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Re: PEMB's and lateral deflection

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> From: Jim Wilson <wilsonengineers(--nospam--at)yahoo.com>

> Are there any official industry position statements or articles on
> coordinating maximum lateral deflection of PEMB's with rigid masonry
> facades?  I am looking for a little more "oomph" to add into a letter to a
> future building owner.

Very little reference and nothing formalized. Even MBMA avoids setting
numbers for any finish, including cold-formed steel panel cladding.

> From: "Bruce Holcomb" <bholcomb(--nospam--at)brpae.com>

> In addition to Code prescribed deflection limits, we use AISC Design
> Guide 3 - Serviceability and AISC Engineering Journal, First Quarter,
> 1993 for serviceability limits, but neither of these is directly for
> metal building systems.  The Metal Building Manufacturers Association
> (MBMA) has their own manual which (if I remember correctly) has
> recommended deflection and sidesway limits.  I no longer have a copy, so
> I can't quote it.  But you can order one from MBMA.

Spec the same values that you would use for a conventional steel framed
structure. Base your required limits on the materials, assemblies and
details that will be used to interface around the PEMB steel. Make sure
that the PEMB quote echoes back your deflection requirements.

> From: "Harold Sprague" <spraguehope(--nospam--at)hotmail.com>

> Bruce and Jim,
> I had this conversation with some PEMB manufacturers a while back.  The
> authors of Design Guide 3 intended this for use for the PEMB industry.  They
> do a lot of work for that industry.  Some manufacturers will balk, but the
> recommendations of Design Guide 3 are reasonable.  There are no building
> code wind drift limits since this is a serviceability issue and is not a
> safety issue.

As I keep saying, treat the PEMB the same way that you would treat any
other steel framed building. Know the limits and spec appropriately. If
the manufacturer can't/won't meet your spec, drop them. 

> We create a one page performance spec for PEMB.

That's the best. The consultant should make sure that it is modified
appropriately for each project, just as SHOULD be done with every
standard spec.

> The MBMA had some commentary stated that
> you should calculate lateral drift and some owners don't like it when the
> lights sway.

"you" = the PEMB designer

That's almost verbatim. However, as noted above, the MBMA won't set the
limits. Additionally, the manufacturer is not a consultant or the
project EOR. Most manufacturers will not attempt to decide the
serviceability tolerances for your masonry/store-front connections or
cornering details. They assume that the EOR will ensure that all other
elements are suitable for their building - read the fine print in their
quote. 

> The practice for better PEMB manufacturers is to limit lateral drift to
> h/30 for a 10 year wind, and h/100 if you have masonry.  That is not
> restrictive enough.  Again what is reasonable is contained in Design Guide
> 3.

Those seem very low to me, for the "better" manufacturers. Agreed that
it is not nearly stiff enough (even ignoring P-Delta and seismic h/50).
Regardless of the manufacturer, the EOR is responsible for appropriate
specifications. 

> You will have to put it in your specification.  Don't just reference DG 3.
> It is not a manditory code.  You can make it manditory by specifying the
> recommendations in DG 3 as "required", but that creates a layer of 
> ambiguity.  I specify the maximum lateral drift and under a given loading
> that is to occur.  DG 3 uses a 10 year wind for servicability.  I also
> specify the maximum girt and purlin deflections.

I like your specification! Even when deflections are explicitly
identified the consultant will typically NOT identify the load that
generates the deflection and this is left to the manufacturer's
discretion! (That's as good as no spec)

Also, regardless of the ambiguity issues, it is unlikely that the
manufacturer has a copy of DG 3 and may not care to put out the small
cost of buying one. Most PEMB manufacturers will simply qualify their
quote as "supplied to manufacturer's standard material and design
specifications and criteria." Some cannot give you a written document
explaining what that means, let alone providing the
frame/purlin/girt/panel deflection limits and loads separately.

You could go another level:
1) specify the purlin collateral loads and frame collateral loads as
unique values and possibly unique load conditions for deflection,
2) Require continuous span purlins to be designed with load patterning
for collateral UDLs exceeding 5 psf - this could be done in combination
with the roof/snow load patterning.

I could go on ...    

-- 
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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