Design of Bell & Shaft Drilled Pier Foundation for Pre-Engineered METAL BUILDING

[oengineer]

I am working on designing a metal building foundation using bell & shaft drilled piers. The foundation is a slab-on-grade with perimeter grade beams. The net allowable bearing pressure is 3000 psf for dead load plus sustained live load which can be increased 25 percent for total load conditions. The issue that I am having are the reaction forces provided by the metal building fabricator for the columns. For one of the columns I am getting DL(vert.)= 10k, LL(vert.)= 15k, Floor Load(vert.)= 17.2 k, Wind Load (vert.) = -16 k, and Wind Load (horiz.) = 11.0k. Is it normal to have such high horiz. loads for a column? I am new to designing foundations for Pre-Engineered Metal Buildings. For most buildings, I designed the piers to resist the downward gravity loads. Using the reactions provided by the metal Building manufacturer, I am getting that I need a ridiculously large shaft/bell size, like a 34/102. I am interested in any insight in what I may be doing wrong, what I am not considering, or if I am allowed to just mostly rely on the gravity loads for my design. Any suggestions/comments are appreciated.

 

[oengineer]

Also the water table is at 8 ft below grade and my drilled piers need to be 13 ft below grade to reach the net allowable bearing pressure of 3000 psf.

 

[peal724]

1. yes, it is very typical for PEMB columns to have large horizontal reactions
2. 3000 psf is very low for end bearing on a drilled shaft. Are you sure that's correct.

 

[oengineer]

@peal724 - Yes, 3000 psf is correct. It is given in the soil report. The safety factor for the soil is 2.5. The project is located in the Houston,TX area. We have clay soil.

 

[BAretired]

I agree that 3000 psf is very low for belled piles. Also, with a water table at 8 ft below grade and a pile depth of 13 ft, you will be forming the bells under water which makes the work much more difficult. Is there a possibility of using a different type of foundation?

BA

 

[oengineer]

@BAretired - The soil report calls out for drilled and underreamed footings bearing at a depth of 13 ft. The report does mention that caving of piers may occur during construction of the drilled piers due to the presence of Silty Clay Materials. In order to minimize the possibility of piers caving during construction, the geotech engineer says the contractor should be prepared to use cased piers or straight sided shaft foundations if caving occurs. With the loads provided from the PEMB vendor, I would hate to see what size straight shafts are required. How are the horiz. forces dealt with in the design of PEMB foundations? I can not imaging having a 34/102 pier on site.

 

[mike20793]

As low as the allowable bearing sounds for bell piers, the OP is correct. I just did one in Houston with 3600 psf allowable and my report reads very similar to his by the sound of it.

 

[azcats]

You may not want to use belled shafts anyway depending on how you want to resolve the lateral forces as the extra diameter may help you. How does the soils report read for loading the piers laterally?

 

[KootK]

What mechanism of pile resistance are you attempting to use for lateral? Soil springs at the top of the pile? Often, the thrust is taken by the slab on grade rather than the primary foundation.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[oengineer]

@azcats - No information is given regarding the lateral loading of piers.

@KootK - My goal is to have the piers resist the horiz. forces. Next I was going to try and convert the horiz. forces into couple moments. Or just determine the moment caused by the horizontal force to my pier and determine the max moment the pier can handle.

@everyone/anyone - Would the slab-on-grade system (i.e. grade beams, 6" th'k slab) already take care of the horiz. forces? Also, I have read online something about using hairpins to handled the horizontal loads.

 

[KootK]

Alexander Newman has a book out that deals with much of this stuff. It'll be worth the price if you're new to this type of project.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mike20793]

My report gave 1000 psf lateral resistance neglecting the upper 4 feet. It only gave a bell pier recommendation and not a straight shaft pier. For what it's worth, my structure was pretty lightly loaded with a relatively large footprint and we ended up using a combo stiffened slab on grade and mat.

 

[BAretired]

@everyone/anyone - Would the slab-on-grade system (i.e. grade beams, 6" th'k slab) already take care of the horiz. forces? Also, I have read online something about using hairpins to handled the horizontal loads.

If the horizontal forces are balanced across the building, hairpin bars can and have been used to take the tension. Additional slab reinforcement may be required at frame locations. Alternatively, a separate tie beam system can be used under the slab. This is more expensive but is deemed to be a better solution by many as it does not put the slab in tension and it permits the slab to be free floating.

BA

 

[oengineer]

@mike20793 - I think your soil is a little better than mine. My project is in Pasadena,TX. There soil is a little worse than Houston. I am not sure if I can use the 1000 psf for lateral load.

@BAretired - Would hairpins be expensive to use?

 

[BAretired]

Hairpins are often used and are probably the least expensive method of tying the bottom of the frames together.

BA

 

[Sawbux]

Strange that you did not list any horizontal reactions for D or L loads. Does the building have rigid frames? If so, there are constant lateral loads which are better handled by ties across the building.

If there are no rigid frames, I suspect that the lateral loads for wind are from rod bracing system. These I would feel comfortable taking into the piers and resisting by passive soil pressures.

If you are in an area of expansive soils, be careful how the slab/ footing interface is handled.

 

[oengineer]

@Sawbux-this particular column didn't have high horizontal LL but some of the other columns do.If you're depending on the passive soil load to resist the horizontal force then you're saying you would not include it as effecting the pier design? Since it is clay soil, would you still be confident in the passive soil pressure? Even for the 11 kips horizontal force?

@everyone - Is it typically common to have hairpins in a metal building foundation?

 

[BAretired]

Very common.

BA

 

[jimstructures]

Yes it is very common.

Jimstructures

 

[oengineer]

Does anyone happen to have a hairpin detail to resist the lateral thrust from a metal building frame that gets installed correctly?

I have the PIP STE05121 Anchor Bolt Design Guide. I have never designed/used hairpins before but I am hoping it is the same as what is mentioned in PIP. Would any body familiar with hairpins be able to let me know what direction I should go in designing and installing hairpins for a PEMB?