Campus Square – Buildings C & D Lehigh University

Campus Square – Buildings C & D Lehigh University, Bethlehem, PA

Matthew Kutzler Advisor: Dr. Memari Structural October 8, 2003

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Structural Technical Report 1

Structural Concepts / Structural Existing Conditions Report

Structural Overview Campus Square, Buildings C and D utilize two somewhat different framing systems, but are similar in most other respects. Building C is composed of steel beams, girders, and columns on the second and third floors, with typical bays being 15 ft x 20 ft. Spans range from 5 ft. to 20 feet. Floors four and five consist of load bearing masonry walls and few small steel members. Typical bays are approximately 48 ft x 20 ft. Building D has a framing system similar to that appearing on floors four and five of Building C. Load bearing masonry walls assume all of the gravity loads for floors two and three and most bays are again approximately 48 ft x 20 ft. Both buildings have gabled roofs supported by trusses ranging anywhere from 12 ft to 45 ft in length and placed at 2 ft o.c. Both buildings have a 4” concrete slab on grade on the ground floor with a strength of 4000 psi. The foundation also consists of a 4000 psi continuous concrete footing along the perimeter and interior spread footings. With the exception of the ground floor in each building, all floors are constructed of an 8” prestressed concrete plank at 5000 psi. The concrete masonry units are 1500 psi and the structural steel has a strength of 50 ksi. Shearwalls comprise the lateral resisting system in both buildings. They are present on floors three through five in Building C and all three floors in Building D. The shearwalls are also constructed of concrete masonry units and are generally spaced 48 ft. apart in both buildings. They are laid parallel to the direction of the hollow cores in the precast concrete plank. Also, they appear perpendicular to the longest portions of both buildings. Pages 16 and 17 depict a functional layout of the buildings as well as photos and shearwall locations.



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Detailed Structural Description Foundation Buildings C and D each have a continuous concrete strip footing system running the length of the perimeter. Strip footings are basically utilized under every load bearing masonry wall appearing in Building D. The shearwalls have been tied into these. Concrete spread footings are used in the interior of Building C, providing support for columns protruding through the bookstore. Footings supporting one column are 7 ft x 7 ft and are placed under residential areas of the building. Footings appearing under corridors, however, are as large as 11 ft x 20 ft with two columns for the increased live load. The footings range from 1 ft to almost 4 ft in depth. The buildings are resting on virgin soil with a bearing pressure of 3 ksi. The foundation system will need to be analyzed in the future, particularly the overturning moment and uplift forces, in order to determine its adequacy. The soils report will also need to be checked for any unusual aspects of the soil and how they affected the design of the foundation. Framing As stated earlier, Buildings C and D have two somewhat different framing systems. Building C is composed of steel beams, girders, and columns on the second and third floors, with typical bays being 15 ft x 20 ft. Generally, W16’s and W18’s are used to support the floors in dorm room areas. W14’s are used for the corridors, though their longest span is only 6 ft. W12 x 26’s are placed every 5 ft to support the flat roof over the 2 story bookstore. W18 x 31 columns line the perimeter of the bookstore, but W12 x 79’s are utilized everywhere else in the building. The columns, which only rise as high as floor three, are all bolted to the beams and girders with full depth connections. Floors four and five consist of load bearing masonry walls and few small steel members. Typical bays are approximately 48 ft x 20 ft. The masonry walls only line the corridors and perimeter of the building in most cases. Shearwalls and partitions are used to divide the dorms into their room allotments. The gabled roof is supported by trusses ranging anywhere from 12 ft to 45 ft in length and placed at 2 ft o.c. They are designed for a top chord loading of 30 psf live and 15 psf dead, plus the self weight. The bottom chord is designed for a 10 psf dead load plus the self weight. Continuous lateral bracing on the top and bottom chords are spaced at a maximum of 10 ft o.c.



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Building D has a very similar load bearing masonry wall system to floor four and five of Building C. Load bearing masonry walls assume all of the gravity loads for floors two and three and most bays are again approximately 48 ft x 20 ft. As in Building C, the masonry walls line the corridor and perimeter and room divisions are created by the partitions. The roof is also a gabled roof with the same type of truss system present. Lateral System Lateral loads are resisted by a series of concrete masonry unit shearwalls in each building. Building C has shearwalls on floors three through five and all three floors in Building D. The shearwalls extend the entire height of each floor and are generally spaced 48 ft. apart in both buildings. In addition, the 8” thick walls are perpendicular to the longest portions of both buildings. Each shear wall is 20 ft long and laid parallel to the direction of the hollow cores in the precast concrete plank. All masonry walls are anchored to all floors and roofs to provide lateral stability. Three #8 bars reinforce the shearwalls at all jambs. Plus, #4 bars are spaced at 48” o.c., which are grouted solid with 3000 psi grout at all locations.



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National Code

BOCA National Building Code, 1996 ASCE 7-95 Standard

Design Codes and Standards

Masonry o American Concrete Institute (ACI 530) – Building Code Requirements

for Concrete Masonry Structures o National Concrete Masonry Association (NCMA) – Specifications for the

Design and Construction of Load-Bearing Concrete Masonry

Steel o American Institute of Steel Construction (AISC) o Allowable Stress Design (ASD), 9th edition

Gravity Loads The dead loads have been determined from the Minimum Design Dead Loads tables listed in ASCE 7-95 and typical loads used by the structural engineer. The self weight of the precast concrete floor plank was calculated by first determining the percentage of material present per cubic foot (it was hollow). That percentage was then multiplied by the thickness and cubic weight of normal weight concrete. Live

Floor o 40 psf residential dwelling units o 20 psf partitions o 100 psf corridors and stairwells

Roof o 20 psf (16 psf is the BOCA minimum) o 30 psf snow + drift



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Dead: Building C

Floor 1 o 49 psf 4” slab on grade o 3 psf Flooring

Total: 52 psf

Floors 2 and 3 o 58 psf 8” hollow, precast concrete plank o 6 psf M/E/P o 1psf Acoustical fiber tile o 3 psf Flooring o 3 psf Spray-on fireproofing o 8 psf Steel framing

Total: 79 psf

Floors 4 and 5 o 58 psf 8” hollow, precast concrete plank o 6 psf M/E/P o 1psf Acoustical fiber tile o 3 psf Flooring

Total: 68 psf Roof (flat - lower portion of bookstore)

o 10 psf membrane without ballast o 5 psf Deck and insulation o 1psf Acoustical fiber tile o 3 psf Spray-on fireproofing o 8 psf Steel framing

Total: 27 psf

Roof (gabled) o 25 psf truss system w/ finishes and MEP o 3 psf insulation o 2 psf asphalt shingles

Total: 30 psf



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Walls o 50 psf 8” masonry walls (grout is 48” o.c.)

Dead: Building D

Floor 1 o 49 psf 4” slab on grade o 3 psf Flooring

Total: 52 psf

Floors 2 and 3 o 58 psf 8” hollow, precast concrete plank o 6 psf M/E/P o 1psf Acoustical fiber tile o 3 psf Flooring o 3 psf Spray-on fireproofing o 8 psf Steel framing

Total: 79 psf

Roof (gabled) o 25 psf truss system w/ finishes and MEP o 3 psf insulation o 2 psf asphalt shingles

Total: 30 psf

Walls o 50 psf 8” masonry walls (grout is 48” o.c.)



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Snow Loads Utilizing the ground snow load for Bethlehem is 30psf, but needed to be modified due to the gable roof. As a result, the load was reduced to 17.4 psf with a balanced snow height of 1.17 ft. The flat roof has a snow load capacity of 21 psf. These two loads are less than the 30 psf, unreduced snow load specified on the structural drawings. Snow drift is only present on the flat bookstore roof spanning over the second floor of Building C. At this juncture, it is possible for snow to drift off of two sloping, perpendicular rooftops. However, as per the BOCA code, drift is only taken into account from the roof creating the most pressure on the flat roof below. Consequently, the larger roof, both in width and length, was analyzed for snow drift. The drift width was calculated to be 14 ft, allowing for a larger amount of snow blowing off the gable roof, as opposed to a flat roof. The intensity of the drift is 83.6 psf. Calculations for snow drift can be found in the Appendix (pages 18 - 19). Wind Loads Wind loads were calculated for each building in the North-South direction and East-West direction. The pressures calculated were lower than what was detailed in the structural drawings, which was conservatively estimated as 30 psf for all faces of the buildings. In future calculations, wind’s affect on the exterior cladding will need to be analyzed as well as uplift forces on the gable roof for each building. The diagrams on the following pages detail the pressures and forces experienced by Buildings C and D. Calculations for wind loading can be found in the Appendix (pages 20 - 23 ).



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Seismic Loads Seismic loads were calculated for each building in the North-South direction and East-West direction. According to the structural drawings, wind is the controlling lateral force. However, as can be seen in the force comparison below, it was not always the case on each floor. This can be attributed to the fact that my calculated wind pressures were used in this analysis instead of the 30 psf listed in the structural drawings, lowering the total wind force experienced by Buildings C and D. Also, the dead loads in calculating the seismic forces may be somewhat high. It should be noted that the seismic forces are around 5% of the total dead load. Further investigation is needed to determine why wind does not control in all instances. The following diagrams detail the forces experienced by Buildings C and D. Calculations for seismic loading can be found in the Appendix (pages 24 - 25 ).



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Spot Check Beams and Columns Beams were check on the first floor of Building C for moment and shear capacity using EnerCalc. W12 x 26’s with a tributary width of 10 ft and a length of 20 ft were check on the flat roof of the bookstore. Also, a W16 x 26 beam with a length of 20 ft and a tributary width of 10 ft were checked in the residential area of the floor. Similar framing exists on the first three floors of Building C. In both instances the moments and shears were relatively larger then those required. However, maintaining smaller deflections (they were less than 0.75”) could have been an influence. Also, the dead loads may be on the lighter side for each floor. This could also have resulted in overdesigned columns. A W12 x 79 as checked for an area of 400 sq ft. Loads were accumulated for the upper floors and roof and were within the capacity of the column checked. Almost all other columns are W12 x 79’s and have similar areas of influence. Masonry The load bearing masonry walls were check in EnerCalc on the first floor of Building D. Loads from floors two and three as well as the roof loads were taken into account in calculating the load capacity. The combined stress ratio of masonry and steel bending stresses in addition to the masonry axial stress were totaled and found to be less than that allowed for 8” CMU walls with #6 bars grouted solid at 48” o.c. Masonry walls were used for Building D due to the low floor to floor height and using steel beams would have raised deflection issues. It also would have compromised the plenum space of the floors with a deeper beam. Shear wall An 8’8” high shear wall (26’ total for the entire building) was analyzed from Building D for base shear. The base shears were totaled from the wind and seismic loads, which were nearly 430 k. Ten shear walls are present to counteract this lateral force. To find the shear capacity, the equation 1/2 * (4 – h/d) * (1500 psi)^1/2 was used. 1500 psi is the strength of the CMU block, h is height of the wall (26 ft), and d is the length of the wall (20 ft). The capacity of each wall is 52 psi which, when multiplied by 7.625” and 20ft, is 95 k per wall. This is greater than the total shear determined. This can be attributed to lower wind pressures used than the actual design, but further investigation is needed to determine other factors involved.



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APPENDIX



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Exterior shots of Campus

Square are all courtesy of

Barry Isett & Assoc. Inc.

.



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Documents

Campus Square – Buildings C & D Lehigh University