Single Span Bridge Seismic Requirement per CHBDC

[tmalik3156]

Good day all.
Canadian Code (CHBDC) says that single-span Girder bridges with small skew don't need to be analyzed for seismic demands. It does not say anything about single-span trusses.
In my opinion, trusses are comparable to girders in the sense that top chord = top flange in compression, bottom chord = bottom flange in tension, diagonals = girder web to carry shear.
Can we extend this Clause to single-span Truss bridges as well?

 

[KootK]

That sounds entirely reasonable to me.

 

[bugbus]

Isn't a truss just a girder with really big holes in it?

 

[Lomarandil]

Eh, while this may pan out from an engineering perspective (depending on the method of truss fabrication — many truss connections are more complex and less redundant than a fully welded plate girder), I think you’re sailing toward a fight with the owner’s engineer. I’d start that discussion to get their buy-in now.

 

[BridgeSmith]

I think the article means what it says. Unless your trusses are actually open web girders, you should do the analysis for seismic loads as specified. Girders are fairly robust for lateral loads; most truss types, not so much. The trusses may be fine for the transverse inertial forces due to ground motions, but then again, they may not, or they may require additional lateral bracing. I wouldn't put my stamp on the design without analyzing the out-of-plane bending forces and displacements from seismic loading.

 

[AS_PEng]

Reading the commentary on the CHDBC for this section, it clearly defines a single-span girder bridge and excludes or does not mention truss bridges.

 

[BridgeSmith]

The commentary is in line with my understanding of the reasons for the exclusion of single span girder bridges. For the type of bridges excluded, the deck is strong and stiff, and provides substantial lateral bracing for the girders. This may not be the case for truss superstructures, which may have members too far from the deck to effectively braced against lateral bending/buckling.

AASHTO's exclusion is for all single span bridges. However, it does say "not required", not that you shouldn't or can't do it or require it to be done if the owner decides it's warranted. Also, AASHTO may have requirements for lateral stability that preclude critical cases for seismic load for single span bridges, that the CHBDC does not. Of course, none of that really matters, since your design will be governed by the Canadian code, which is fairly clear on what is and is not excluded from the analysis requirements..