Wind loading during deck pour/curing

[WJA]

Is there a consensus on the wind load to be applied to steel girders in combination with wet concrete loads? AASHTO Guide Specifications for Wind Loads on Bridges During Construction specifies wind speeds for active (i.e. during erection) and inactive work zones. I'm assuming we don't need to consider the full inactive wind speed during the wet slab condition (typically 115 mph x 0.65 reduction factor for 0-6 weeks duration), since it only lasts a day or two and the slab wouldn't be poured in inclement weather. But the 20 mph "active" condition seems too low, since it's intended for erection only.

 

[BridgeSmith]

There may not be a consensus, but I've used the loading for 75 mph wind speed during the deck placement. It seems plenty conservative, considering our upper limit on wind speed for proper curing of the concrete is 25 mph. Anything over that, the surface dries too quickly for it to cure. If winds over 25 mph are expected during or in the 24 hours following the scheduled deck placement, they'll postpone it.

 

[WJA]

Thanks BridgeSmith - I agree that's definitely conservative as this corresponds to the 0.65 reduction factor (good for up to 6 weeks) on the 115 mph speed.

 

[WJA]

I just found the following paragraph in FHWA Engineering for Structural Stability in Bridge Construction Reference Manual, which indicates that I'm maybe overthinking this:

When investigating the girders in their final erected condition for the loads due to deck placement, the wind load may be neglected. While restrictions on equipment operation and worker safety would preclude placement in winds exceeding perhaps 20 to 25 miles per hour, restrictions on wind speed to preclude rapid water loss from the concrete deck (associated with deck cracking) limit winds to much less than even these values. The strength gain of the deck concrete is such that even if stronger winds should occur only a few hours after deck casting, the deck will have gained adequate strength to provide lateral bracing to the girders.

https://www.fhwa.dot.gov/bridge/pubs/nhi15044.pdf

It does seem a bit presumptuous to assume zero wind in combination with the wet concrete, but maybe the 20 mph active work zone condition is reasonable.

Thoughts?

 

[Lomarandil]

I agree that 20mph (3s gust, service) concurrent with wet concrete is appropriate for that very reason -- the evaporation limit. About half the time I'll use that, sometimes I'll stick with the 0-6wk wind for my own convenience (e.g. the bridge isn't sensitive to the difference) or to build in a little extra margin for lateral stability if the bridge is sensitive to lateral.

Additional nuance -- when the deck is ready for concrete placement, I believe that the girder shielding condition better matches AASHTO LRFD than AASHTO Guide Spec for Wind Load on Bridges during Construction. So I'd use the permanent Cf and shielding rules in that condition.

 

[BridgeSmith]

Additional nuance -- when the deck is ready for concrete placement, I believe that the girder shielding condition better matches AASHTO LRFD than AASHTO Guide Spec for Wind Load on Bridges during Construction. So I'd use the permanent Cf and shielding rules in that condition.

I agree with that. I believe there's wording in the guide spec that makes it clear the loading specified no longer applies once the deck forms are in place.