Broms - Overload Factor/Undercapacity Factor vs. Load Factors of Ultimate or Service Wind loads

[engjg]

In regards to Broms analysis for laterally loaded piles it is my understanding he recommended a 2 to 3 overload factor and a 0.7 undercapacity factor which is alternately considered a 3/0.7=4.3 or 2/0.7=2.9 safety factor. It is my understanding Broms is a considered an ultimate strength method. In dealing with the design of a drilled shaft foundation where one has lateral load design reactions (shear and moment) resulting from a service(asd) wind load combination, I am questioning whether one should increase the service (asd) wind load reactions to a strength level or not. Obviously the Broms method is older but I assume the "overload factors" were load factors and the undercapacity factor was like a strength reduction factor as a corollary to LRFD.

In example in dealing with load combinations pre asce 7-10 where service wind speeds were used load combinations were generally:

ASD: W
LRFD: 1.6w

The question in dealing with service asd wind load reactions should you apply the 1.6 in addition to the saftey factor? i.e. 1.6x4.3 or 1.6x2.9?

Or ASCE 7-10 and older:

ASD: 0.6W
LRFD: W

4.3/0.6
2.9/0.6

This seems over-conservative to me and the overload factors should be considered the load factor and you should stick with an overall factor of safety using service asd reactions of just 4.3 & 2.9.

Interested in others thoughts

 

[BridgeSmith]

In addition to being an ultimate strength method, Broms is also a very approximate method. In my cicles it is rarely used, except as a quick and dirty hand method until I can run a P-y analysis.