1

Traci Peterson Option: Structural Faculty Consultant: Memari The Del Monte Center at The North Shore− Pittsburgh, PA Structural Technical Report 3− Lateral System Analysis and Confirmation Design Date of Submission: 11/18/04

Executive Summary

Currently under construction on Pittsburgh’s North Shore, The Del Monte Center is a 285,000

square foot, six-story office complex including 41,000 square feet of retail and restaurant space on

the lower floors. The building is a steel braced-frame structure.

The third technical report deals with the existing lateral system of The Del Monte Center At

The North Shore. This system, which is composed of a network of 50 ksi steel braced frames, is

described in detail in the report. To summarize this description, the braces in the north-south

direction are simple non-eccentric knee braces and the braces in the east-west direction are mostly

inverted K-braces, also known as chevrons. The braces connect to the frames in such a way that

they can be modeled as pin connections. The lateral load combinations and distributions are also

described in detail.

The results of an analysis of the lateral system, performed in part by hand and in part using

STAAD, are included in this report. Some of the issues considered are strength, drift, story drift,

overturning, impact on foundations, and torsion. Spot checks of critical members have also been

performed.

Listed below is a summary of the findings:

• Seismic lateral forces are larger than those caused by wind. • 1.2D + 1.0E + 0.5L + 0.2S is the controlling load combination. • A simplified distribution of loads to the braced frames can be assumed, whereby for analysis

purposes the lateral loads in the North-South direction can be divided by 4 and applied to a single frame and the loads in the East-West direction can be divided by 8 and applied to a single frame.

• The building has an eccentricity of 29’-8” in the East-West direction, which induces torsional loads. However, these torsional loads were found to be minimal.

• Drift in the North-South direction was found to be acceptable, while drift in the East-West direction was not. Errors in the assumptions used may possibly be the cause.

• The critical members that were spot checked all passed their strength requirements. • Overturning is found not to be a significant problem.

2

Introduction

Construction of The Del Monte Center began on Pittsburgh’s North Shore on June 10, 2004

and is expected to be completed in late 2005. The office complex is six stories plus two rooftop

mechanical penthouses, totaling a height of just over 100 feet. The building is divided into two

distinct parts, called Units A and B, with a total square footage of 285,000. The lower floors include

41,000 square feet of retail and restaurant space, while Del Monte Foods is the major tenant of the

upper floors.

The typical floor systems are 3 ½ inch lightweight concrete slabs with a strength of 4000 psi

on 2”-18 gage composite steel deck, making them 5 ½” total. These composite slabs are supported

by a composite beam system. The slab-on-grade foundation is 4 inches thick with a strength of 3000

psi and is founded on 18 inch diameter auger cast piles installed to a depth of 1 foot into bedrock.

The piles have an axial capacity of 285 kips and a lateral capacity of 32 kips.

The steel structure is composed of braced frames, which are the focus of this report. In the

north-south direction, the braces are mostly inverted K-braces, also known as chevrons. In the east-

west direction, they are simple non-eccentric knee braces . The braces are connected to the frames

with pin connections. The girders are connected to the columns with shear connections, which can

also be modeled as pins. The framing does not vary significantly from floor to floor, and the bays in

both units are relatively consistent. The structural steel shapes used for the framework are typically

ASTM A572 or A992 grade 50. In the typical bays, the beams used most commonly are W18x35,

W18x40, and W21x144 and their typical spans are 40-43 feet. The typical girders are W24x62 and

W30x90, with spans of 28’-8” to 36’-8”. The columns are continuous and are most often W12’s and

W14’s. The diagonal bracing elements are typically W8’s and W12’s in the North-South direction

and W10’s and W12’s in the East-West direction. Elevators and stairs are located in the center of

Unit A and on the east side of Unit B, and clustered around them on each floor are mechanical and

electrical rooms. These areas, along with the connecting bridge between the units, account for most

of the departures from the typical bays.

The sections included in this report are as follows:

• Bracing Elevations

• Lateral Loads and Load Cases

• Distribution of Loads

• Analysis- Torsion, Drift, Member Strength, Overturning

• Conclusion

3

Bracing Elevations

4

Lateral Loads and Load Cases

Wind and seismic loads on the building have been determined by the method outlined in

ASCE 7-98 “Minimum Design Loads For Buildings And Other Structures.” Computation of wind

loads can be found in Appendix 3.1 and seismic loads in Appendix 3.2. The resulting story forces on

the building are shown below:

The load combinations considered, based on ASCE 7, are as follows:

1.4D 1.2D + 1.6L +0.5S 1.2D + 1.6S + (0.5L or 0.8W) 1.2D + 1.6W +0.5L + 0.5S 1.2D + 1.0E + 0.5L +0.2S 0.9D + (1.6W or 1.0E) It was found that 1.2D + 1.0E + 0.5L + 0.2S is the controlling load combination, which is not

surprising since the seismic load is larger than the wind load. This load combination reflects the

assumption that the simultaneous occurrence of earthquake loads with full live loads is unlikely to

occur.

5

Distribution of Loads A simplified, yet very reasonable, approach to the distribution of lateral loads has been

adopted for the analysis of the structure. Based on the bracing elevations and the locations of those

braces (refer to page 3), it has been determined that the small Frame E’s, when compared to the

much larger Frame D’s, do not contribute that substantially to the lateral force resisting system, and

are therefore ignored. This means that the lateral force resisting system in the North-South direction

is essentially composed of 4 braced frames similar to Frame D. These 4 frames are symmetrically

placed with respect to the center line of the building, and all have equal stiffnesses. Therefore, the

lateral story forces for the entire building in the North-South direction can be divided by 4, and

applied to the individual Frame D’s. Using the same logic, the smaller parts of Frames B and C can

be ignored. Therefore, in the East-West direction, the lateral force resisting system is composed of 8

frames similar to Frame A. Then the story forces in the East-West direction can be divided by 8 and

applied to Frame A for analysis. A description of all the loads, lateral and gravity, acting on the

individual Frames D and A can be found in Appendix 3.3. The continuous composite slab floor

systems in the building act as a rigid diaphragm to transfer shear forces due to wind and seismic

loads on the façade of the building to the braced frames. These braced frames transfer the load to

the beams, which in turn carry the load to the columns and down to the foundation system.

analysis

Computer analyses of both Frames D and A were performed using STAAD. Hand

calculations which accompany the STAAD analysis can be found in Appendix 3.4. Stiffnesses of the

individual frames were found using the equation k=P/∆. The deflections were found by applying a 10

kip load to the frame in STAAD. Then those values were plugged into k=P/∆, using 10 kips as the

value for P. Next, the building’s center of rigidity was found. The braced frames in the North-South

direction are symmetrical with respect to the building’s center line, therefore there is no eccentricity

in that direction. The eccentricity in the East-West direction was found using the equation

d=(Σdiki)/(Σki) and equals 29’-8”. Next, the torsional effects due to seismic lateral loads were

calculated using the equation Fi=M(kidi)/(Σdiki2), where M=Pe. Seismic loads were analyzed instead

of wind loads because they are larger, and are therefore the controlling load case. These torsional

effects were found to be negligible when compared to the direct shear values. If this were not the

case, the torsional forces would have been added to the direct shear forces to find the total shear

forces on the building.

6

Drifts and story drifts of Frames A and D were also checked. The values listed in the table

below were found using STAAD, when the controlling load case of 1.2D + 1.0E + 0.5L + 0.2S was

applied to the frames.

It is standard practice to compare drift values to a limit of H/400. This is a value derived in

The Gaylord & Gaylord Handbook. The drift values obtained in STAAD for Frame D seem very

reasonable, and are within the H/400 limit. However, the values obtained for Frame A far exceed

H/400. Reasons for this discrepancy could include errors in assumptions used for load calculations

and distributions. Perhaps the small sections of Frames B and C, which were assumed to be

negligible, are not.

Strengths of critical members were also checked using values obtained by the STAAD output.

The members checked are the columns and diagonal braces on the ground floor. The columns were

analyzed as beam-columns, using the equation Pu/b + Mu/m < 1. Table 6-2 of the AISC Steel

Manual was used to find coefficients b and m. The results are summarized below:

Both columns satisfy this strength check.

Frame

1st story drift (in.)

2nd story drift (in.)

3rd story drift (in.)

4th story drift (in.)

5th story drift (in.)

6th story drift (in.)

Drift (in.)

D 1.537 0.173 0.199 0.248 0.24 0.215 2.612

A 1.776 1.837 1.787 1.668 1.553 1.152 9.773

H/400 (in.)

2.654

2.654

Column Pu

(k) Mu

(ft-k) b

x103 kips-1 m

x103 (kip-ft)-1 Pu/b + Mu/m

Frame D W14x145

133.743 624.204 0.653 0.924 0.880

Frame A W14x120

185.375 443.689 0.808 1.15 0.615

7

The axial loads on the diagonal bracing elements, as well as their axial capacities found in

Table 4-2 of the AISC steel manual, are listed in the table below:

Both of these members have capacities that exceed their required strengths for the controlling

load case.

Overturning is another issue considered. Appendix 3.5 shows a calculation of the overturning

moment for the building. Because the dead load of the building is so great, it easily resists the

overturning moment, even when uplift is considered. Also, the drilled auger cast piles used for the

foundation system, which extend 1 foot into bedrock, further resist overturning. Therefore,

overturning is not a significant problem for this building.

Conclusion

This report has found that the seismic lateral forces are larger than those caused by wind, and

therefore, the controlling load combination was found to be 1.2D + 1.0E + 0.5L + 0.2S. A simplified

approach to the distribution of lateral loads to the individual braced frames was adopted, and was

proven to be very reasonable. All checks were found acceptable in terms of strength, drift, torsion,

and overturning. The only exception is the drift check of the braced frames in the East-West

direction. While the reason for this discrepancy is not completely clear, possible errors in the

assumptions used for determining the loads and/or distributions may be the cause.

Diagonal Bracing Member Axial Load (k)

Axial Capacity (k)

Ground Level, Frame D W12x87

486.202 817

Ground Level, Frame A W10x60

369.384 497

8

9

10

11

12

13

14

15

16