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RE: Wood truss design criteria

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I agree with you in most of your topics in the summary that you have made. 
Here in the Philippines, we adopt the National Structural Code of the Phil.
(NSCP) in designing wood structures. Based on my experience with wood
trusses, I usually analyze the top chord members as a continuous frame
member and not as a truss member (meaning it is subjected to both positive
and negative moments plus axial load). 
For the metal connectors like fishplate and buttplate, I analyze it as a
hinge. I believe that connection for wood member cannot stand the moment
since it will still rotate not like connections for steel structures. I'll
share with you my other insights and techniques later. Thanks !!

Allan Yango
Structural Engineer

> ----------
> From: 	NRoselund(--nospam--at)[SMTP:NRoselund(--nospam--at)]
> Reply To: 	seaint(--nospam--at)
> Sent: 	Thursday, March 25, 1999 10:24 AM
> To: 	seaint(--nospam--at)
> Subject: 	Wood truss design criteria
> I'm reviewing the drawing for metal plate connected wood trusses.  This is
> my
> first project with this kind of truss.  The design submitted to me is
> based on
> the 1997 UBC, which requires ANSI/TPI 1-1995 as the design standard.
> The rules of the design standard seem to defy structural mechanics.
> Here's my
> summary of the design method for the top chord:
> 1)	The top chord, continuous from eaves to ridge over a single panel
> point, is
> analyzed as a continuous beam for flexure with compression.
> 2)	The positive moment analysis is based on assuming a simple span
> between the
> point of inflection and the pin-connected support (I concur with this).
> 3)	The compression stress used to determine the combined stress effect
> in the
> zone of maximum positive moment is based on the true length of the member
> from
> the panel point to the pin-connected support (I concur with this too).
> 4)	The analysis for negative moment at the panel point is based on 0.9x
> the
> actual horizontal distance  between the panel point and the pin-connected
> support; (how is the reduced span justified?)
> 5)	The allowable compression stress used to determine the combined
> stress at
> the panel point does not take into account the unsupported length from the
> point of inflection to the panel point (why not?).
> 6) 	In the combined stress computation at the panel point, the allowable
> flexural stress is not required to be reduced by (1 minus the compression
> stress ratio) as is required by formula 3.9-3 of the 1997 NDS.  
> 7)	The connectors are designed to provide pinned connections with no
> moment
> capacity assumed, so that I see no justification for assuming that the
> connectors provide unaccounted-for continuity that justifies shorter-than-
> actual spans for moment calculations, and increased member stability for
> compression bucking stress allowable.
> The result of these rules seems to be that excessively high stresses are
> concealed by the calculation method; high stresses that would be
> determined by
> conventional structural mechanics.
> Is there testing or rational analysis behind the ANSI/TPI Standard?  Has
> anyone else questioned the standard?
> Based on my requirement that the design of the trusses comply with the
> UBC, it
> appears that I am locked into accepting the design since it is based on
> the
> UBC-specified standard.  But I'm not comfortable with it.
> Nels Roselund
> Structural Engineer