"The shallow the truss, less longer the
members and hence more axial capacity."
BUT this is only true for the web members and the increase in axial force will be greater than the increase in capacity (I think), therefore the truss capacity will decrease.

May you please elaborate a little more, CROWN06?
Are you trying to demonstrate that a beam or column made of a single profile will reasist loads better (or with less weight? or at smaller cost? or what?) than a truss over the same span and with the same loads?

prex
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one would have thought that a shallow truss would be inefficient in carrying bending loads

Apsix and Prex
What will increase the axial force?
Make a 24" deep truss and place loads at panel points than reduce depth to 12" with same configuration. Do not change diagonal angles (slope) or any geometry. You can shorten the truss in length and add one more panel to match original length.
Prex, all I am trying to say that truss is analyzed by axial forces only. So what I am missing in my opening statement.

If you are dealing with axial tensile loads only, then the load capacity is due only to the net total area of section, so it is substantially the same for two trusses with different geometries (but of course one should also see how loads and supports are distributed).
In any other situation (that is almost all situations where trusses are employed), the load capacity will decrease by decreasing the depth (assuming the total section area or weight is conserved), as of course tension and especially compression in the longitudinal members will go up fast.

prex
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CROWN06,
If you reduce the depth of the truss by 1/2,  then your chord forces will increase by 2.  Of course this is assuming a load placed at a panel point at the centerline of the truss.  So unless the original engineer overdesigned the truss top and bottom chord, chances are they will fail.  Also, you have to think about deflection.  By reducing your truss depth by 1/2, you have reduced your effective Ix (neglecting web members) by approximatly a factor of 3.5.

Think about if you were designing a new building.  hey lets use shallower trusses because there stronger...just doesn't make sense.

CROWN06,

I think people are just a little confused by your grammar.

I think what you say is basically correct.

regards

csd

"The shallow[er] the truss, less longer the
members [will be] and hence more axial capacity."

true enough for compression members (thinking about euler); but the shallower the truss the higher the chord loads (like twinnell above).

i'm sure there is an optimum height of a truss to accomplish a particularly design, higher (taller) trusses increase the length of elements, reducing the euler allowable, reducing the internal loads (generally), generally reducing the cross-sectional areas of the elements, so i can see a trade-off in terms of weight.

CROWN06
Some of this discussion may be lost in the translation.

" Do not change diagonal angles (slope) or any geometry. You can shorten the truss in length and add one more panel to match original length."
For the same span and the diagonal angle to remain the same the number of panels has to be doubled and, yes, the axial force in the diagonals will then remain constant (ignoring self weight).

However, as stated above, the top and bottom chord forces will double for half the original depth. Therefore the tension chord may need to be larger and the compression chord either also increased in size, or have additional lateral restraints.

Deflection, as noted by twinnell, will also greatly increase.

apsix,

I think I'm missing something.  If the truss is subjected to axial loads, then why does halving the height have much to do about anything with the chord loads.  Would they not remain the same (or close to it)?

Thanks,

jetmaker

Jetmaker,

The axial loads that we are speaking of are the chord axial loads created by the moment in the truss.  Not axial loads on the truss as a whole.