Deflection Limits for Bridges

[pbc825]

Hello bridge engineers,

I've been asked to design a bridge for a client who makes temporary bridges. The client had another engineer making drawings for him, and I've been provided with a sample drawing. These bridges are very basic and are composed of longitudinal beams with a lumber deck. The abutments are out of scope.

I've read through CSA S6-00 and I can't seem to find a deflection limit for the girders. The beam sized by the other engineer appears to undergo excessive deflection under live loads (something in excess of the L/300 range). Although I'm wondering if the deflection limit is less stringent for temporary structures. Any guidance on where to find deflection limits in S6 would be appreciated. Also, I'm wondering if S6 would define what is temporary. I'm thinking the client may be using this on a more permanent basis, but I'd like to know what the threshold is.

Thank you in advance.

 

[bridgebuster]

In AASHTO the live load deflection is limited to L/800 & L/1000 for bridges without and with pedestrians. Deadload deflection is taken up by camber.

"Temporary" is subjective. At times, we take chances when something is short term. Maybe a day or a week or two, what will go wrong? As Dirty Harry would say " you feeling lucky?"

 

[pbc825]

Thank you bridgebuster. Much appreciated.

Any S6 folks care to weigh in?

 

[cooperDBM]

The deflection limits in S6 are based on the first flexural frequency of the bridge in order to limit uncomfortable vibration. In the latest edition, S6-14, you should look at clause 3.4.4.

 

[bridgebuster]

I was just looking through S6 and noticed that it uses a different approach from AASHTO. Regarding how temporary is temporary: in Section 1 Temporary is defined as a service life of less than 5 years & Section 1.4.4.4 b seems to imply - at least to me - that there is some "flexibility" with the code compliance. Perhaps someone can tell me if I'm reading that provision correctly.