Truss/Joist End Moments 1

[GalileoG]

I am currently designing an expansion to an existing steel-framed building. The existing building is framed with trusses in both orthogonal directions and are moment frames.

The original engineering drawings specify end moments for the truss connection design, the drawings state that these moments are due to lateral loads only. The connections of the chords to the column are standard WT bolted/welded to the columns.

What is throwing me off is: how can the end moments not include moments due live or snow loads? I imagine that these moments would be higher than those created by lateral loads.

When specifying joist rigid frames (joist design being by specialty engineer), I have always provided a table that shows moments due to snow, live, wind, and seismic.

Your thoughts are appreciated.

 

[cal91]

We design STMFs and we include end moments due to gravity loads as well, but they are usually 10% or less than our end moments due to lateral loads. Take into account that we're in California however.

 

[DST148]

The approach you mentioned in your post is called FMC - Flexible Moment Connections or Wind Moment Connections. The basic principles of FMC approach are to treat the beams as simply supported under gravity loading but as moment connected under lateral load. If you have single-bay frame, assume beams at one end as moment connected and pinned at the other ends under lateral loading.

 

[KootK]

how can the end moments not include moments due live or snow loads?

I've run into a few of these myself. My thoughts:

1) Truly, it's probably an oversight.

2) It might be mitigated somewhat by the potential to lock in the bottom chord connection after much of the dead load is in place.

3) Designers may have been attempting something like the partially restrained moment frame system that we sometimes use with steel beams. It may not have been justified but may have been the intent none the less.

How old is the existing building?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dhengr]

GalileoG:
RE: your second para., “the drawings state that these moments are due to lateral loads only.” Mightn’t this be interpreted to mean, that these are the lateral load frame moments, and that the truss designer should add these to the other loads/moments caused by the various other loads which they would apply to design their joists and trusses? These ‘other loads’ would be dictated by the AHJ/local codes and likely be shown in the structural general notes too. Do a rough check of the ends of the trusses/truss chords and connections to see if they would actually carry the total load reactions and moments. With a few notes and tabulations the original engineer could have been much more clear about their intentions.

 

[hokie66]

It's all in the design philosophy. I think DST148 has it right. The building is designed for gravity loads, then tied together to resist lateral loads.

 

[Civil_Geek]