Isolated footing design considering seismic load

[SilverBeam75]

Hi,

When you design an isolated footing, the area required (based on the bearing capacity of soil) is using the unfactored gravitary load. But my analysis seismic analysis indicates that I have a pressure load up to 6-10 times my gravitary load. How exactly do you design the footing. I guess that when you deal with seismic loading the settlement is not governing the design. Any suggestions ? Not sure where it is explained in the code.

Thank you,

 

[Prestressed Guy]

A secondary question should also be, if you have that much seismic load, what is the net uplift and how are you going to hold it down? When I have an isolated column with significant reversible vertical loads from wind/seismic, I usually put it on a grade beam footing that extends to the adjacent column on each side so that their dead load helps hold the whole mess on the ground. This usually will fix the problem of soil pressure. BTW, you can increase soil bearing capacity by 1.33 for wind/seismic.

 

[strucguy]

Haydenwse -

I am not sure if the 1/3rd increase in bearing capaciy is allowed anymore by the code if the soil pressures are evaluated based on the load combinations given in Section 2.4.1 of ASCE 7-05. Again, I am talking purely in terms of what's allowed by the code. Please correct me if I am wrong.
Thanks.

 

[JAE]

strucguy,

For seismic, if you are designing footings (in ASD usually) you need to use ASD load combinations which use 0.7E for the loading.

Whatever force the combination with 0.7E produces in the footing, you need to design for that as you would any other load.

 

[Prestressed Guy]

Soil bearing pressure is based on service loads rather than factored loads and the 1/3 increase is an allowable increase in bearing pressure due to transient loads. Also remember that the seismic load E is a factored load and so it should be divided by 1.4 to get service level loads.

 

[strucguy]

JAE -

That is what I exactly meant...when I said "Section 2.4.1 of ASCE 7-05". From what I understand, the load combinations already account for combined action of gravity and lateral loads, and that there is no need to use 1/3 rd stress increase in the allowable soil pressure for the design of footing.

Haydenwse -

I agree with you on E being factored load, and thus shall be reduced to service level to be used in allowable stress design. But, that reduction is already included in the load combinations I mentioned above by use of 0.7 factor. So, we are on the same page in that regard. And, coming to one-third stress increase, the increase cannot be applied when the design uses the above mentioned equations. Here's the exerpt from ASCE 7-05.

"Increase in allowable stress shall not be used with the loads or load combinations given in this standard unless it can be demonstrated that such an increase is justified by structural behavior caused by rate or duratino of load."

I did use one-third stress increase in the allowable soil pressure values, but, that was when I was using alternate load combination given in IBC-2006. This increase is permitted by the code, provided one uses those specific load combinations.

Again there may be different schools of thought here. And, may be I am being conservation. More comments welcome.

 

[strucguy]

Typo in the above post
"And, may be I am being conservative"

 

[WillisV]

See ASCE 7-05 Section 12.13.4. If ELF method is used you can reduce pressure due to seismic event by 25% so your actual service level seismic force is due to 0.7*0.75*E = 0.525E. Additionally the effect of the vertical component on bearing pressures may be ignored per Exception 2 of 12.4.2.2.

 

[Gumpmaster]

This is a question I've wondered about for a while. The bearing capacity given by the geotech is often based allowable long term settlement for the framing system used. It's not typically (but may sometimes be) based on a bearing failure of the soil.

The near instantaneous loads from seismic will not increase the settlement of the footing. Based on the individual soil type/water table, you may have higher or lower bearing capacities. It differs on a case by case basis. You can't just arbitrarily increase the allowable by 33%. It may be lower because of liquifaction or other issues.

Your geotech should be able to give you an allowable soil pressure under seismic conditions (it may require him to do more analysis). 99% chance though, the allowable pressure under seismic will not be 6-10 times your allowable static capacity.

 

[Prestressed Guy]

Does anyone have any additional comments on the seismic uplift? Seismic is always a reversible load. If you are getting 6-10 the bearing pressure you will almost certainly get a significant net uplift as well. What to you do to hold the footing down?

In a previous position I was an engineer of a precast manufacturer. On one project, I was required by the Engineer of Record who was a Principal at one of the largest firms in this area to connect a couple of footings to my precast columns with 16 # 14 dowels to resist an Em load of 3.5 million pounds. The footing was 10’ square by 24” deep.

I asked him how a 30,000 lb footing was supposed to hold down 3.5 million pounds of uplift, I was tersely informed that Em was only required to go to the footing not the soil. WHAT???

Even dividing out the omega-0 you still have 1.4 million pounds of net uplift according to his calculations. So I guess that under a design seismic event, it is okay for these footings to “do the stomp”! Rock-On.

 

[strucguy]

Willis -

Thanks for the clarification. But, the verical component is still considered in equation 12.4-1, and used in load combinations 5 and 6 of Section 2.4.1 that are critical for evaluating soil stresses during a seismic event. And more over, the 25 percent reduction in seismic load is only applicable for over-turning checks. Where in the code does it mention that the same reduction can be applied for evaluating soil stresses as well? Not yet convinced.

 

[Prestressed Guy]

It isn't mentioned in the code, it is commonly allowed by the Geotech engineer. The following note is from the geotech report for a current project. "The net allowable bearing pressure may be increased by one-third for transient wind or seismic loads.”

This is not the old 1/3 increase for ASD steel design. It is a soil dependent increase in the capacity of the soil to resist short term loading as assessed by the geotechnical engineer.

 

[strucguy]

Haydenwse -

That is exactly where I am getting to. From my understanding, the 1/3 rd stress increase for soil pressures shouldn't be taken for granted, and shall only be be used after due consultation with the geotechnical engineer.

 

[Prestressed Guy]

Strucguy, I agree. It is the geotech's call.

 

[Gumpmaster]

Haydenwse,

It sounds like he may have been incorrectly applying something like AISC 341-05 C13.3a which says, in reference to the maximum force that can be developed in a system, to design the members for "how much force can be resisted before causing uplift of a spread footing". The members don't necessarily need to be designed for the omega force if the system can't physically develop it.

The overall foundation stability still needs to be designed for the applied load.

If you think about it though, the second a footing uplifts, the period of your building lenthens dramatically, placing you further along the response spectrum and reducing your loads. That probably isn't a good thing to base your design on though.

 

[Prestressed Guy]

Gumpmaster,

Exactly, especially when you are using it to laterally support a 4 story concrete parking garage with a 7 story steel structure on top of that. These columns support the vertical loads from the interior concrete elevator core for the steel building. The core stopped at the upper level of concrete due to conflicts with the parking layout. The lateral loads were redistributed by the slab to the exterior walls.

Which irregularity do you prefer. On this building you can take your pick!

 

[WillisV]

@strucguy - agreed Ev =0 only allowed for uplift condition.

The 25% reduction applied to the overall overturning moment at the base of the structure due to lateral seismic forces (talking about the big picture here - not moment on an individual footing). As the structure overturning moment is generally resolved into a force couple in braced frames or individual moments in footings, I read this as allowing a reduction in either the force couple or the individual moments for design, both strength and stability, of the footings.

As basis see the 2009 NEHRP Provisions which basically provide a commentary to ASCE 7-2005 (downloadable - big file - here:

http://www.fema.gov/library/viewRecord.do?id=4103)

which says for this provision (emphasis added):

C12.13.4 Reduction of Foundation Overturning. Since the vertical distribution of forces prescribed for use with the
equivalent lateral force procedure is intended to envelope story shears, overturning moments are exaggerated. (See
Section C12.13.3.) Such moments will be lower where multiple modes respond, so a 25 percent reduction is permitted for
design (strength and stability) of the foundation using this procedure.

 

[SilverBeam75]

Hi,

Thank you ALL very much for all this information. Its very interesting.

I will contact my GEOTECH and see with him.

 

[strucguy]

@ Willis -

Thank you for your explanation. Also, thanks for sharing this valuable resource.