can be easily seen from the results in the Appendix A, the base shear for the building is 1,038 kips and the overturning moment at the ground floor is 96,698 kip-ft. These results are nearly 3 times the largest values obtained from the wind loading, thus the wind loads were ignored and the seismic loads were used as the controlling design lateral loads. Additionally, unlike the wind loading, the lateral systems in both directions experience the same loading and thus must both be designed for the same load effects.
3.0 Structural Design
3.1 Gravity System
3.1.1 Gravity Columns
The gravity columns which make up all of the interior columns were designed for axial load only. These columns have beams framing into them and have simple shear connections, which are modeled as pins so that virtually no moment is transferred into the column. Thus, in order to design the columns we first had to determine the axial loads due to dead and live loads only. These loads were based on the tributary area of the column and gravity loads including the column self-weight. The resulting loads are summarized in Appendix B.
The dead and live axial loads were summed cumulatively from the roof down to determine the total axial load at each floor. These loads were then factored according to the load combinations provided in the UBC (1997) to obtain a design load, Pu. However, before we could continue with the design, two engineering decisions were made. First, due to the column layout and symmetry of the building we determined that we could reduce all of the interior gravity columns down to two typical columns; one on the corner next to the elevators, and the other towards the middle of the building closer to the shear wall. This consistency provides a simplification during construction. The second engineering decision is that the columns would be spliced at 4 feet above every second level. This decision is based on the transportation constraints of the columns as well as the constructability of the building.
With these decisions in mind, the columns were then designed using a K= 1, Fy = 50 ksi and = 0.85 for compression. Column sizes at each story were chosen so that the ratio of axial compression from the loads to the axial compression capacity of the size, , was less than or at most equal to 1.00. We used W14 sections for the gravity columns. The final preliminary design of the gravity columns were taken as the sizes designed at the 1st, 3rd, 5th, 7th, and 9th floors. These are summarized in Table 3-1.
Table 3-1. Gravity Column Design
GRAVITY COLUMNSFloor / Column 5 / Column 6
Roof
10
W14X53 / W14X53
9
8
W14X90 / W14X90
7
6
W14X120 / W14X109
5
4
W14X159 / W14X145
3
2
W14x211 / W14X176
1
3.1.2 Interior Girders
The interior girders are designed for 1.2 D + 1.6 L. Refer to the previous load key sheetsfor the various load areas. For interior girders only, we analyzed the girders with distributed loads and tributary areas. We looked at both the strength and deflection, calculating the minimum section modulus as well as the minimum Ix before deciding the girder sections. The deflection limits for live loads and dead loads are L/240 and L/360 respectively. Sample calculations can be found in Appendix B.