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Re: MBMA Code and Wind Loads

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This is not the first time I've heard of this happening. In fact, it has
happened to me. Scrutinize the criteria presented in the shop drawings. It
looks like the building supplier has provided a design for a much lower
wind velocity. Using that old phrase, "Revise and Resubmit". You are the
EOR and remind them of that. When I do this, I admit I am not very PC, but
effective.

Regards,
Bill Allen

----------
> From: LibertyEng(--nospam--at)aol.com
> To: seaoc(--nospam--at)seaoc.org
> Subject: MBMA Code and Wind Loads
> Date: Thursday, February 12, 1998 2:41 PM
> 
> 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
> 
>