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RE: Deflection limits for skylight support framing.

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The full title is - Maximum Allowable Deflection of Framing Systems for
Cladding Components at Design Wind Loads, TIR A11-96.  It is available from
the American Architectural Metals Association  It
only costs $6.00.  I do not have a copy.  But your manufacturer should, and
he may give you a copy.  

Deflection for skylights is a serviceability issue.  The manufacturers will
complain, but it seldom a major expense to stiffen them up.  You are correct
in your assessment of the L/750 it does appear to be overly restrictive.  It
depends on how he is designing the skylight.  I would guess that he is
designing it as a 3 hinged arch in which case the vertical deflection should
not be a major issue.  

Talk to his engineer.  The sales guys profess knowledge, but possess little.

Harold O. Sprague

-----Original Message-----
From: Bruce Holcomb [mailto:bholcomb(--nospam--at)] 
Sent: Friday, October 17, 2003 12:53 PM
To: Seaint (E-mail)
Subject: FW: Deflection limits for skylight support framing.

I have sent this before, but have not gotten a response.  It was about the
time that several people have indicated problems receiving email from the
list, so I thought I'd try again.

>  -----Original Message-----
> From: 	Bruce Holcomb  
> Sent:	Wednesday, October 08, 2003 11:58 AM
> To:	Seaint (E-mail)
> Subject:	Deflection limits for skylight support framing.
> I am looking for deflection limits for designing skylight support framing.
I have a 40' x 180' gabled skylight bearing on a perimeter of W-beams
spanning between steel columns at various spacings of 15' to a maximum of
40'.  The skylight MFR. has told me I must maintain a max. deflection in any
direction of L/750.  That seems somewhat reasonable for horizontal
deflection of a gabled skylight, but that seems way to strict for vertical
deflection.  In AISC Design Guide #3, on page 21, I find a DIFFERENTIAL
vertical deflection limit of L1/240 (max of 1/2") where L1 is the span of
the skylight.  This would be no problem to achieve for most beam spans.  I
also see L/300 for live load deflection (p. 4)... again, no problem.
> I understand that AAMA has a reference: TIR A11-96.  Does anyone have this
document?  Any experience with vertical deflection limit of skylight support
framing would be helpful.
> Bruce D. Holcomb, PE
> Butler, Rosenbury & Partners
> 300 S. Jefferson, Suite 505
> Springfield, MO 65806-2217
> ph (417) 865-6100
> fax (417) 865-6102

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