Drilled Pier Foundation on Expansive Soil

[S R P]

Hi,

I am designing a foundation on medium expansive soil (PI~30)for a church building. Got couple of questions.

1. Will a church be considered as a commercial building? In taht case, should the design live load be 100psf based on IBC code? I guess that this is a location where lot of people gather in the same time, I considered it as a office building - lobbies and first floor corridor. (

)

2. Per geotechnical engineer's report, soil Bearing Capacity is 6000psf with SF=2. Bell Diameter to be 72in or less with 1:3 bell to shaft diameter. if bell diameter exceeds 72in, bell angle to be 60deg with 1:2 bell to shaft diameter. Few columns are to carry around 170kips gravity loads.

Bearing area=(170000lb*2/6000)=57sq.ft
Footing dia = 8.5ft
Shaft dia = 4.25ft

This is too beg for such a small load in structural engineering point of view. Please guide me with an option to handle this case or will I have to stay with the same diameter? I can manage all loads with max 24in dia. shaft per structural calculations. Please advise.

 

[JAE]

First - did the geotech tell you that the 6,000 psf was an "allowable" pressure (with a safety factor of 2 already in it)?
If 6,000 psf is an allowable, then you don't need to apply the safety factor again.

Second, if your column load is too big, then reducing pier spacing might be an option. Your column numbers and locations may be fixed in the church design so it may not help much but could reduce pier loads to some extent.



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[S R P]

JAE,

Thank you very much for your quick reply.

Do you think my selection based on IBC is correct (LL=100psf)? or can I go for a lower live load?

Soil report says" Utilizing a min factor of safety of 2 for total load, allowable bearing capacity of the foundation soil at the recommended depth is 6000psf for total load.

I also thought of adding an extra line of columns, but it is not possible.

 

[jayrod12]

I would say 100psf is correct. It does read to me like you don't need to doubly factor your loads. It sounds to me as though the 6000 psf has the safety factor baked into the cake.

If you needed two piers to keep the sizing down, you could just design an appropriate pile cap.

 

[S R P]

Thanks a Million to you, Sir.

 

[JAE]

Yes - for public assembly like churches use 100 psf...non-reduceable live load.

Also - based on your posted statement from the soil report - this is an allowable soil load of 6,000 psf. You don't add another safety factor to it.



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[S R P]

Thanks a lot.

 

[oldestguy]

If you have ever been in a church at the time of a holiday celebration and watched as the crowd moves to get out or other move of a group? Very jammed up people.

 

[S R P]

Yes. That is why I wanted to go for a highwe load which is 100psf, ven though not listed in the code as a church. But was little doubtful, if I did was right, when looked at the pier dimensions required.

 

[JStephen]

I see "Assembly Areas", with options for "Fixed seats" at 60 psf and "Lobbies" and several other subcategories at 100 psf, and believe that would be the category for churches, rather than offices.

 

[S R P]

I also saw that. But, I wasn't sure if a church would fall in a different category, as it has the requirement that seats to be fastened to the floor.

 

[JAE]

Many times church seats aren't attached.

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[JStephen]

"Unattached seats" is another category, also at 100 psf.