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# Wood truss design criteria

• To: seaint(--nospam--at)seaint.org
• Subject: Wood truss design criteria
• From: NRoselund(--nospam--at)aol.com
• Date: Wed, 24 Mar 1999 21:24:44 EST

```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

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