Wind Loads on Building

[ImpostorSyndrome]

How do you calculate the wind loads (specifically, the uplift forces) on an enclosed building (36'L x 15'W x 12'H) that is resting 17' above water on 2 cantilevered beams? ASCE 7-10 shows how to do this with a building on solid ground, but how do you determine the effects of a 160-mph gust on a building with an exposed underside? Thanks for your help

 

[ztengguy]

I would think if you get a 160MPH gust, you need to look at the water pressure effects more than wind. Houses in NJ during Hurricane Sandy were being overtopped by waves along the shore.

Otherwise, ASCE has books on loads on buildings.

 

[ImpostorSyndrome]

Water pressure effects aren't part of my scope - just the wind loads. Can you direct me to where i should be looking in ASCE that shows how to calculate the loads on this type of building configuration?

 

[SlideRuleEra]

Look in Chapter 8 of FEMA P-55 Coastal Construction Manual, 4th Edition. It is based on ASCE 7. FEMA P-55 is available as a free download here:

http://www.fema.gov/media-library/assets/documents/3293?fromSearch=fromsearch&id=1671

Since your scope of work is only wind loads, this chapter should help.

Note that ztengguy is right that designing a building for what resembles a coastal hurricane event involves a lot more than just wind. For a complete design, the all off FEMA P-55 needs to be considered in its entirety. However, I understand that is not what you need at this time.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[racookpe1978]

Unless somehow (1) the building is so far "up" off of the ground that there is no resistance to wind going "under" the building (to being trapped or slowed by the ground and by the underside of the building) , and (2) there is no resistance to the wind from the support structure and stairs and service buildings between the building and the ground under the building, then you have to prudently and conservatively ignore what little wind goes "under" the building and consider it to be solid-sided from ground to top of building.

Further, the wind speed at ground level is very low (going asymtopically to zero at zero elevation) so the effect of "raising the top" of the building up into ever-faster and faster wind speeds far overwhelms to little bit of reduction you get by trying to assume the building is resting in mid-air.

Now, water resistance is another story at all regards: there, minimizing the "between ground and building floor" water resistance IS critical because water is 1000x the weight of wind, though far slower, and water is 100% concentrated down low under the building against the foundations and posts and stairs.

 

[ImpostorSyndrome]

thanks for the help SlideRuleEra and racookpe1978.

i read somewhere that to find the uplift forces due to wind underneath the house, just treat the floor like a roof overhand and C_p = 0.8. However, if i do it this way, the uplift force is greater than the force of the wind hitting the windward wall, which doesn't make any sense to me. is there a way to figure out the predicted velocity pressure on the underside of the house from the velocity pressure on the windward wall?

 

[ztengguy]

Are you doing MWFRS, or just C&C?

 

[ImpostorSyndrome]

ztengguy,

MWFRS

 

[racookpe1978]

If I am reading you properly, do you expect a measurable "lift" from Buernulli's principle based on the increased flow of air speeding up as it passes over the building? (Compared, that is, to the building up of pressure uder the building as it "squishes" under the airway between the building and ground?)

For a cube, you don't get any airfoil effect.

 

[SAIL3]

I have not looked at the FEMA documents referenced above so the following is kinda off the top of my head...the lateral wind load on an elevated structure may come out less than a similar bldg on grade because of the wind pres escape at the bottom of the elevated structure thus influencing the press on the leeward wall...on a bldg at grade the wind press on the leeward side is calculated using a velocity that is calculated at the total height of the bldg and the resulting press is applied to the total height of the leeward wall...the height of this bldg is only 12ft so the difference is not that great...wind escaping over the top of the bldg creates uplift but this is conteracted by wind escaping underneath the bldg creating a downward load...the net uplift will be based on the difference in the wind velocity calculated for each...what would concern me more is the following....
1. wind velocity vector underneath the bldg may have a vertical component for a considerable distance as it enters the space below the bldg.
2. the density of the air this close to the water may be heavily influenced by the entrainnment of water thru spray, etc. and affect(increase) the calculated wind loads.
3.wave action...depending on the site exposure, reach,etc ...17ft clearance may not be enough..

 

[SAIL3]

if one side of the open space is blocked off by a bulkhead or embankment, then the uplift from wind for velocity vector perpendicular to that obstruction would be more like values obtained for an open bldg and could increase uplift significantly...anyway, there are alot of different design cases here that should be looked at...

 

[TDIengineer]

I would possibly treat this as a flat roof and use zone 3 pressure at the complete underside. When determining Lambda, be conservative and use the total height of the structure, not the height to the bottom.

 

[DST148]

@Impostor...> Your situation is something similar to elevated water storage tanks supported on a framework of beams and columns.
American Water Works Association has a publication D100 devoted to wind loads. Although not directly related to building structures,
it may give some insight in the calculation of wind loads.