# AISC Seismic Provisions

• To: seaint(--nospam--at)seaint.org
• Subject: AISC Seismic Provisions
• From: "Connor, John A NWK" <John.A.Connor(--nospam--at)nwk02.usace.army.mil>
• Date: Fri, 9 Apr 1999 13:30:07 -0700
```I would appreciate any comments and clarifications on the following problem.

Design code:  1997 NEHRP (seismic provisions), 1997 AISC (steel building
Project Location:  Kansas
Building type:  Aircraft hangar

I am designing a steel braced frame for an aircraft hangar using the codes
listed above.  My problem is determining the required capacity for the
bracing connections.

NEHRP directs me that my seismic requirements shall be in accordance with
AISC's Seismic Provisions for Structural Steel Buildings.  Based upon the
significance of this building, I have decided to design the steel framing to
meet the requirements of AISC's chapter 13, "Special Concentrically Braced
Frames".

The braced bays of this building consists of x-braces.  The bracing members
are approximately 50' in length.  Assuming I use a steel tube shape, chapter
13's requirements for slenderness and also limiting width-thickness ratios
leads me to select a TS 10x6x5/8.

I used this size as my preliminary member size in my STAAD model.  After
running all of the different load cases, the output for this member is about
60 kips for both compression and tension.  The "controlling" load case for
this output was from the wind loads.

Chapter 13 says that the required strength of my bracing connection shall be
the LESSER of the following:

a)  "The nominal axial tensile strength of the bracing member, determined as
RyFyAg."
For this tube: (1.1)*46*16.4=830 kips.

b) "The maximum force, indicated by analysis, that can be transferred to the
brace by the system."
I interpret this as my STAAD output which equals 60 kips.

According to chapter 13, the "lesser" would be a connection designed for 60
kips.

Did I interpret the criteria correctly?  Did I miss the "fine print"
somewhere?

What I don't understand is, why do I have to have such a "huge" bracing
member, when I only have 60 kips going through the member?  In the event of
a seismic event, my connection would fail before the member would "buckle"
or "snap".  My seismic event forces are less than my wind forces.  It
doesn't seem right requiring a large brace and only have a 3-bolt
connection.

Also, when would condition "a" control over condition "b"?  For example, if
the analysis says my maximum tension is 500 kips, I would have to select a
member for (0.9)*Fy*Ag.  0.9<1.1 so I would never have this as a controlling
case.  Does the code have a mistake in wording, or is it correct?

Any information or comments would be appreciated.

John Connor, EIT
Kansas City, MO

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