Technical Assignment #1 Structural Concepts/Structural Existing Conditions Report Executive Summary: This report contains investigations into the structural concepts used in the design of the Hyatt Center located at 71 South Wacker Drive; Chicago, Illinois and is broken down into four sections as follows: Building Description: The Hyatt Center is a 49-story, 1.7 million gross square feet commercial office building located in the heart of Chicago’s business district. It has a unique curvilinear façade and oval-shaped footprint really a “football shape” which is supported laterally by a central core. The core walls mimic the curvilinear facade producing a constant core-to-exterior spandrel/column dimension, an issue of economy. The floor and roof floor systems are composite wide flange beams with composite metal deck. Beam spans of 43’ to the exterior columns and perimeter column spacing of 38’- 3 ¾” are typical on most floors. Lateral bracing of the tower consists of shear walls in the central core connected by link beams at wall openings, this type of system is referred to as a “coupled shear wall.” All lateral loads as well as gravity loads were designed to be resisted by the core leaving the exterior columns as primarily gravity members. Lateral forces are transferred to the core beginning at the curtain wall, into the structural framing then into the concrete slab which acts as a rigid diaphragm. Large grade beams transfer the force from the core into belled and rock caissons which are bearing directly onto the “hardpan” and bedrock, respectively. Design Model Codes and Standards: The City of Chicago Building Code controlled the design of the Hyatt Center. Additional standards referenced are AISC LRFD for structural steel, ACI 318-02 for concrete design and ASCE 7-95 for design wind loading to name a few. ASCE 7-02 will be used for the analysis and calculation of loads for the remaining of this project. Calculations: Gravity (roof snow loads, column take-down of R7-SR1, typical framing weight per floor, total weight of the structure) and lateral (wind and seismic loads) are summed and calculated. The typical dead load weight per floor ranged from 5280 kips at the roof – 44,840 kip at the mezzanine levels. The critical un-factored base shear due to wind (7770k) is less than the un-factored seismic base shear (36,637k). The overall height of the structure paired with 3 levels of mechanical spaces throughout the height of the building and the structures massive weight all contribute to the large variation between seismic and wind base shear. Conclusions and Additional Consideration: Patrick L. Hopple Faculty Consultant : Dr. Linda Hanagan Date: Tuesday, October 05, 2004 Project: Hyatt Center (71 South Wacker Drive : Chicago, IL)
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This report contains investigations into the structural concepts used in the design of the Hyatt Center located at 71 South Wacker Drive; Chicago, Illinois and is broken down into four sections as follows:
Building Description: The Hyatt Center is a 49-story, 1.7 million gross square feet commercial office
building located in the heart of Chicago’s business district. It has a unique curvilinear façade and oval-shaped footprint really a “football shape” which is supported laterally by a central core. The core walls mimic the curvilinear facade producing a constant core-to-exterior spandrel/column dimension, an issue of economy. The floor and roof floor systems are composite wide flange beams with composite metal deck. Beam spans of 43’ to the exterior columns and perimeter column spacing of 38’- 3 ¾” are typical on most floors. Lateral bracing of the tower consists of shear walls in the central core connected by link beams at wall openings, this type of system is referred to as a “coupled shear wall.” All lateral loads as well as gravity loads were designed to be resisted by the core leaving the exterior columns as primarily gravity members. Lateral forces are transferred to the core beginning at the curtain wall, into the structural framing then into the concrete slab which acts as a rigid diaphragm. Large grade beams transfer the force from the core into belled and rock caissons which are bearing directly onto the “hardpan” and bedrock, respectively.
Design Model Codes and Standards:
The City of Chicago Building Code controlled the design of the Hyatt Center. Additional standards referenced are AISC LRFD for structural steel, ACI 318-02 for concrete design and ASCE 7-95 for design wind loading to name a few. ASCE 7-02 will be used for the analysis and calculation of loads for the remaining of this project.
Calculations:
Gravity (roof snow loads, column take-down of R7-SR1, typical framing weight per floor, total weight of the structure) and lateral (wind and seismic loads) are summed and calculated. The typical dead load weight per floor ranged from 5280 kips at the roof – 44,840 kip at the mezzanine levels. The critical un-factored base shear due to wind (7770k) is less than the un-factored seismic base shear (36,637k). The overall height of the structure paired with 3 levels of mechanical spaces throughout the height of the building and the structures massive weight all contribute to the large variation between seismic and wind base shear.
Conclusions and Additional Consideration:
Patrick L. Hopple Faculty Consultant : Dr. Linda Hanagan Date: Tuesday, October 05, 2004 Project: Hyatt Center (71 South Wacker Drive : Chicago, IL)
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Patrick L. Hopple Dr. Linda Hanagan Tuesday, October 05, 2004
Hyatt Center (71 South Wacker Drive : Chicago, IL)
Overall Structure:
Composite System: The Hyatt Center is a 49-story commercial office building in the
center of the Chicago business district. The entire building is
composed of a 49-story tower and a 7-story mezzanine.
The mezzanine structure and the tower have some
similar characteristics in terms of employed gravity
structural systems. The mezzanine is comprised typically of
rectangular bays (36’x46’) and use specially detailed open-
web steel trusses and wide-flange beams with web-
openings in the retail areas to accommodate ducts and
overhead electrical runs. The trusses are comprised of
back-to-back angles for the diagonals and verticals with chords of “WT”
sections which support an 11-inch slab on composite metal deck and 4
½” – ¾” diameter shear studs. Lateral loads in the mezzanine are resisted by rigid moment
frames in both directions. My focus for this report is primarily on the tower of the Hyatt
Center.
The tower structure consists of an oval-shaped footprint on the site which produces
some difficulty when trying to find a square bay. However, the tower was designed to allow
for a constant span dimension from the core to the perimeter spandrels. This was
accomplished by designing the central core walls to mimic the
profile of the curved façade, in doing so a constant beam span
(varying from 42’ at the base to 45’ in upper levels) can be
obtained across the floor plate, resulting in the same beam
spans. A constant perimeter span of 38’-3 ¾” was also upheld
by the radial grid dimensioning.
A typical bay used in calculations and in framing checks,
(R14 to R15 on SR2) can be seen to the left. The framing
consists of composite wide-flange beams (W18x50 – interior
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Hyatt Center (71 South Wacker Drive : Chicago, IL)
and W27x84 – perimeter) with a 5 1/2” light-weight concrete slab on
20 gage composite metal deck. Simple shear connections are located
where the beams meet the spandrels; however, the connection to the
concrete core wall is unique. Embedded steel plates are used to
transfer the reactions from the beams into the core wall. Loads from
the spandrels are carried into the W14 columns which vary in size
from 90#/lf to 808#/lf on level 5 at which time built-up columns PL 24x930 and BX
36x1140 are used for the remaining of the column length until the base plate. This column
run, R7-SR1 in particular, was checked for adequacy by determining loads and cumulating
them down the structure.
Lateral System: Lateral loads on the tower are resisted by a combination of shear walls located in the
central core of the building. The shear walls are comprised of two “C-sections” and four “I-
sections” all linked by rigid concrete link beams. This system of shear walls linked by beams
is referred to as a coupled shear wall. Lateral loads get to the central core by means of the
rigid concrete slab as a diaphragm. All the lateral loads were designed to be carried by the
central concrete core leaving the exterior columns resisting only axial forces. The concrete
core wall dimensions vary along the height of the building, as well as its compressive
strength. A core diagram can be viewed below:
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Hyatt Center (71 South Wacker Drive : Chicago, IL)
Rock Caissons 8000-12,000 psi 145 pcf Normal Weight
F’c = 12,000psi, U.N.O. in caisson plan
Foundation Walls 5000 psi 145 pcf Normal Weight
Blast Walls 5000 psi 145 pcf Normal Weight
Shear walls & link beams 5000 – 10000psi 145 pcf Normal Weight
Caisson Caps & grade beams 6000psi 145 pcf Normal Weight
Columns/Pilasters 8000 psi 145 pcf Normal Weight
Column Encasement 6000psi 145 pcf Normal Weight
Lower Level Framed beams and slab
6000psi 145 pcf Normal Weight
Slabs-on-grade 4000psi 145 pcf
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Hyatt Center (71 South Wacker Drive : Chicago, IL)
Normal Weight Slab on steel deck 4000psi 115 pcf
Light Weight Mechanical rooms = 145 pcf
Curbs, Fills and Equipment Pads
4000psi 145 pcf Normal Weight
Metal pan stairs 4000psi 115 pcf Light Weight
Codes and Code Requirements:
Applied Codes and Standards: The City of Chicago Building Code
ASCE 7-95 – Design Dead and Live Loads & Wind Loads (Strength Design: Method 2;
Serviceability Design: Wind Tunnel Analysis)
AISC – Load Factor and Resistance Design (LRFD) 3rd edition
ACI 318-02
Codes for Thesis Research: IBC 2003
ASCE 7-02 – Design Dead and Live Loads & Wind Loads (Strength Design: Method 2)
AISC – Load Factor and Resistance Design (LRFD) 3rd edition
ACI 318-02
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Design Methodology:
Required Loads: General Design Live Loads (ASCE 7-02):
Parking 50 psf
Plaza 100 psf
Lobby/Retail 100 psf
Cafeteria 100 psf
Fitness Center 100 psf
Function Space 100 psf
Office 50 + 20 psf partitions
Restrooms 60 + 20 psf partitions
Mechanical 150 psf
Electrical vault 250 psf
Battery/UPS 300 psf
Roof 25 psf + Drift
General Design Superimposed Dead Loads:
Plaza 100 psf
Lobby/Retail 75 psf
Raised Floor 15 psf
Elevator Lobbies 40 psf
Restrooms 40 psf
Ceiling/MEP 5 psf
Roofing 10 psf
Roof Garden 40 psf
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Design Methodology: The design of the Hyatt Center is quite simple when you break
down the parts of the building. Long spans from the central core to
the perimeter beams require a floor system which is light and meets
the required capacity to hold the design loads. Ordinary structural
steel beams, with its excellent strength to weight ratio, has an economic limit of
approximately 45 feet without modification of section properties in high-rise buildings.
Composite beam construction made perfect sense because by using the mechanical
properties of each material and supplying shear studs across the interaction plane, the
composite system is able to transfer the shear stresses between the steel and concrete which
limits the slipping across the concrete-steel interface. This concrete-steel interaction is what
gives steel the added span length without increasing member sizes to make it economical
for use in the long spans of the Hyatt Center. Concrete spans are typically in the range of
30-35 feet unless post-tensioning is involved. The pure weightiness of a concrete high-rise
would place foundation costs for this building through the roof.
Now in evaluating the lateral wind force resisting elements it can see from the
dimensions of our building core (height approximately 700 ft and width 50ft) produces a
building height/width ratio of 1:14 which is a slender building. A building this “thin”
requires a very rigid structural element to keep story drifts to a minimum, but more
importantly, keeping the serviceability and motion perception to a minimum. A braced
frame has the potential to meet these requirements, however, the steel required to meet the
drift limitation could exceed the tonnage used in just part of the tower itself. Also braced
frames may interfere with interior spaces rendering them useless. In general, with the
requirements of serviceability being dependent on human perception to motion, the
material of choice to resist the given lateral forces, coupled with current construction
practices in Chicago, was chosen to be a concrete core due to it’s rigid in-plane resistance
to loads; given the slenderness ratio of the Hyatt Center. By coupling the core walls
together with link beams, as in the Hyatt Center, the overall stiffness of the core wall
system is increased because instead of each core wall acting independently of each other to
resist the load, they interact with each to resisting the load.
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An analysis of the shear wall will have to be completed to
adequately determine the amount of drift associated with the
coupled shear wall as compared with other systems. To model the
shear wall, a computer analysis package will have to be used to
accurately reflect what is happening within the walls of the core. A
simple analysis can be completed which involves using equivalent
sections and ignoring the link beams then analyzing the core as a cantilevered beam with
equivalent section properties. This assumption will yield higher than permissible story
drift; however, it will provide a starting point onto which the interaction which coupled
shear walls afford can be compared.
By designing the central core to carry lateral loads the steel columns are free to carry
only their respective axial loads. The simplicity of the column design is an advantage when
analyzing the Hyatt Center. Column loads can be summed and cumulated from floors
above and each column can be checked for adequacy quite simply by hand calculation. A
more advanced analysis of the entire structure in a computer analysis package will be
required to analyze exactly how much lateral load is resisted by the core and if any lateral
load is getting resisted by the column members.
Calculation of Loads: Calculations are available by request.
Snow Loads: Calculation of the snow loads came from ASCE 7-02 Section 7.3. My assumptions of a
flat roof, a building in Chicago, IL (exposure B) a fully exposed roof and Occupancy
Category III yielded a ground snow load of 25 psf from Figure 7.1. An exposure factor of
0.9, thermal factor of 1.0 and importance factor of 1.1 gave a snow pressure of 25 psf +
drift. Calculation of the drift from lower roof and over the parapet walls (Pd=40.6psf) can
be found in Appendix A. The calculated snow load matched the design snow load
provided by Halvorson Kaye Structural Engineers.
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Hyatt Center (71 South Wacker Drive : Chicago, IL)
Wind Loads: Calculation of wind loads on the Hyatt Center came down to
determining if my building was considered rigid or flexible. Method 1: Simplified Procedure was not applicable for the Hyatt Center due to the exceeding of the 66ft height limit. Therefore, Method 2: Analytical Procedure was required to be used so using my dimensions of my building and referencing ASCE 7-02 I determined the natural frequency of my building to be 0.369 Hz which is less than 1.0 Hz. Rigid buildings are defined by ASCE 7 as buildings with natural frequencies are greater than or equal to 1.0 Hz, therefore I had to calculate my building as a flexible building as I assumed from the start. The equation for Kz was used to determine values higher than the 500ft published. Data gathered from the wind tunnel analysis was used to determine the gust factor, topographic factor and verification of mean wind speeds in Chicago, IL. Wind pressure were then gathered and plotted on the diagram on the next page. Wind story force and wind pressure calculations and diagrams can be found in full size in Appendix B1, B2, B3 and B4. (Note: the 7-story mezzanine was ignored in wind pressure calculations for simplicity.)
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The overturning moment of 2,381,810 ft-kips produced by the critical N-S wind load direction is balanced by the deep foundation caissons (Ks = 15 tsf) and the self-weight of the structure.
Seismic Loads:
The seismic load check involved many steps before the actual dynamic loads could be
produced. First I needed to calculate and approximate typical framing loads on each floor
to be used in a grand calculation of the entire structure weight. I used the typical bay of
R13-R14 on SR2 as my typical bay location. I then formulated expressions for the beam
weight/length, tributary area and calculations for the weight of composite deck and
concrete. This value was then imputed into the Total Weight spreadsheet which also
accounted for superimposed dead loads, column weights, core weights and superstructure
dead loads throughout the building on each floor. Calculations are available and loads for
input into the Seismic Load can be found in Appendix C1, C2 and Seismic load in
Appendix C3 with a diagram in Appendix C4. Calculated seismic story forces and base
shears are located on the diagram to the right.
Seismic base shear controls the lateral design,
however, the City of Chicago Building Code
does not require seismic loads to be designed
for, and therefore, a wind analysis will be used
instead for any lateral checks.
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Hyatt Center (71 South Wacker Drive : Chicago, IL)
Member Spot Checks:
Beam/Framing Element: I chose to analyze a typical bay which kept repeating member sizes on many floors to
simplify the procedure. The interior beams, and in this case, the girders spanning from the
core to the perimeter column are W18x50’s with 40 shear studs spaced along its length,
cambered 1 ¼” and spanning 43 ft long. The required number of shear studs needed for
fully composite beam was calculated to be 62; therefore the beam was designed as only
partially composite member (65% composite). With the beam at 65% of its full capacity it
still provides adequate strength to resist the given loading but uses less shear studs per
beam which is a cost savings in steel and economic design. The calculated factored load
(Mu =575.5kip-ft) was still less than the capacity of the partially composite beam (ΦMn
=600kip-ft) so the beam is adequate for the given loading.
Column: A check of column R7-SR1 at level 2 indicates the column is sized satisfactory for the
calculated loading. Level 2 was chosen as the location to spot check the column because the floor to floor height is 22’-11”, the highest story height in the tower. At this location the column will be more susceptible to buckling than at other levels. Column R7-SR1(at right) is a built-up box column composed of A572, Grade 50 (Fy = 50ksi) plates. The plate dimensions are 36” x 36” out-to-out with plate thickness of 2 ½” all around. The calculated factored load on level 2 from the column take-down spreadsheet in Appendix E is 9326.5 kips. Column BX 36x1140 was found to have a capacity of 13,835.5 kips. A 48% over-design in strength was calculated.
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Hyatt Center (71 South Wacker Drive : Chicago, IL)
Approximation of loads on each floor may be a large cause of this high percentage in strength. The Hyatt Center was also designed to incorporate blast mitigating programs on lower level structural elements. Column R7-SR1 is actually a concrete encased and concrete filled column up to level 3, however, this was not taken into account during the column check. The effects of blast loading on the column may account for the remaining strength. The concrete encased and filled built-up box columns will need to be analyzed in more depth to get an adequate understanding of the exact design loads put on the column element.
Lateral Element:
The main lateral force resisting system is the central reinforced concrete core walls. The coupled shear walls act as rigid plates or “vertical diaphragms” to resist the lateral loads imposed on them. The methodology behind a simplified check is to treat each core wall as a separate element with calculated MOI, areas and section properties. These section properties can then be modeled as an equivalent cantilevered beam with a unit load at the top which can be used to calculate the relative stiffness (Unit Force/Deflection) of each wall. Forces can be applied and proportion onto each of the walls according to their relative stiffness. A simple run in a computer analysis package such as STAAD or RISA-3D will determine the maximum deflection of each wall and can be checked again the story drift limit of h/480 (N-S) or < 17” total displacement at roof level.
A second procedure can be used for more advanced analysis using a finite element
computer package such as RAM Advanse, ETABS or SAP 2000. Many techniques can be used to model the coupled shear wall. Meshes and/or plates can be used for the piers and spandrels of the coupled wall as well as object modeling using ETABS.
These procedures were not performed due to lack of background research into the
computer analysis programs mentioned.
Conclusions:
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Patrick L. Hopple Dr. Linda Hanagan Tuesday, October 05, 2004
Hyatt Center (71 South Wacker Drive : Chicago, IL)
The Hyatt Center located at 71 South Wacker Drive in Chicago, Illinois, has many different structural systems and concepts in design. A raised concrete slab above Level B1 serves as the ground level slab for which the outdoor plaza and landscaping planters rest. Extensive use of doubly reinforced concrete beams and 12” thick R.C. slabs are used for the outdoor plaza. These beams are beyond the scope of this thesis project, however, analysis of the raised slab may provide more accurate information on the loading of columns instead of using approximate loads and methods described in this report.
The concrete encased and filled box columns are of interest to be looked at further in
depth. The 48% calculated over-design from column R7-SR1 may be caused by blast loads which cause the column to act like a beam-column more than a purely axial member. Blast loading is one such loading which has not been looked at in-depth for the column check but the lateral forces imparted onto the column could cause the column capacity to decrease due to the combined axial bending in the member. Not accounting for this phenomenon could be the reason why the calculated axial capacity was so high.
In conclusion, further analysis of additional lateral loads caused by blast pressures and
in-depth analysis lateral load distribution among the columns and the central core walls may reveal that the columns actually do take a small portion of the lateral loads instead of the assumption that the core takes all the lateral load.
Project: Course:
Consultant: Date:
By:
pg= 25 psf
Ce= 0.9
Ct= 1
Is= 1.1
pf = 25
hc= 13.8 ft γ = 0.13*pg + 14 but <= 30psf
hd= 2.35 ft γ= 17.25 < 30psf therefore OK!
lu= 50 ft hb=pf /γ hb= 1.43 ft
w = 4hd hc /hb= 9.58
w= 9.41 ftpd= 40.6 psf pd=γ*hd (max. intensity)
(ASCE 7-02 Table 7-2)
(ASCE 7-02 Table 7-3)
(ASCE 7-02 Table 7-4)
psf + Drift
Fully Exposed Roof (Ce)
Roof heated by Mech. Rm. (Ct)
Category III structure (Is)
> 0.2 therefore drift needs to be calculated!
AE 481W - Tech. Assign. #1
Tuesday, October 05, 2004
Drifts on lower roofs (ASCE 7-02 Section 7.1)
hd< hc therefore:
Assumptions:Flat Roof
Exposure B
Chicago, IL (pg)
pf = 0.7CeCtIpg (Section7.3 - Eq. 7-1)
(ASCE 7-02 Figure 7-1)
Snow Loads - ASCE 7-02
Hyatt Center (71 South Wacker Drive )
Dr. Linda Hanagan
Patrick Hopple
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Appendix A
Consultant: Dr. Linda Hanagan Building Loads.xlsASCE 7-02 Wind Load Method 2
By: Patrick L. Hopple
Basic Data: V(mph)= 90 I= 1 Table 6-2
hn(ft)= 673.83 B(ft)= 322 Exposure α zg(ft) â^b α b c l(ft) ε zmin(ft)*
Period (sec.) L(ft)= 143 B 7.0 1200 0.14 0.84 0.25 0.45 0.30 320 0.33 30
Ta = Ct*hnx (EQN 9.5.5.3.2-1 C 9.5 900 0.11 1.00 0.15 0.65 0.20 500 0.20 15
Elevator shafts were assumed to be slabs (conservative)
TYP. framing (psf)Total Framing Load
(kips)Level
General Design Superimposed Dead Load
Self-weight of
Tot. Column DL/fl (kips)
SDL (kips) on core##
Floor Area (sf)
Core Area (sf)
Core (kips)
## 50 psf (restroom) floor load assumed on each floor in core (conservative)Constant in tower floors, 1/2 at 49-roof levelAssumed constant cross-section for Tech. #1 (Varies at 3 height levels)
Total SDL (kips)
Total Weight (revised).xlsTotal Weight of Structure 1 of 1
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Appendix C2
Consultant: Dr. Linda Hanagan Building Loads.xlsASCE 7-02 Seismic Loads
35410.50 7560.00 10204.3LWC= 115 pcfNWC= 145 pcf ** = SDL values taken from drawings on each floor.
*** = DL framing taken from Total Weight Spreadsheet and estimated by calculation of a typical bay.
42970.50
Design DL (kips)
* = Values taken from each floor off of drawings.
Column Wt. (plf)
Slab thick. (in)
SDL (psf)Cumm. DL
(kips)Cumm. DL
(kips)Ultimate Column
Load (kips)Slab Conc.
(NW or LW)Infl. Area (sf)
Total LL (kips)
LL Reduction Factor
DLcol.-self
(kips)
DLframing
(psf)
Design LL (kips)
Slab DL (psf)
SDL (kips)Reduced LL (kips)
Column R7-SR1
LevelStory
Height (ft)Tributary Area (sf)
Mezz. Trib. Area (sf)
Column Take-downs.xls 1 of 1
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Appendix E
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Polygonal Line
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Oval
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Note
Typical Bay to be checked for capacity and member sizes. This bay was also used to determine the framing dead load for use in the total weight calculation which in turn was used in both the seismic and column take down calculations.
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Note
Typical exterior column checked for capacity at ground level. Column was found to be oversized by 48%. Blast loads and the approximation of loading on the column are just 2 possible reasons for this over-design.