33% Increase on Allowable Bearing Capacity for Basic and Alternative ASD Load Combination 1

[33mmrrdd]

Does the increase of 33% on allowable bearing capacity is applicable for both Basic and Alternative ASD Load Combination?

I remember this increase of allowable stress is only applicable to alternative load combination. For soil capacity, it is allowed for both?

 

[EireChch]

This may be a silly question due to my lack of knowledge on US codes but...why would you increase the allowable capacity?

 

[Doctormo]

The rule of thumb increase in allowable bearing capacity is usually for the seismic condition (Extreme Event) and reflects a reduction in the factor of safety (or increase in resistance factor) for the short term seismic loading condition if I understand the question.

 

[JAE]

33mmrrdd - you need to specify which code or standard you are talking about.

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[Okiryu]

33mmrrdd, I was looking at my old files and just found the attached from the Geotechnical Earthquake Engineering Handbook from Robert Day. I thought it can be a good reference for you.

 

[MotorCity]

The allowable bearing capacity in your geotechnical report should not be adjusted. It should be a bottom line number that includes all adjustments, reductions, increases, etc.

 

[EireChch]

MotorCity - that was my thought too. the clue is in the title...allowable as in its what your allowed to use.

 

[wannabeSE]

When using the presumptive load bearing capacity in the 2015 IBC, section 1806.1 allows a 1/3 increase for wind and seismic when using the "alternative basic load combinations" in 1605.3.2.

The OP has a good question. More often than not, the soils reports I review include a 1/3 increase for short term loads (wind and seismic). I have used this increase with typical ASD load combinations. Next time I want to use the 1/3 increase, I will check with the geotech. I assumed the increase is reasonable because bearing pressures are often governed by settlement which may not be a problem with short term loading.

 

[33mmrrdd]

JAE, I'm using IBC.

Okiryu, thanks for the reference.

wannabeSE, that's the point of my question. In the soil report usually it says you may increase the bearing pressure. But in the IBC code, only the alternative basic load combination is stated to increase. Are the geotechnical engineers aware of this? Or they are allowing you to increase whatever combination as long it has short term loads.

 

[Okiryu]

33mmrrdd, I don't have the current IBC 2015, but the 2012 version indicates in Section 1806.1 that this increase is only for the presumptive values shown in Table 1806.2 when the alternative load combination is used. See attached. Although, the IBC limits this increase to the presumptive values, I think that most of geotechs just have adopted that statement in their recommendations.