Wind Load Factor for Construction Loading

[OSUCivlEng]

I am trying to interpret another poorly worded sentence in the 9th Edition of the AASHTO LRFD Bridge Design Specs.

In Section 3.4.2.1 it says The load factor for wind during construction in Strength Load Combination III shall be as specified by the Owner. Any applicable construction loads shall be included with a load factor not less than 1.25.

The load factor for wind in Strength III is normally 1.0. It clearly states the owner may specify their own load factor for wind in Strength III when considering wind during construction loading. My question comes with the next sentence. I interpret it as stating that any construction loads considered when checking the Strength III load case shall have a minimum load factor of 1.25. There is no other reference to load factors for construction loads for Strength III in this section. The previous paragraph does state that the load factor for construction loads in the Strength I load case should not be less than 1.5

However, the NSBA document I am looking at says the load factor for wind in the Strength III load case should be 1.25 when considering construction loading and the load factor for the construction loads should be 1.5. That doesn't make any sense to me.

So what say you? Am I missing something here?

 

[BridgeSmith]

AASHTO made major changes to the how the wind load is calculated and the load factors (Strength III Wind load factor was previously 1.4), starting with the 8th Ed., so if the NSBA document is more than 5 years old, it may not be valid for use with the 8th or 9th edition spec.

Anyway, NSBA is recommending load factors that are more conservative than the minimum required by AASHTO. Nothing strange about that. It's also not in conflict with the spec, which uses the wording "not less than 1.25". As it states in the commentary "Often the construction loads are not accurately known at the time of design."

AASHTO doesn't attempt to define what the wind load should be during the various stages of construction, because there are just too many possible permutations. Sometimes, if the combined stresses are too high throwing everything at the erected steel girders, we use a reduced wind load in combination with load of the wet concrete. During the deck pour, and for the 24 hours afterwards, the wind needs to be below 25 mph for the concrete to cure properly, so assuming 70 mph wind during that phase seems reasonably conservative.

 

[BridgeNerdGuy]

I use the AASHTO Guide Spec for Wind Loads on Bridges during construction for this case. We typically analyze as a regular Strength III load however the wind pressures would be calculated with this document which as expected are lower than the Strength III speeds/pressures calculated with the 9th edition of the code. This is just the table of contents but may be helpful.

file:///C:/Users/BrianWatkins/Downloads/GSWLB-1_TableOfContents%20(2).pdf

 

[BridgeNerdGuy]

^ Revised link to the TOC which can be accessed for free:

https://store.transportation.org/Item/PublicationDetail?ID=3728