structural concepts / structural existing conditions (s-1) · 2002-09-30 · structural concepts /...

September 30, 2002 Structural Concepts / Structural Existing Conditions (S-1) Executive Summary: The building utilizes both concrete and steel to form the structural steel system. A concrete strip footing supports the walls, while concrete spread footings transfer the column loads to the ground. The basement walls are a combination of concrete masonry units and reinforced concrete walls. The basement and retail space on the first floor are slabs on grade, while the rest of the building uses steel framing to support the floors. Each floor is almost identical in framing with the exception of a few mechanical opens. The floor system uses simply supported composite beams that run in the east – west direction that frame into girders that run from north to south. The roof of the building is framed the same way as the floors, but does not use composite beams. Spliced columns take the floor loads down to the foundation and span all the way up to the roof. The building is designed for loads that are in compliance with the 1998 Ohio Basic Building Code and was designed using the Allowable Strength Design philosophy. To determine the loads the building was subjected to, I consulted ASCE 7-98. From this document, I got the minimum live loads, as well as the wind, seismic and snow loads. After determining the loads, I was a little surprised that the seismic loads control the design of the braces in the braced frames. After I determined the loads, I was able to spot check a few of the members. I selected a braced frame, floor beam and column below the 5th floor. After performing some calculations, my designs were very close to those of the original designer even though I used LRFD. The exception to this was the brace. My size was much larger than the original design, because it was governed by the seismic load. After further reviewing the structural notes, I found that the original design used different assumptions in the seismic loads and was able to reduce the load by taking some special precautions and designing the members and connections specifically for seismic loads. After designing the member for wind load, my design was very close to that of the original design. In each case, my design was slightly smaller than the original design. I think that is a reflection on the advantages on using LFRD design vs. ASD.

The structural framing system for 250 West Street utilizes a steel superstructure to carry

the loads the building is subjected to and a concrete foundation to transfer the loads to the

ground. The basement of the building allows for parking and storage and takes up about half of

the footprint of the building. It consists of a slab on grade and reinforced concrete walls. The

first floor of the building is composite beams in the southern half of the building where it was

designed for office space. The northern end of the first floor is a slab on grade. The rest of the

floors are almost identical. They have composite beams running in the east –west direction that

Kevin A. Sponsler S-1

frame into composite girders. The floor system only carries the gravity loads because of the

presence of braced frames. There are 2 brace frames in each direction of the building that carry

the lateral loads. To complete the design, the roof, upper roof and penthouse roof all use non-

composite steel framing.

The building was designed to be compliant with the 1998 Ohio Basic Building

Code and was designed using Allowable Stress Design procedures for the design of the steel

framing.

In addition, the following codes were also followed:

Concrete

• ACI 301-96 – Specification For Structural Concrete For Buildings

Structural Steel

• AISC Specification For The Design, Fabrication and Erection of Structural Steel For

Buildings

• AISC Code of Standard Practice

• Structural Welding Code, AWS D1.1 of the American Welding Society

• Specification For Structural Joints Using A325 or A490 Bolts

Metal Studs and Joists

• AISI Specification of The Design of Cold-Formed Steel Structural Members

• Structural Welding Code, AWS D1.3 of the American Welding Society

The foundation of 250 West Street consists of concrete strip footings that support the

basement walls, spread footing to support the columns and some grade beams. The strength of

the concrete used in the foundation is 3000 psi, while the reinforcing bars have a yield strength

of 60 ksi. The strip footings range in width from 1’- 4” to 2’- 0” and have a depth of 12”. Each

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strip footing has 2 #4 reinforcing bars running continuously. Under each column are square

footing that range in size from 6’- 0” square and 18” deep with 6 #6 bars each way to 14’- 6”

square and 40” deep with 12 - #10 bars running each way on the top and bottom. There are also

several grade beams that run from the column footings on the exterior of the west side into

footings approximately 25 feet in from the west side of the building. Each grade beam is

approximately 3’- 0” wide by 4’- 0” deep with 6 #8 bars in the bottom and 17 #10 bars in the top.

The walls of the basement/foundation are for the most part concrete masonry units.

Twelve inch CMU’s are used in the perimeter of the foundation from the top of the footings to

the first floor. The only exception to this is the south, east and west walls of the parking area.

The construction of these walls are 12”, air entrained, reinforced concrete that has a strength of

4000 psi. The floor of the basement is a 4” slab on grade made of 3500 psi concrete.

The rest of the structure is framed with steel. The steel strengths are summarized in table

1.

Table 1 - Steel Grades and Yield Strengths

Shape Steel Grade Yield Strength Wide Flanges A572 Gr. 50 50 ksi

Angles, Plates, Channels A36 36 ksi Pipes A53 Gr. B 35 ksi Tubes A500 gr. B 46 ksi Bolts A325

Electrode E70 Anchor Bolts A36 or A307 Shear Studs A108 60 ksi

The first floor of the building uses composite steel beams and concrete slabs on the southern

end of the building and a slab on grade in the north. The steel beams span 40’ from the center of

the building to the east side and are spaced at 10’- 4” at the southern end and gradually end up at

a spacing of 7’- 0” in the center of the building. Almost every beam is a W21x44 with 18 shear

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studs in this area. The beams on the western side of the building are much shorter and span

approximately 18’. A W12x14 with anywhere from 6 – 8 shear studs are used to frame this area.

On top of the steel shapes is a 3” metal deck that is covered with 3.25” of 3000 psi light weight

concrete to complete the composite design.

Floors 2 – 7 are very similar to each other and can be seen in Figure 4 on page 11. There are

4 lines of girders that run in the north- south direction that have a spacing of 40’, 18’, and 40’

with the 18’ span being in the middle. Each girder spans from column to column with spans of

20’ and 30’. The girders are composite wide flange beams that range in size from a W18x35 to

W24x55. Each girder supports beams that run in the east-west direction and range in size from a

W12x14 in the short span of 18’ and W21x44 in the 40’ span. The beams have a tributary width

of approximately 10’. Each beam and girder has between 6-45 shear studs through the 3” metal

deck to give it composite action with the lightweight concrete slab. Each beam is simply

supported at each end with the exception of the beams in the corners. Since there are no columns

in the corners of the building, the beams are field welded to the columns to support the loads and

give the beams stability.

The roof is framed the same as floors 2-7, except that it utilizes non-composite beams and

only covers part of the building. The beams and girders range in size from W12x14 to W30x90.

The roof is covered with 3” metal roof deck to complete the structural framing.

The gravity loads are transmitted from each floor to the ground by a series of columns.

The locations can be seen in figure 4 on page 11. The columns run from the basement all the

way up to the roof. Each column spans two floors and is spliced 4’ above the top of steel on that

level. Bolted splice plates connect the columns’ flange at most of the splice locations with the

exception of the columns in the braced frames. Figure 5 on page 12 illustrates this. These

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columns use double angle shear connections in the web of the member. Each column is

supported by base plates that range in size from 14” square by 1 ¼” thick to 30” square and 4”

thick. Each base plate has 4 anchor bolts with the exception of 2 columns which use 6 anchor

bolts.

The lateral loads are resisted by 4 braced frames. Figure 5, on page 12 illustrates 2

frames and is similar to the other 2.

The frames have the following locations.

• Column line D from 4.4 – 5.1

• Column line C from 5.9 – 6.4

• Column line 4.4 from D – C

• Column line 7.4 from D – C

Each frame uses steel tubes to take the

lateral loads that range in shape from HSS

8x8x1/4 to HSS 8x8x5/8. Each brace is

field welded in place to a gusset plate

located at the beam column connection

location or a gusset plate welded to the beam

depending on where it frames into. Figure 1 shows a t

In order to design the structural system for a

the system will be subjected to. To determine these

minimum design loads. The minimum live loads are s

n

5

Figure 1 - Typical Bracing Connectio

ypical connection.

building, you have to determine the loads

loads, ASCE 7-98 was consulted to get the

ummarized in table 2.


Table 2 - Minimum Live Loads by ASCE 7-98

Live Load Minimum Design Load(psf)

Corridors (1st Floor) 100 Other Corridors 80

Office 50 Retail 100

Terrace 100 Storage (light) 125

Partition 20

All the loads listed in the table are the minimum loads that the system must be designed

for. However, because most of the building is unfinished, an increased load should be used in

the office space. It is inevitable that some of the space will be used for storage and hallways will

be built in the space. As a result, a live load of somewhere between 80-100 pounds per square

foot should be used in my opinion. To determine the exact amount, a little more research will

have to be done on buildings of this nature.

The roof load is another gravity load the building will face. It is the maximum of the

minimum roof live load and snow load. The minimum roof load for the building is 20 pounds

per square foot. For the snow load, figure 7-1 was looked at to determine the ground snow load

of 20 psf. Since all of the snow load factors were determined to be 1.0, the snow load becomes

14 psf. For this building the minimum roof live load is used for the roof load if drifting is not

considered. To determine the effects of drifting snow on the roof, the code needs to further be

explored.

Wind loads are another load the building must be designed for. The basic wind speed for

Columbus, Ohio was determined from figure 6-1 to be 90 mph. According to the code, the

building is exposure type B, because of its urban location. The site is also relatively flat and has

no hills or depressions in the surrounding topography, giving it a topographic factor of 1.0. The

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importance factor is also 1.0, because of the function of the building. The frequency of the

building was determined to be 1.47 seconds, qualifying it as a rigid structure and simplifying the

gust factor calculations to a conservative .85. The external pressure coefficient, Cp, varies

around the building and whether the windward or leeward side of the building is being

considered. Table 3 summarizes the coefficient.

Table 3 - External Pressure Coefficients

Location Cp Windward .8

Leeward (North – South) -.35 Leeward (East – West) -.5

Side -.7

The wind load calculations can be found in Appendix 1 and are summarized in Figure 2.

The final lateral

loads the building must

withstand are seismic loads.

The structure is a category

II building with a seismic

use group of I. The site

class was assumed to be D,

because of the uncertainty

in the site conditions and is

also conservative according

to the code. A response

modification factor of 3

Figure 2 - Wind Load Distribution (psf)

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was used, because the building is not specifically designed for seismic resistance. The base

shear for the building was determined to be 637.8 kips. The distribution of the base shear can be

seen in figure 3.

The calculations for the seismic loads

can also be found in Appendix 1.

The dead loads must also be

determined to design the building.

These loads are dependent on the types

of building materials used. Since the

building is unfinished, some

assumptions had to be made for the

floor dead loads. Besides the loads of

the steel framing system and concrete

deck, an assumption of 15 psf was used

to account for additional finish materials

supported by the floor framing. Figure 3 - Vertical Distribution of Seismic Forces (in Kips)

Some additional loads must be determined in order to completely design the building.

These loads are: Wind uplift force, soil pressure, the effects of drifting snow and construction

loads.

Several members were checked in the existing building. All members were checked

using the Third Edition of the LRFD manual, even though the building was originally designed

using ASD. This will result in slightly different member sizes with LRFD typically being a little

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smaller. All members were also checked using the minimum design loads according to ASCE 7-

98. The detailed calculations are not included in this report, but can be requested for view.

The first member checked was a brace between floors 2 and 3 in the braced frame on

column line D from column lines 4.4 – 5.1. The brace was only checked for wind and seismic

lateral loads since that is all the member will be subjected to. The tributary area method was

used to determine the force the frame must resist. The loads from ASCE 7-98 were factored to

come up with the critical load case for the brace of 1.6, the wind load, and 1.0, the earthquake

load. From the calculations performed, the earthquake load governs the design of the member

and a HSS 10x10x1/2 should be used. This is, however, much bigger than the HSS 8x8x5/16

that is currently being used in the building. After reviewing the design notes for the building, a

response modification factor of 5 was used to determine the seismic loads. This will reduce the

seismic load and cause the wind load force to control the design of the member. When the

member is designed for wind load a HSS 8x8x1/4 is required and is slightly smaller than the

existing member. However, that can be expected since the building was designed using the

allowable stress methodology.

Another member checked was a beam running from A8-C8 on the third floor. This is a

typical member that can be found on almost all floors of the building. It is a composite beam

with a span of just over 40’. My design resulted in a W18x35 with 30, ¾” diameter shear studs

as the most economical combination of shape and shear studs. The existing beam is a W21x44

with 18 - ¾” diameter shear studs. The difference in size is due to the difference in design

methodology and possibly slightly different loading.

The final member checked was column A-6 below floor 5. This is an exterior column

which is subjected to only gravity loads. After determining the loads and taking into account

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live load reduction, because the column caries multiple floors, I came up with a W12x45. The

existing design calls for a W12x53. These column sizes are very similar with most of the

difference as a result of using LRFD, while the original design was done by ASD.

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11

Figu

re 4

- T

hird

Flo

or F

ram

ing

Plan

(Flo

ors 2

-7 S

imila

r)


12

Figure 5 - Braced Frames

Appendix 1

structural concepts / structural existing conditions (s-1) · 2002-09-30 · structural concepts /...

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