Pile design load factors 3

[phuduhudu]

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?

 

[Teguci]

When I've encountered this problem before, I gave the Geotech a phone call and had the discussion followed by appropriate documentation of the conclusions.

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.

 

[slickdeals]

I recently discussed the issue of using battered piles with a Geotech and his suggestion was that in seismic zones, it is better not to use them because they can cause punching issues on the pile cap because of their slope. I have been meaning to look into the issue in detail, but have you found anything to the contrary?

 

[swivel63]

for compression, i'd use just one. for tension in the pile, i'd use both. for lateral loads, battered piles work. i've seen people use passive pressure when in a pinch.

@ 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.

 

[slickdeals]

His logic was that if a pile is normal to the surface the horizontal shear waves would have no component. But if a pile was battered, there would be a component of this horizontal force traveling diagonally to the pile cap causing punching shear issues.

Like I said, it makes some sense but I have not yet looked for published reports that suggest this phenomenon.

 

[phuduhudu]

Teguci
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?

 

[swivel63]

it'll increase the compression load if you use one, and increase the tension load if you use them both. in both cases, at best you'll be spot on, at worse you'll be conservative.

@ slick deals

wouldn't the critical section also be inclined giving more of a shear area?

 

[phuduhudu]

swivel63

Not really. If I reduce earthquake loads with both factors then I will get less tension uplift by those factors too.

 

[Teguci]

The case study for the battered pile punching through the pile cap occurred in a high seismic zone and has been addressed in the IBC code with the use of the Seismic Overdesign Factor (the one where you multiply your seismic load by 2.5).

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.

 

[Teguci]

Piles are required to be load tested. Load tested piles only need a FOS of 2 per IBC. Other similar piles can be based off of the load tested pile capacity.

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.