Lateral Bracing for Steel Beams with Timber Deck.

[JRR0809]

I’m completing the load capacity rating for an older steel beam bridge with a 4” thick timber deck. Steel beams are S15x42.9 @ 2’-4” O.C. Span length is 30 ft (Simple Span). As best I can tell, there is no connection between the timber deck and the steel beams and no intermediate diaphragms. Check of Lat. Torsional Buckling for 30 ft unbraced length severely reduces rating and drops posting requirement to 8 tons. I don’t feel I can consider the compression flange to be braced with no physical connection between beams and deck, but I hate to tell my client that his bridge needs to be posted at 8 tons after it has stood for the past 70 years with no posting.

Any thoughts?

 

[ishvaaag]

Since the rating methods are regulated you shall have to go along the method. That some structure is standing is good thing but no warrant of it having the required standard of safety and reliability.

 

[msquared48]

What has gone across it in the past?



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

 

[apsix]

Suggest a lateral bracing system that will result in a reasonable capacity.
It won't be the first time that an old bridge has to be retrofitted to comply with current requirements.

 

[JRR0809]

The bridge is on a relatively low volume county road. It does not get a lot of traffic, but it is in an agricultural area and carries a lot of farm to market traffic, especially in the fall of the year. Farmers typically have little concern for legal load limits, and it is quite likely that this bridge has carried its fair share of overloaded (40+ tons) semis and grain carts.

 

[bridgebuster]

You didn't mention diaphragms. They would reduce Lu.

Aside from that, the rating is what it is.We've all seen structures that shouldn't be standing yet they are for one reason or another. However, codes are codes.

 

[apsix]

Perhaps a load test will satisfy.

 

[BAretired]

JRR,

You say "As best I can tell, there is no connection between the timber deck and the steel beams..."

Is there any chance you are mistaken? It seems unlikely that there is no connection. It may be worth a closer look.

BA