Ultimate vs. Allowable Passive Pressure - which to use with a safety factor

[skimboard20]

Hi all,

I recently received corrections comments from a building official regarding a small shoring wall design. The wall consists of driven wide flange piles and 4x12 timber lagging. It retains about 5 ft of soil.

We have a soils report for the project where the soils engineer gives us an ultimate passive pressure of 400 pcf to use for the design. I used this value to design the piles, along with a safety factor of 1.5, and a safety factor of 1.2 when I included a seismic surcharge.

The building official is saying that we need to use an allowable passive pressure in our design, and points to IBC 1802.1. This has a few people in my firm confused. Should we be using an allowable passive pressure + a safety factor of 1.5? Is the safety factor of 1.5 meant to be used with an ultimate passive pressure? The soils engineer, who is experienced in this type of project, says that using an allowable passive and a safety factor is overkill and is disagreeing with the building official.

Looking back at soils reports from other firms on different projects, we were definitely given an allowable passive pressure in the report, which was then used in our design, with the corresponding safety factor.

I'm wondering if anyone has insight into how to think about this. Thanks!

 

[driftLimiter]

1802.1 just says that allowable pressures shall be used with allowable stress design load combos.

Though I haven't yet had a geotech provide ultimate values, there is precedent for using them for foundation at least in ASCE7-22 12.13.5. Granted this section is in reference to seismic loading.

My general rule is if the building official is saying they want something I just do it. These are not enemies I want to make, and I don't want to burn a bunch of time going back forth with someone who ultimately gets the final say. I have found they very seldom backdown from a position once they have put it in writing anyway.

 

[Aesur]

Safety factors are required for retaining walls and use ASD combinations which uses allowable pressures, therefore the building official is correct, ultimate pressures need to be converted to allowable and then the safety factor applies on top of that. I don't know what value to use for ultimate to ASD for soils, I haven't had a geotech provide ultimate yet, but maybe one of the Canadians here can shed some light on that as I believe ultimate is more common in their area now days. When I have looked this up before, I have seen it tends to range from a 2 to 4 with an average of 3 for ultimate to allowable.

 

[Celt83]

Retaining wall stability is governed by 1807.2.3 (IBC 2018) which is a little more onerous than just the ASD load combinations and notes the use of nominal loads.

 

[MTSOE]

I would inquire of the geotechnical engineer whether or not the ultimate passive pressure they provided has already been factored (reduced).

I approach cantilever pile designs like this by comparing resisting moment to overturning moment. I set the embedment depth such that the ratio of the factored (reduced) moment to factored overturning moment is equal to 1. In the case of the Canadian bridge code, the factors for overturning soil and hydrostatic loads are 1.25 and 1.1, respectively. The factor for a resisting soil load is 0.5. I don't try to arrive at an additional factor of safety if I'm already using factored loads.

If I wanted to use unfactored loads, I would set my embedment depth until the ratio of my resisting moment to overtirning is equal to the desired factor of safety.

I hope this helps.

 

[OldDawgNewTricks]

Without seeing the exact wording of the Building Official's comment, it's hard to definitively say what they mean. As driftLimiter said, 1802.1 just says that if you are using allowable stresses, you must use allowable stress design load combinations. You are allowed to use ultimate strength/LRFD design, but you have to be consistent. Your design seems to be mixing ultimate strength and allowable stress methodologies (factor of safety is typically allowable stress design terminology). So perhaps the Building Official's comment just means that you need to use a consistent design methodology. I recommend asking the geotechnical engineer to provide allowable soil criteria that already include appropriate factors of safety so that there is no confusion and then using allowable stress design as requested by the Building Official.