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# MBMA Code and Wind Loads

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• To: seaoc(--nospam--at)seaoc.org
• Subject: MBMA Code and Wind Loads
• From: LibertyEng(--nospam--at)aol.com
• Date: Thu, 12 Feb 1998 17:41:35 EST

```How does the MBMA (Metal Building Manufacturer's Association)'s "code" or
design manual compare to model codes such as BOCA, UBC or ASCE-7 with respect
to wind loads?  my suspicion is that MBMA's requirements are less to provide a
economic advantage to metal building suppliers over other types of
construction.

I have a project to design a foundation for a metal building in the Azores on
a USAF Base.  The contractor has ordered the metal building. (24' x 60' by 20'
high) The supplier has provided reactions for foundation design.  My dilemma
is that the sum of the horizontal reactions at the base of a rigid frame works
out to less than 25 psf on the frame's tributary area, much less than I
believe a usual building code would require.  The design wind speed is 140
MPH.  The suppplier used an MBMA standard to get his wind loads.  Using ASCE
7-93 (fastest mile wind speeds)  I'd use:
V = 140 mph
I = 1.05
Kz = Kh = 0.87 (Exposure C)
Gh = 1.29
Cp = 0.8 Windward
Cp = -0.3 Leeward

to get:  q = 0.00256 Kh ( I V )^2  = 48 psf
and p = (48) (1.29) (0.8) = 50 psf windward wall
and p = (48) (1.29) (-0.3) = -19 psf leeward wall

for a total wind load of almost 70 psf on the main wind force resisting bent.
The supplier's number is less than 25 psf.

If the building is in Exposure D Kh = 1.27, Gh = 1.14 and the total factors
out to about 88 psf.

Is the MBMA Manual this liberal to reduce wind loads to a third of what ASCE-7
would require?  The supplier of course quoted based on "MBMA Code" and feels
he is adequately meeting that criteria.  He's checked his reactions based on
my comments and says he's OK. I haven't got the MBMA Manual to compare it to
BOCA  or UBC.  The MBMA Manual is intended to provide direction to metal
building suppliers rather than replace a building code but it seems somthing
is amiss in this case.

Here my cleint may unknowingly provide the USAF with a structure that is much
weaker than they are used to receiving because they have assumed that the
MBMA's trade standard is the same as the Air Force's requirements.  This
project was initially bid as a cast in place concrete building.  The metal
building route has been taken to reduce cost.  As often seems the case when
comparing metal building system buildings to "stick built" or "red iron"
buildings, the metal building supplier's promise of an "equal system" doesn't
pan out as equal when closely srcutinized.  Here, lower wind loads may have
helped to reduce the cost.

Anyone else see it this way?  Any comments appreciated.

Jan M. Harris, PE
Liberty Engineering, PC
Virginia Beach, VA

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