Executive Summary Lateral System Analysis and Confirmation ... · North Carolina A&T State...

North Carolina A&T State University Mick Leso School of Education Building Structural

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Executive Summary Lateral System Analysis and Confirmation Design

The lateral system of the existing James C. Renick School of Education Building located in Greensboro, North Carolina was the prime consideration when it came time to design the structural elements. The sixty-four thousand (64,000) square foot building has two separate structural systems which are divided by an expansion joint. The northern section of this building supports itself laterally with a complex system of braced frames and concrete shear walls. The reason for this intricate mix of structural elements is owed to the immense fifty-two foot (52’) long cantilever that it must support. Due to the fact that the cantilever may initiate some pretty nasty load distributions throughout the building, this part of the structure was isolated to spare the southern wing of the Renick Building from undesirable forces. The southern section is much simpler in design, constructed using steel girders with moment connections in the north-south direction. In the east-west direction, there are a few diagonal braces but most of the loads are carried by a large number of floor-supporting, framing members. This report is to show exactly how wind and seismic loads affect the Renick Building and how they are dispersed throughout its structural arrangement.

This structure was initially designed in RISA 3-D and RAM Structural using both ASD and LRFD load combinations. For this report, the analysis was completed using the existing structure and the original design loads computed from ASCE7-98 and IBC 2000. The steel frame members and concrete shear walls were input into ETABS 8 Nonlinear and loaded using those forces under a bunch of different circumstances. The self weight and superimposed dead loads were also included in the calculations that were produced by ETABS and are held constant in each load combination. The variations in loadings come both when the proposed lateral loads are either wind or seismic and when the direction of these forces acting on the building are rotated about the building’s axis by 90°. The computer output was able to display similar results to the calculations I did by hand for the first technical report. Both sets of results are given in this report.


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Technical Report 3 Lateral System Analysis and Confirmation Design

Table of Contents

1. Introduction……………………………………………………………..3 2. Existing Lateral System – Northern Wing…………………….4 3. Existing Lateral System – Southern Wing……………………5 4. Lateral Analysis & Comparisons…………………………………6 5. Floor Diaphragm Displacements/Story Drifts…………………11 6. Pier Forces……………………………………………………………12 7. Appendix A - Floor Plans……………………………………….14 8. Appendix B – Design Loads…………………………………….17 9. Appendix C – Code Materials, Load Combinations………17 10. Appendix D – Wind Loads………………………………………19 11. Appendix E – Seismic Loads………………………………….21 12. Appendix F – Framing Plans/Elevations……………………22


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Introduction The Renick School of Education Building, located at North Carolina A&T State University in Greensboro, N.C., is classified for instruction and business uses. The 64,000 square foot building has three stories of classrooms and assembly halls, permanent and partitioned offices, and a few laboratories. The primary structural element in the building is a two story high, 52 foot long cantilever and the immense trusses that support it. To isolate this portion of the building, an expansion joint was needed to contain the cantilever’s forces within its own structure. This way, the remainder of the Renick Building would only require providing support for itself. For this report, I will refer to the two sections as the northern and southern wings, the northern part including the cantilever. The diagram below shows a detailed sketch of the building’s framework.


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Existing Lateral Systems – Northern Wing The northern wing of the James C. Renick School of Education Building’s lateral system is significant because of the massive cantilever that extends above the sidewalk as a gateway to the central campus of North Carolina A&T State University. The Freelon Group architects toyed with their original design in many ways until a feasible structural system could be built to withstand both gravity and lateral loads in the large cantilever. A few ideas that had been thrown around, including supporting the cantilever with a single column at the very end of its span, were put to rest bearing in mind that the preferred design is far more intriguing to the everyday person.

Renick Building’s Northern Wing - Framing plans are located in Appendix F of this report.

This view provides a significant impression on pedestrians and was an element that could not be dismissed in design. To support such a large cantilever, the engineers found that massive trusses constructed with steel wide flange members would be the ideal solution. These trusses could be assembled offsite and put into place by crane, saving a lot of construction time and labor. However, the major problem is lateral loads that occur perpendicular to the direction of the cantilever. Drastic deflections from these forces could easily cause the façade to crack or the building to deform beyond the elastic limit generating an ever present sag in the structure. And if such a large cantilever were to sag a distance viewable to the naked eye, the building’s popularity dwindles. The trusses are bulky, yet provide substantial support for the building’s gravity loads. To give it enough strength, sections such as W14x90s and W14x211s are used for the cantilever girders. The vertical and diagonal members are primarily W14x43s, offering a cost reduction when dealing with the steel provider due to repeating members. The complications accumulate in the decision of how to tie the two trusses together with cross bracing. Eventually, wide flange beams and HSS braces were placed to connect the cantilever girders in the perpendicular direction. However, these members are subject to numerous different load conditions and failure possibilities. They must carry not only the floor dead, superimposed dead, and live loads, but are needed to provide structural stability against lateral loads. What is also alarming is the vast forty foot (40’) length the wide flanges span. The beams could easily buckle or yield under the floor loads alone. To be considered as lateral supports means a potential lateral torsional buckling failure among others.


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Existing Lateral Systems – Southern Wing

Typical Southern Wing Framing Plan

The southern wing of the Renick building is a much less complicated clutter of steel wide flange members. The structure, being only 3 stories high, is affected very little by wind forces. For this reason, the extent of the lateral supports in the north-south direction happens in the connections between the columns and the girders. Shown on the framing plan above, these connections are labeled as welded moment connections. In the other direction, the number of wide flanges supporting the floors provides much of the stability against lateral loads. These composite beams are working in turn with the composite metal decking and lightweight concrete floor system to provide a stiff lateral system. There are also a few inverted V-braced frames, formed with HSS hollow tubes in the east-west direction.


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Lateral Analysis & Comparisons The lateral analysis was completed primarily with the help of the computer program, ETABS 8 Nonlinear. Initially, for the first technical report, hand calculations were reported giving values for the wind and seismic loads on the Renick Building. These hand calculations are shown at the end of this report in Appendices D and E. Also, the original design work done in office at Stewart Engineering, Inc. included computer models in both RISA-3D and RAM Structural. Although the output data and applicable results were unavailable for this report, it can be said that most of the original design work was completed using these models. A few of the cantilever truss members had to be bulked up from their earliest design due to the results compiled from RISA and RAM’s lateral load analysis. ETABS was chosen for this report for its reputation of integrating a braced frame system with concrete shear walls. There were a few slight differences in the results that ETABS provided, but these are likely negligible, given the input required to run the program. ETABS calculates the wind loads based on the size of the actual structure that is input. So, differences in my wind values are presumably based on the fact that there are small differences in the actual building dimensions. Part of the southern wing is buried underground which was factored into my hand calculations and ETABS does not take that into account. This is most likely why ETABS gave a larger value for the y-direction.

Also, the program does not distribute any wind load from the lower half of the ground floor directly into the slab on grade. Normally it would be factored directly into the structure at ground level rather than assumed supported by the 2nd floor. Since no foundation was provided for the ETABS analysis and all of the columns were assumed fixed connections at the top of the foundation, it is uncertain how exactly ETABS distributes this extra load. My assumption is that the 2nd floor carries the entire load from the foundation up to halfway between the 2nd and 3rd floor. The ETABS values are as follows:

Case Type Story FX (kips) Shear WINDX ASCE7-95 ROOF 58.29 3RD FLR 112.98 2ND FLR 124.31 295.58 FY (kips) WINDY ASCE7-95 ROOF 15.41 3RD FLR 31.72

2ND FLR 36.48 83.61


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From the ETABS output’s story shears, the overturning moments can be calculated as: X-direction: Base Shear = 295.58 kips Overturning Moment = (124.31 k)(17’) + (112.98 k)(31.17’) + (58.29 k)(45.33’)

= 8277.14 ft-k Y-direction: Base Shear = 83.61 kips Overturning Moment = (36.48 k)(17’) + (31.72 k)(31.17’) + (15.41 k)(45.33’)

= 2307.41 ft-k Both of these values turn out to be slightly larger than my hand calculations produced in Appendix D, so I will assume a conservative approach and just use ETABS’ overturning moments. The x-direction has a large moment; however it is spread out over a length of 320 feet. Considering the overturning moment a couple placed on the exterior column line: Typical bays span 30 feet in north-south, 40 feet in east-west direction. (See Appendix A for floor plans) X-direction: (8277.14 ft-k)/(45.33’ height) = 182.60 kips dead load required on exterior wall. (182.60 k)/{(320’)} = 570.6 plf along north-south direction Check: From Appendix B: Ground Floor Dead Load = 51.0 psf 2nd/3rd Floor Dead Loads = 55.0 psf Roof Dead Loads = 45.6 psf Σ Dead = 51 + (2)(55) + 45.6 = 206.6 psf (206.6 psf)(40’/2) = 4.132 klf > .571 klf OK Y-direction: (2307.41 ft-k)/(45.33’ height) = 50.90 kips dead load required on exterior wall. (50.90 k)/{(102.5’)} = 496.6 plf along east-west direction Check: From Appendix B: Ground Floor Dead Load = 51.0 psf 2nd/3rd Floor Dead Loads = 55.0 psf Roof Dead Loads = 45.6 psf Σ Dead = 51 + (2)(55) + 45.6 = 206.6 psf (206.6 psf)(30’/2) = 3.099 klf > .497 klf OK


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From Appendix D, the figures produced by hand confirm an x-direction base shear of 318 kips and a y-direction base shear of 74 kips. Due to the proximity of these values, I decided to move on in ETABS with the values listed above in this chart. ETABS was spot on with the hand calculations for the seismic loads:

Case Type Story FX (kips) Shear QUAKEX IBC2000 ROOF 30.41 0 QUAKEX IBC2000 3RD FLR 105.29 0 QUAKEX IBC2000 2ND FLR 67.91 0 203.61 FY (kips) QUAKEY IBC2000 ROOF 0 30.41 QUAKEY IBC2000 3RD FLR 0 105.29 QUAKEY IBC2000 2ND FLR 0 67.91 203.61

Appendix E shows that the x and y directions have identical earthquake loads, which is to be expected. The shear was found separately for the north (54.36 kips) and south (150.23 kips) wings, making the sum of the two shears is 204.59 kips. Obviously the northern wing will be subjected to a lesser seismic load since the structure is smaller and carries not as much weight. The highlighted base shears show the larger of the two lateral loads in the each direction. The north-south direction is controlled by seismic loads, while the east-west direction is controlled by wind loads.

Typical Renick Building Braced Frame


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Braced frame check in the x-direction: From Table 4-6 LRFD Steel Manual: HSS 6x6x½ Compressive Strength ΦPn = 267 kips From Table 3-4 LRFD Steel Manual: HSS 6x6x½ Tensile Strength ΦPn = 403 kips Roof: (7.08’)(15.05+9.54 psf) = 174.10 plf 3rd Floor: (7.08’)(15.05+9.54 psf) + (7.08’)(13.92+9.54 psf) = 340.20 plf 2nd Floor: (7.08’)(13.92+9.54 psf) = 166.10 plf; (8.50’)(12.27+9.54 psf) = 185.38 plf

166.10 plf + 185.38 plf = 351.48 plf Foundation: (8.50’)(12.27+9.54 psf) = 185.39 plf Check: Worst case at 2nd Floor: (351.48 plf)(30’ span) = 10.54 k tan-1(14.17’/13.5’) = 46.39° F = (10.54 k)/ cos43.61° = 14.56 k < 267 k OK From this check, such a great difference in compressive strength leaves me to believe that the HSS braced frames are not only providing strength laterally, but also in the vertical direction. And from the sketch above, it is clear that the frames are also providing support for the large beams spanning across from column line B.5 to C. It becomes clear that the lateral bracing in the cantilever also provides the extra support for gravity loads helping to relieve the vertical members of the truss


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Some assumptions were made in ETABS before running the full analysis: A. It was chosen to use LRFD load cases and combinations to stay consistent with the RISA

and RAM studies. LRFD load combinations can be found on the next page. B. All supports were assumed pinned unless the member was continuous or the connection was

shown on the framing plans as a fully-welded moment connection. These moment connections were assumed to resist the entire moment presented to them.

C. All lateral loads were assumed applied a floor level directly on the structure. The components and cladding of the building were not accounted for in this report.

D. Gravity loads are designed as uniformly distributed loads across square foot areas. The live loads were also allowed to be reduced where applicable.

The following LRFD load combinations were calculated in the ETABS analysis:

Combo Factor Case Combo Factor Case LRFD1 1.4 DEAD LRFD5A 1.2 DEAD LRFD2 1.2 DEAD 1 QUAKEE 1.6 LIVE 0.5 LIVE 0.5 ROOFLIVE 0.2 SNOW LRFD3A 1.2 DEAD LRFD5B 1.2 DEAD 1.6 ROOFLIVE 1 QUAKEN 0.5 LIVE 0.5 LIVE LRFD3C 1.2 DEAD 0.2 SNOW 1.6 ROOFLIVE LRFD5C 1.2 DEAD 0.8 WINDXE 1 QUAKES LRFD3D 1.2 DEAD 0.5 LIVE 1.6 ROOFLIVE 0.2 SNOW 0.8 WINDYN LRFD5D 1.2 DEAD LRFD3E 1.2 DEAD 1 QUAKEW 1.6 ROOFLIVE 0.5 LIVE 0.8 WINDXW 0.2 SNOW LRFD3B 1.2 DEAD LRFD6A 0.9 DEAD 1.6 ROOFLIVE 1.6 WINDXE 0.8 WINDYS LRFD6B 0.9 DEAD LRFD4A 1.2 DEAD 1.6 WINDYN 1.6 WINDXE LRFD6C 0.9 DEAD 0.5 LIVE 1.6 WINDXW 0.5 ROOFLIVE LRFD6D 0.9 DEAD LRFD4B 1.2 DEAD 1.6 WINDYS 1.6 WINDYN LRFD7A 0.9 DEAD 0.5 LIVE 1 QUAKEE 0.5 ROOFLIVE LRFD7B 0.9 DEAD LRFD4C 1.2 DEAD 1 QUAKEN 1.6 WINDXW LRFD7C 0.9 DEAD 0.5 LIVE 1 QUAKES 0.5 ROOFLIVE LRFD7D 0.9 DEAD LRFD4D 1.2 DEAD 1 QUAKEW 1.6 WINDYS 0.5 LIVE 0.5 ROOFLIVE


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Lateral Floor Diaphragm Displacements From analyzing the computer model, I found that the structure holds up well against all of the load combinations. Some of the worst displacements of each of the floor diaphragms can be found below (values for displacement are in inches). For load combination reference, see page 6.

Story Diaphragm Load X Y 2ND FLR NORTHBLD LRFD4B -0.0024 0.0574 2ND FLR NORTHBLD LRFD4C -0.0205 0.029 2ND FLR SOUTHBLD LRFD4A 0.0437 -0.0037 2ND FLR SOUTHBLD LRFD4C -0.0428 0.0045 3RD FLR NORTHBLD LRFD4C -0.0441 0.0584 3RD FLR SOUTHBLD LRFD4A 1.3344 -0.0188 3RD FLR SOUTHBLD LRFD7C 0.0074 0.1723 ROOF NORTHBLD LRFD4C -0.0632 0.0828 ROOF NORTHBLD LRFD4B -0.0034 0.1242 ROOF SOUTHBLD LRFD4A 1.8844 -0.0376 ROOF SOUTHBLD LRFD7C 0.0114 0.2077

As the ETABS output shows, most of the worst case scenarios occur under the load combination LRFD4 which is 1.2(Dead)+1.6(Wind)+0.5(Live). The only other load combination that controls is LRFD7, which consists of 0.9(Dead)+1.0(Quake). So, both wind and seismic loads have the potential to affect the Renick Building. Overall, the values of diaphragm displacement are very low. The deflection limits for the exterior wall panels and façade are listed as L/240. The only real problems concerning deflection then would be in the south building due to a large wind load in the x-direction. Story Drift Given that the structure proved to be quite stiff, the story drift values should be very minute. ETABS provided matching results, shown below. (values in inches)

Story Item Load Drift 2ND FLR Max Drift X LRFD4C 0.004096 2ND FLR Max Drift Y LRFD7C 0.000798 3RD FLR Max Drift X LRFD4A 0.008519 3RD FLR Max Drift Y LRFD7C 0.000979 ROOF Max Drift X LRFD4C 0.006219 ROOF Max Drift Y LRFD4D 0.000735


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Below is a screen shot of the ETABS model, specifically the northern wing. The green lines represent wide flange members, as columns, girders, and beams. The blue members are the HSS hollow rectangular tubes used for bracing. The floor diaphragms are colored to specify their particular compositions. The light blue are the 2nd and 3rd floors, made of 3¼” thick, 4000 psi lightweight concrete slabs on 2” 20-gage composite metal deck. ¾” diameter, 4” long shear studs welded to the beams help to provide the extra composite strength needed in the floor structure. The green floor plates are the roofs consisting of 2” thick, 4000 psi lightweight cellular concrete on 3” 20-gage metal deck. The two walls are the aforementioned 12” thick shear walls surrounding the fire escape stairwell. The pier forces of these shear walls are shown below the screen shot.

Pier Forces The pier forces are found to be the axial loads and shear that are taken by the two shear walls in the north building. (Values in kips)

Story Pier Load P V GRD LVL SHEAR WALL1 LRFD4C -212.8 -216.42 GRD LVL SHEAR WALL2 LRFD4A -199.1 103.81 2ND FLR SHEAR WALL1 LRFD4A -110.8 75.11 2ND FLR SHEAR WALL2 LRFD4C -110.1 -86.7 3RD FLR SHEAR WALL1 LRFD5D -51.66 8.43 3RD FLR SHEAR WALL2 LRFD4C -51.17 -23.14

Once again, the major contributor to loading the shear walls in the north building comes from LRFD4, the greatest effect of wind loads on the cantilever.


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Conclusions

The Renick Education Building in Greesnboro, North Carolina has an exceptionally

strong lateral bracing system, especially in the cantilever. However, what is more intriguing is the building’s ability to utilize members capable of providing strength against both lateral and vertical loads.


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Appendix A

2nd Floor Plan


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North Building


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Typical Floor Bays


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Appendix B Design Loads

Dead Loads 1st Floor 4" normal weight slab on grade 50.0 psf (4/12)ft * 150 pcf Floor finish 1.0 psf Value from ASCE-07 51.0 psf TOTAL 2nd/3rd Floor

3-1/4" lightweight concrete slab on 2” - 20 gage composite metal deck 41.0 psf

Value from United Steel Deck Manual pg 38

Suspended ceiling grid 2.0 psf Value from ASCE-07 5/8" gypsum board panels 6.0 psf Value from ASCE-07 MEP 5.0 psf Value from ASCE-07 Floor finish 1.0 psf Value from ASCE-07 55.0 psf TOTAL Roof Built-up roofing 2.0 psf Values from ASCE-07 3/4" fiberboard 1.1 psf 1.5 per inch 3" rigid insulation 4.5 psf .75 per 1/2 inch Suspended ceiling grid 2.0 psf Value from ASCE-07 MEP 5.0 psf Value from ASCE-07

2" lightweight concrete slab on

1-1/2" – 22 gage metal deck 31.0 psf Value from United Steel Deck Manual pg 36

45.6 psf TOTAL

Appendix C Design Codes General The North Carolina State Building Code (IBC) 2003 North Carolina State Construction Manual – 8th Edition 1993 ANSI/ASCE 7-98 Minimum Design Loads for Buildings and Other Structures 1998 Steel AISC Manual of Steel Construction – 9th Edition – ASD * AISI Design for Cold-Formed Steel Structural Members 1996 SJI-92 Standard Specifications, Load Tables, and Weight Tables for Steel Joists and Joist Girders AISC Load and Resistance Factor Design (LRFD) Specification for Structural Steel Buildings * * Cantilever truss analyzed with both ASD & LRFD specifications Concrete ACI 318-99 Building Code Requirements for Reinforced Concrete ACI 301 Manual of Concrete Practice 1998 Concrete Reinforcing Steel Institute Manual of Standard Practice


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Materials Steel Structural Steel ASTM A992, Grade 50, Fy = 50 ksi HSS ASTM A500, Grade B, Fy = 46 ksi Misc. Steel ASTM A36, Fy = 36 ksi Reinforcing Steel ASTM A615, Fy = 60 ksi

Concrete Light Weight f’c = 4000 psi (elevated slabs) Normal Weight f’c = 3000 psi (slab on grade, retaining walls, footings)

Bolts ASTM A325 – 3/4” diameter

Deflection Limits S or W D + L Roof Members: Supporting plaster ceiling L/360 L/360 Supporting nonplaster ceiling L/240 L/240 Not supporting ceiling L/180 L/180 Floor members: L/360 - Exterior walls and interior partitions: With brittle finishes - L/240 With flexible finishes - L/120

Load Combinations

STRENGTH DESIGN – according to ANSI/ASCE 7-98 (sect 2.3.2, p. 5) & IBC 2000 (sect 1605.2, p. 296)

LRFD-1 1.4(D + F) LRFD-2 1.2(D + F + T) + 1.6(L + H) + 0.5(Lr or S or R) LRFD-3 1.2D + 1.6(Lr or S or R) + (0.5L or 0.8W) LRFD-4 1.2D + 1.6W + 0.5L + 0.5(Lr or S or R) LRFD-5 1.2D + 1.0E + 0.5L + 0.2S LRFD-6 0.9D + 1.6W + 1.6H LRFD-7 0.9D + 1.0E + 1.6H

D = dead load H = soil load * S = snow load E = earthquake load L = live load R = rain load * F = fluid load * Lr = roof live load W = wind load Fa = flood load * T = self-straining force (temperature) *

* - neglected for this report


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Appendix D

Wind Loads basic wind speed 90.0 mph importance factor 1.15 Conservative approach see spreadsheet for tabulation of windward and leeward wind loads


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East-West Direction: Roof: (7.08’)(15.05+9.54 psf) = (174.10 plf)(320’) = 55.71 k 3rd Floor: (7.08’)(15.05+9.54 psf) + (7.08’)(13.92+9.54 psf) = (340.20 plf)(320’) =

108.86 k 2nd Floor: (7.08’)(13.92+9.54 psf) = 166.10 plf + (8.50’)(12.27+9.54 psf) = 185.38 plf

(166.10 plf)(320’) + (185.38 plf)(271.5’) = 103.48 k Foundation: (8.50’)(12.27+9.54 psf) = (185.39 plf)(271.5’) = 50.33 k Total Base Shear = 318.38 k Overturning Moment = (55.71 k)(45.33’) + (108.86 k)(31.17’) + (103.48 k)(17’) =

7677.66 ft-k North-South Direction: Roof: (7.08’)(15.05+4.65 psf) = (139.48 plf)(85’) = 11.86 k 3rd Floor: (7.08’)(15.05+4.65 psf) + (7.08’)(13.92+4.65 psf) = (270.96 plf)(85’) =

23.03 k 2nd Floor: (7.08’)(13.92+4.65 psf) = 131.48 plf + (8.50’)(12.27+4.65 psf) = 143.82 plf

(131.48 plf)(85’) + (143.82 plf)(97.5’) = 25.20 k Foundation: (8.50’)(12.27+4.65 psf) = (143.82 plf)(97.5’) = 14.02 k Total Base Shear = 74.11 k Overturning Moment = (11.86 k)(45.33’) + (23.03 k)(31.17’) + (25.20 k)(17’) =

1683.86 ft-k


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Appendix E Seismic Force Resisting System

South Building:

E/W Direction – Steel Concentrically Braced Frames R = 3 N/S Direction – Ordinary Steel Moment Frames R = 3

North Building:

E/W Direction – Combined system of Steel Concentrically Braced Frames and Reinforced Concrete Shear Walls Rmin = 3

N/S Direction – Steel Concentrically Braced Frames R = 3 Design Base Shear: Per Section 1617.4 of IBC Analysis Procedure: Equivalent Lateral Force Procedure Earthquake Loads SS 24.9% S1 11.1% Fa 0.012 Fv 0.0169 Sms 0.0030 Sm1 0.0019 Sds 0.0020 Sd1 0.0013 Importance factor 1.25 Use group II Design category B Site Class C South Building North Building E-W Rx 3.0 Rx 3.0 Cdx 3.0 Cdx 3.0 N-S Ry 3.0 Ry 3.0 Cdy 3.0 Cdy 3.0 Vx 150.23 Vx 54.36 Vy 150.23 Vy 54.36 see spreadsheet for actual calculation of Seismic Base Shear


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Appendix F


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Executive Summary Lateral System Analysis and Confirmation ... · North Carolina A&T State...

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