Sliding and Overturning Factors of Safety during Construction

[Davidmat]

I am performing a stability analysis on an abutment and wingwalls for the design of a bridge.

Considering in-service loading, the minimum factors of safety for overturning and sliding that I typically use are 2.0 and 1.5, respectively.

When looking at "during construction" conditions, I am assuming that since this is a temporary condition, lower factor of safeties could be used. However, I cannot find a reference that would back up this assumption. Can someone help me with this information? Thank you.

 

[GeoPaveTraffic]

I would not use a factor of safety in any overtuning analysis. The only thing you should be looking at on overturning is that the resultant is with in the middle third of the base. I would not use a different criteria for overturning during construction.

As for sliding, a lower FS during construction would be reasonable. However, I would also not expect the calculation to be that different.

 

[bridgebuster]

GeoPaveTraffic:

Do I understand correctly that you never apply a factor of safety for overturning analysis?

Davidmat:

There is nothing in AASHTO that permits a reduced FS, or for that matter overstress (other than what is provided in Section 3)although many engineers apply arbitrary standards for construction and or temporary conditions. It comes down the old question: How temporary is temporary?

 

[VoyageofDiscovery]

Watch out for compaction stresses!

Regards

VOD

 

[GeoPaveTraffic]

bridgebuster,

That is correct, I never calculate a factor of safety for overturning because it has no meaning. The important thing is where the resultant force falls. As long as the resultant force is in the middle third of the base of the wall the requirements are met.

 

[UcfSE]

At first GeoPaveTraffic's statement seemed very odd but honestly when I design retaining walls so that the reaction is in the middle third, the overturning factor of safety takes care of itself and it usually around 3 or so.

 

[bridgebuster]

GeoPaveTraffic, UcfSE:

I was whether you neglect to apply the FS to the permanent condition not the temporary condition.

 

[GeoPaveTraffic]

bridgebuster,

I would not consider a difference in this requirement for temporary versus permanent.

 

[Davidmat]

I have a thought.

When performing stability analyses for proposed retaining walls, I usually neglect passive forces because of potential future disturbances such as utility excavations or scour.

If the passive force is there during construction, why not use it and keep the same factor of safety? Some kind of verbage could be included in the specifications and/or plans saying something like "do not disturb the soils on the front side of the retaining wall until the work has been completed....."