Pile design load factors
Pile design load factors
(OP)
I have a load factor question. I am doing pile design and geotech have given me an allowable pile service load based on an FOS of 3. However, they say this can be reduced to 2.5 for the transient portion of the load e.g. wind and earthquake. For wind there is no problem but ASCE also allows overturning moments for soil structure interface loads to be reduced by 0.75. Does that mean I can effectively reduce my earthquake loads by 0.75*2.5/3 = 0.6225? Or am I combining 2 factors doing the same thing?






RE: Pile design load factors
How are you handling the lateral loads in the piles? For design categories B and C I usually went with battered piles. Otherwise, the shadowing effect of in-line piles becomes a problem.
RE: Pile design load factors
RE: Pile design load factors
@ slickdeals
did he say what slope had to do with pile punching shear? i always figure punching shear is what it is, it's either there or not.
i've always designed in very high wind zones, so it might be different in seismic zones.
RE: Pile design load factors
Like I said, it makes some sense but I have not yet looked for published reports that suggest this phenomenon.
RE: Pile design load factors
I am in category B and can handle the lateral loads as I have a raft pilecap over all the piles that shares the load out pretty effectively and is within the limits given by the geotech for pile service lateral loads.
Swivel63
Why use just one factor for compression and both for tension?
RE: Pile design load factors
@ slick deals
wouldn't the critical section also be inclined giving more of a shear area?
RE: Pile design load factors
Not really. If I reduce earthquake loads with both factors then I will get less tension uplift by those factors too.
RE: Pile design load factors
The problem occurred because we design structures using an equivalent static load based on a reduced ultimate dynamic seismic load. For the higher seismic zones, the reduced load is allowed because plastic hinges absorb the overstress. When we use a battered pile, the reaction is very stiff compared to laterally loaded vertical piles (axial vs bending respectively). Consequently, the reaction on the pile cap is closer to the ultimate dynamic load for a battered pile reaction and no plastic hinging can be allowed to occur in the pile cap.
RE: Pile design load factors
FHWA has a pile design guide which is loaded with information. It also notes that piles that are statically load tested should have a FOS of 2. Further installed piles should match up with the test pile as to blow counts and penetration.
If you have an uplift pile(doesn't sound like you will), ask the Geotech if you need an uplift pile test or, for friction piles, the compression test is enough for both types of loading.