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RE: TRUSS SHAPES

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I was speaking mainly of parallel chord trusses, however the principle is
still the same. With an inclined top chord, you have to keep in mind that
some of the shear comes down the top chord in compression. As a result the
diagonals near the center are normally reversed to keep them in tension. In
other words, near the center of the truss, the shear in the truss is small
enough that it cannot overcome the vertical component of the top chord
force.

Jim K.

-----Original Message-----
From: Jim Kestner [mailto:jkestner(--nospam--at)somervilleinc.com]
Sent: Thursday, November 21, 2002 3:29 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: TRUSS SHAPES


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Your following message has been delivered to the list
  seaint(--nospam--at)seaint.org at 13:52:53 on 21 Nov 2002.
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A Howe Truss puts the diagonals (longest members) in compression (and the
verticals in tension). I would think that you want a Pratt truss that puts
all the diagonals in tension (and the verticals in compression). A Howe
truss is normally used for wood trusses. A Pratt for steel trusses. Use a
Warren Truss if you have moving loads that cause stress reversals in the
diagonals. Other considerations should include simplifying details such as
elimination of gusset plates or minimizing the number of them.

Don't forget about wind uplift.

I hope this helps.

Jim K.

-----Original Message-----
From: Juan José Treff De la Mora [mailto:jjtreff(--nospam--at)hotmail.com]
Sent: Thursday, November 21, 2002 2:33 PM
To: seaint(--nospam--at)seaint.org
Cc: solutions(--nospam--at)aisc.org
Subject: TRUSS SHAPES


Dear Colleagues,
I need to design an A-36 steel trusses to cover a warehouse. The span is 82
ft, in a seismic zone and with winds of 65 mph. The only trusses the load
will be supporting is the roof and lamps. I have two questions:

1. What is more efficient structuraly speaking, a Howe type truss with
gabled top chords and straight bottom chord or a Fink type truss with both
chords gabled and parallel to each other?

2. And speaking of weight, which one of the two will require less material?

Thank you,

Juan






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