ASDIP FOUNDATION is a structural engineering software for the design of concrete footings. It includes the design of uplift footings, based on the ACI 318 provisions. This document covers the design of footings subject to uplift loading using ASDIP FOUNDATION.
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Uplift loads may be applied to a footing for a number of reasons, but the most common uplift loads are due to wind. For example a light warehouse building in a hurricane zone can be exposed to this kind of uplift loading. In such cases, the gravity forces due to footing self-weight, pedestal and soil cover should counteract the applied uplift load, as shown below. These gravity forces are known as the overburden loads.
How do you check the stability of uplift footings?
Before 1998, the design for uplift footings included the stability check of the applied service loads with a minimum safety factor of 1.5, as shown above. Thousands of structures were designed following the 1.5 safety factor criteria, but some questions needed clarification, such as whether the 0.9D portion included the footing weight or not, or whether the 1.5 safety factor should be applied also to the factored loads.
The ASCE 7-98 included for the first time the required load combinations to perform the uplift check directly, without the need of additional stability calculations and comparison to a minimum safety factor. In the old equation, If we divide 0.9 / 1.5 = 0.6, we will obtain the load factor of the combination 0.6D-W. Basically the safety factor is already implicit in this load combination. Furthermore, the corresponding combination for factored loads was also published.
In ASCE 7-10 the wind loads were calculated as ultimate loads, therefore the load factors of the uplift load combinations were adjusted accordingly, as shown above, but the design criteria remains the same.
Design a simple spread footing for an uplift wind load of 20 kips (ASCE 7-10). For simplicity, no other dead or live loads are specified. Consider 1'-0" of soil cover and a pedestal 18" square x 2'-0" high.
Click to enlarge
Enter the uplift load as a negative axial value. Try different footing sizes and verify the uplift analysis until the design is satisfactory. In this case a footing 9'-0"x9'-0"x1'-4" with soil cover of 1'-0" is adequate to counteract the applied uplift load. Note that this footing will have a very limited capacity to resist overturning, since the net gravity load is also very small. If there were other lateral loads this footing would need to be larger.
The uplift safety factor calculated below is 1.19 > 1.0. It should be noted that the old minimum safety factor of 1.5 is not used anymore for the uplift load combinations.
ASDIP FOUNDATION includes the design of spread footings for uplift loading, with multiple options to optimize the design easily. The design of such footings may be difficult if the concepts behind the code provisions are not clear. This example shows that the design of uplift footings can be completed and optimized within minutes.
For engineering background, please read the blog post Spread Footings Under Biaxial Bending: A Complex Design Subject. For our collection of blog posts about foundation design please visit Structural Footing Design.
Detailed information is available about this structural engineering software by visiting ASDIP FOUNDATION. You are invited to download the Free 15-day Software Trial, or go ahead and Place your Order.
Javier Encinas, PE
ASDIP Structural Software
Indeed, this is a interesting topic since stability check in ASCE is not clear.
I have some questions:
1- Since the uplift safery factor of 1.5 is included in the combination 0.6D+W, why are you using a safety factor of 1.5 for the overturning verification? What is your Code reference?
2- Are you performing the Stability check for seismic combinations? And if yes, what combinations are you using?
3- Why the contact surface checking is not performed? I mean is it allowed to have partial uplift of the foundations under service combinations? And if yes, to which extent?
4- Finally please note that cracks width limitations are of importance in the design of footings. In some cases the flexural design is controlled by service moments in case cracks width are limited to a certain value.
1.- The article focuses on uplift, not overturning, but the design philosophy is the same. The load combinations have been calibrated to check the stability. ASDIP Foundation lets you specify the safety factor of both conditions, so you can easily set 1.0 if you are using the ASCE 7 combinations, or 1.5 for your own pre-combined load. For industrial design it’s common to see safety factors of 2.0 or more.
2.- For seismic, the load combination to check stability is 0.6D+0.7E. Note that the seismic vertical component can be ignored for footing design.
3.- I assume that your question is related to overturning, not to uplift. The Code is silent to the allowable extent of surface contact. If your bearing pressure is under the limits and your stability check is ok, then your engineering judgement will determine the contact limit.
4.- I agree. At least As min should be provided for crack control.