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TRUSS SHAPES

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Juan,

A Howe truss can either be pitched or have parallel (or nearly parallel) 
chords.  Under gravity loading, the diagonal web members are in compression.

A Pratt truss is similar to a Howe truss except the diagonal web members are 
in tension.

A Fink truss has a sloped top chord, and web members are arranged so that 
they are perpendicular to the top chord and evenly spaced along the top chord.

In order to minimize weight for a steel truss, I would have the shorter 
web members in compression, and the longer web members in tension.

For efficiency, a bowstring truss would be most efficient under uniform 
gravity loads with the top and bottom chords having essentially the same 
force from support to support and the web members very lightly loaded except 
for partial LL and wind loading conditions.

For parallel chord trusses spaced about 10 ft (3m) apart, the depth at 
mid-span should be about 1/10 to 1/12 the span.

For a pitched truss, the depth at mid span should be about 1/4 the span.

The bowstring truss approximates the shape of the uniform load moment 
diagram, therefore the depth at mid-span is 1/8 the span.

HTH

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Juan Jose wrote:

. > 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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