Uplift on Foundation 1

[Lina]

Thanks for Oshake, breaks, Dik, Polecat, and Ron for your support on my previous questions. I just completed a 2 days seminar about Wind Loads for Buildings and other Structures. Now I am very careful designing for wind loads and uplift. The seminar was great and every structural engineer should have it.
My question is that my college textbooks are not any help in this area. To make the long story short. I have an outside wooden stair going from the first floor to the second floor of a two story house. The platform of the stair is supported by two columns on one side and the wall on the other. I have an uplift load on the platform that I have to consider when I am sizing the square foundation of the column. My fondation text book did not cover this area. What text book or published documents do you recommand. If you subtract the uplift load from the load on the column you may have a negative load and my concrete textbook does not cover this area. I know you have recommended several sources please let me know of a good source to start with. Thanks

 

[phuduhudu]

Lina,

The only way to resist an uplift load is to hold it down with something. With a concrete base you can use the weight of the base plus the weight of any soil cover over it. You should be careful using the soil cover though as someone might do some landscaping one day and remove it not knowing it is holding a column down.

The other thing to remember is that all the joints (e.g.holding down bolts) should be able to take a tension equal to the uplift force.

My question however is, how have you managed to calculate an uplift force on an external stair to a building? I wouldn't have thought you could get any significant uplift on an external stair so as long as the concrete base to the column was adequately sized for the gravity loads it should hold the stair down. Carl Bauer

http://www.bauerconsultbotswana.com

 

[Lina]

carlbauer.
Thanks for your help in this matter. Your answer was very helpful. Please be advised that I live in South Florida and that we have to meet the requirements of ASCE Standard.
To the best of my knowledge this structure whether it is a closed structure (by framing all around with concrete or with an impact resistant type of framing) or Partially open structure by keeping it without framing all around. The structure will be subject to an uplift presure in the first case and to an overhang pressure in the second case. Therefore I think that the structure should be designed for an uplift and I agree with you that the weight of the foundation must be greater than the uplift pressure. What about ASCE SECTION 5.2 (Uplift on Floor and Foundation) the uplift pressure due to the water table. In Florida we have a very shallow water table (about 3' from finish ground) and during a hurricane the water table rises creating an uplift on the foundation in addition to the uplift wind force. Do you have to consider the water pressure due to water table by adding it to the uplift pressure due to wind and size the foundation to exceed the total uplift load?
Do you recommend any good text book that covers this concept or that calculates the pressure due to water table. I am not very happy and confutable with what I have. Thanks for your help...

 

[Ron]

Lina,
You are not likely going to find a textbook that sufficiently covers your topic. In reviewing a lot of similar work, I see differences of opinion among experienced engineers on this topic.

I will try to answer your questions from an allowable stress perspective as you mentioned you are in South Florida and I am familiar with the South Florida Building Code and ASCE 7, and ASD is easily applied to these conditions.

The difficulty with an outside stair is determining the wind load in several different directions as well as using multiple factors applied to the load. The easy part is that your net uplift load is likely very small or negative if you are using a steel stairway as the mass of the steel is great compared to the "sail" area.

The projected area of the stairs works a lot like a sloped roof. Assuming you are complying with accessibility standards and have no open risers in the stairway, you will have a larger wind-induced lateral load on the stair than the uplift load. Assuming the stair treads are not solid and allow drainage, the uplift area is quite small compared to the mass of the steel (are you using grating for the treads?). The landing, if solid, is the largest area with any uplift potential. Again, the dead load is subtracted from the uplift load to produce the net effect.

Now let's assume you have calculated a net uplift on the system of 500 lbf. Your foundation must resist at least 1.5 times this amount (750 lbm.) as the stability requirement for South Florida (and most other areas in the US) is that the total uplift cannot exceed 67% of the resisting dead load. It is often written in a different form to read something to the effect...."the overturning moment shall not exceed 2/3 of the dead load resisting moment", but unless you have a significantly odd geometry, my simplified version works fine. Since you are attaching half of your landing to the building and half is supported by the columns, your foundation uplift requirement is less since some resistance to uplift is also provided by the shear connection at the building.

Also, keep in mind that under ASD procedures, the wind load is transient so you are allowed to consider an overstress factor of 1.33. This means that if your allowable stress on a member is 10 ksi, then under a wind load, the allowable stress goes to 13.33 ksi.

Now the procedure for applying factors for the wind load...
Use the "Other structures" procedure in ASCE 7. The factors you choose should be close to the geometry of your structure, but they won't necessarily be exact. For example, you might choose the factors for a solid sign for the lateral load and the factors for a monoslope roof for the uplift load. If you can reasonably justify your choices, it is called "Engineering Judgment" and, assuming you are appropriately qualified and registered, that is certainly allowed. That doesn't mean you won't get criticized for your choices, just that you have equal footing (no pun intended!) for your basis.

Good luck.

Ron

 

[Lina]

Ron.
Thanks for your help in this matter.
The platform is assumed to be an overhang. When the wind direction is on the face of the building where the stair is located the platform will see pressures on top and bottom surfaces. I am using the external roof pressure coeficient for the top pressure with a zero internal pressure (p=qXGXCp-0)Eq. 6-15 in ASCE. The bottom pressure is the overhang pressure (P=gXGXCp) where Cp is equal to 0.8 (Roof Overhang) section 6.5.11.4.
ASCE 98 does not allow 1/3 stress increase for wind and seismic loads anymore.
The stair must be a wood stair and the total uplift load on the column came out to be about 412 lb (you were very close with your assumption). Base on your advise I am designing the foundation to resist 1.5 the uplift load (618 lb), this will yield to a square foundation 30"x30"x12" (the weight of this foundation is 937lb) assuming this foundation meets all other requirements.
Ron thanks for you advise please reply if you do not agree.

 

[phuduhudu]

Lina,

I am intrigued. Do you really get uplift due to a high water table on a solid concrete foundation? I can understand that basements get uplift forces due to floatation but surely not a solid concrete foundation? Or are you concerned about swelling of the bearing strata?.

As Ron said the uplift is only half resisted by the column foundation but since it is likely to be such a small uplift anyway couldn't you transfer all the uplift force to the building? Somehow I can't see an external stair to a building flying up into the air. If I was concerned about that then I would be even more concerned about walking on the stairs! Carl Bauer

http://www.bauerconsultbotswana.com

 

[JAE]

Carl and Ron and Lina:

A solid concrete footing in soil that is 100% saturated will only have a bouyant density (net) of 150 pcf - 62.4 pcf = 87.6 pcf. This is the in-place density of the footing in the condition Lina indicated. When the uplift occurs, the footing will only resist the uplift based on a weight calculated with the 87.6 pcf. As the footing raises out of the water/soil condition, the portion of the concrete "out" of the water will revert to 150 pcf.

I would recommend designing for the 1.5 factor that Ron mentioned, and use the lighter density.

I agree, that it would be good if the building could resist the uplift (with diagonal struts?) if possible without distressing the supporting wall.

The 1/3 stress increase doesn't apply to overturning/uplift calcs. (I know you didn't mean that, Ron....just wanted Lina to be clear on it).

 

[Ron]

JAE...thanks for the clarification and I agree with your buoyancy comments. In South Florida there are generally no expansive soils that give a problem so the buoyancy issue is the only one to deal with in that respect.

Lina...the South Florida Building Code (1999 edition)still specifically refers to ASCE 7-93 as the prevailing load code, not ASCE 7-98. The new Statewide Building Code (the latest promise is Jan. 2002)does reference 7-98 or later. While ASD has been severely compromised by 7-98, it isn't completely dead. My comments were based on the assumption of 7-93 as referenced by the SFBC. If you are working with the 1999 edition, see page 4-17 for the reference (Broward Edition)

 

[JAE]

Speaking of a dead ASD...I noticed that AISC will issue its next specification as a combined spec (ASD and LRFD together).. Maybe its not quite as dead as thought.

 

[Ron]

Hey JAE...that's a good thing. Though LRFD is being taught in school these days, my opinion (humble of course)is that ASD provides a much more straightforward approach to keeping load application and stress analysis as separate entities, which I think is appropriate. Load application is not material dependent....stress analysis is.

Oh well...so much for my flogging of the limit state crowd!

 

[JAE]

Ron:

I was taught ASD in undergrad school, took a plastic design in graduate school (hated it) and proceeded to use ASD for 10 years. When LRFD came out, I bought the blue manual but never used it. It was only when I became a department manager, training young engineers, that I realized the benefit of understanding LRFD to help them grow as engineers.

I converted over and have never looked back. It does required just a bit more effort in setting up load combinations but I am always astounded at the vehement reaction some engineers are having over LRFD. Its not that bad and doesn't really affect the design time to a noticable extent.

But I agree that having the option for both is not a bad idea.

 

[tw]

This is slightly off-topic but was discussed slightly in this thread. It relates to ASCE 7-98 and AISC's 1/3 increase in steel allowables. ASCE 7-98 says:

ASCE 7-98:1. In section 2.4, Allowable Stress Design:

When structural effects due to two or more loads in combination with dead load, but excluding earth load, are investigated in load combinations of Sections 2.4.1 and 2.4.2, the combined effects due to the two or more loads multiplied by 0.75 plus effects due to dead loads shall not be less than the effects from the load combination of the dead load plus the load producing the largest effects.

Increases in allowable stress shall not be used with these loads or load combinations unless it can be demonstrated that such an increase is justified by structural behavior caused by rate or duration of load.


AISC ASD manual says

Section A5.2 – Wind and Seismic Stresses: “Allowable stresses may be increased 1/3 above the values otherwise provided when produced by wind or seismic loading, acting alone or in combination with the design dead and live loads…”

What do you guys do? I use AISC's 1/3 allowable increase but am careful not to design too near the edge, not worth it.

Tom

 

[JAE]

Tom:
My take on this was gained by sitting on my city's code review board which was tasked with reviewing the new IBC 2000. It references ASCE 7-98 and includes the language that you wrote above.

What it essentially does, is negate the 1/3 stress increase for the wind load combinations listed. Keep in mind that the wind loads you get from ASCE 7-98 are not the same wind loads that the older ASD was based on (the newer loads based on a 3 second gust).

The IBC 2000 includes, however, an alternate series of load combinations that allow the 1/3 stress increase. These combinations have a factor, called omega, that is applied to the W (wind) in the combo.

So, if you design under a code that references AISC ASD and uses the older wind maps, you can continue to use the 1/3 increase. If your wind loads come from the new map, and you are designing under IBC 2000/ASCE 7-98, the 1/3 increase no longer can be used.

 

[Ron]

One interpretation of the 7-98 ASD provision (which is consistent with its commentary) is that you cannot use the 1/3 increase if using the FACTORED loadings; however, it does not preclude the 1/3 increase otherwise.

To reiterate a previous point, the load application and stress analysis are separate entities with load application being universal while stress analysis is material dependent. The concept of ASD increases for transient loads should be even more prominent for the 3 second gust than for the fastest mile wind speeds.