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Re: MBMA Code and Wind Loads

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     The MBMA wind load provisions are now part of the ASCE 7-95 
     Provisions.  Regardless of their merit, they are now "code".  Be 
     careful when mixing ASCE 7-93 criteria (fastest-mile wind speed) with 
     ASCE 7-95 criteria (3-second gust speed).
     
     Rick Drake, SE
     Fluor Daniel, Irvine


______________________________ Reply Separator 
_________________________________
Subject: MBMA Code and Wind Loads
Author:  LibertyEng@aol.com_at_-FDInternet at FDINET
Date:    2/12/98 5:41 PM


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

<<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. >>