Surcharge loads and load combinations

[ruready]

What category do surchages fall under in the IBC load combinations? If you have a strip surcharge from a building, do you split the dead and live loads up or would the resulting load applied to the retaining wall be H? IBC 1610.1 "Design lateral pressure from surcharge loads shall be added to the lateral earth pressure load" is interepretd in our office two ways, any surcharge load is classified as H or make sure you include it in your design in combination with H. Thanks

 

[garrettk]

What is the allowable bearing pressure of the footing nearest the wall? Without digging through the code, that seems to be the logical surcharge load that should be appled to your retaiing wall.

You'll need your geotech to come up with the resulting horizontal and vertical pressures.

 

[InDepth]

I agree with garettk, use the bearing pressure as soil surcharge load (good practice and allows for future demo of adjacent building and redevelopment).

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Here is my take on your question:

1. If you are transmitting any load (DL, LL, EQ) through a soil medium to another structure it should be considered a soil load.
2. If you are transmitting a load by underpinning, then the appropriate load is used and transferred to the wall.

If you are using ASD load combos, it really doesn't matter. If you are trying to use LRFD or have a seismic load, then you need to decide how to tackle the situation. You have to think about probability....the 1.6 on H in LRFD is provided because of the uncertainty involved in soils. You can't use 1.2 load factor of a DL surcharge in soil because there isn't that level of certainty.

One item you need to verify is if the building is imposing a seismic surcharge on the permanent structure.

In the load combos, I define "H" as 3 Cases
Case 1: H = Lateral Soil Load + Building Surcharge + Soil Seismic Increment.
Case 2: H = Lateral Soil Load + Construction Surcharge
Case 3: H = Lateral Soil Load + Traffic Surcharge

Notes: Case 1 - beware of seismic directionality arguments and resistances (and seismic return periods....for temporary shoring, use much less)
Case 2 & 3 - Add Building Surcharge if in the zone of influence. All Cases: Ground Water pressure is included in H.

The geotech will probably give you a Bousinnesq surcharge pressure, but beware that this is usually a "rigid" wall distribution that should be reduced per NAVFAC DM recommendations.

You may Also want to consult ASCE 37-02 and AASHTO for further guidance.