pratt truss - angles or W beams 3

[delagina]

on my way home i noticed many pratt truss along the freeway supporting signs/directions. all members are angles.

i have also seen a calculation for a tower like structure, all angles including columns. all members were considered truss (including the columns) in Staad input.

but i have also seen a calculation of a pratt truss with top and bottom beams as W beams.
W beams were not considered as truss.

can someone explain the difference between these approach.

thanks,

 

[BAretired]

When you are on the freeway, watch the road, not the trusses.

Members of a truss can be any shape capable of carrying the load. Availability of material, architectural preference, ease of connection or a flip of the coin may be the deciding factor. It is not rocket science.

BA

 

[msquared48]

It may not be rocket science, but if you hit the truss on the freeway, consider yourelf launched.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Lion06]

The biggest considerations are loads and span.

 

[paddingtongreen]

I don't understand what you mean, I always modeled the chords as continuous regardless of whether it was wide flange or tee, etc. It better models the lateral stability condition.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

delagina,
I can't believe that you capitalized Staad and did not extend the same courtesy to Pratt.

Michael,
Continuous chords don't work well in my Maxwell diagrams.

 

[paddingtongreen]

hokie, I was thinking Staad.

When I was young and still figuring things out, I calculated the deflection as a truss, applied it to the continuous chords, and proportioned out the loads to each. It was a big effort because I wasn't quite sure where I was going with it, and it was not really worth it because the continuity contributed very little to the total.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[msquared48]

I always thought they were Hokie diagrams...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[slickdeals]

Ah.....the perennial question of whether to model truss with pinned ends (like in the early pre-computer days) or model them with fixity and account for all the crazy moments in all directions, which will go away if the section yields (moment hinge) without an axial hinge.

See Larry Muir's page on this one.

http://larrymuir.building.officelive.com/Trusses.aspx