Wind Load - Internal Pressure Applicable

[Maturin]

I'm performing a design check on temporary staging used to repair concrete on an existing structure. The staging consists of a 8' wide floor and a 16' high outside wall and encircles the existing 150' diameter structure. The staging is not covered (i.e. no roof). The floor and the wall are completely cladded with no openings. The top of the staging is in line with the roof of the existing structure and is approximately 130' above grade.

Should internal wind pressures be considered to act on the staging surfaces with this configuration?

 

[Ron]

I would consider it an open structure.

 

[a2mfk]

I would model it as a wall component and cladding with no internal pressure. It is not like a sealed building with an AC system that may have an internal negative or positive pressure, the inside of the staging sounds like it would balance out pressure-wise with the surrounding environment. With that tall of a structure you will of course get increased pressure with increased heights...

HTH,
Andrew Kester, PE

 

[PMR06]

Can you post a sketch? Am I reading this correctly, 150' diameter, 130' tall? What is it?!

 

[msquared48]

If I read this correctly, the floor is 114 feet above grade? What is supportoing the floor and wall? 114' high columns or scaffolding?

If I understand this correctly, I can see that the loads that are seen by your structure will HEAVILY depend on the shape (slope) of the existing concrete structure roof.

Sketch would be good... Real good.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

Post a sketch of your structure. We are not mind readers.

BA

 

[hokie66]

The walls would be like building parapets. The floor would be subject to the same uplift (windward side) and suction (leeward, maybe sides) as the existing structure at that height.

 

[Maturin]

Attached is a sketch showing the arrangement of the staging and existing structure. I work at a nuclear plant and the staging is used for repairs to our reactor building.

Since the structure is round I thought that external pressures for stacks, chimneys and tanks may apply.

 

[msquared48]

Although it has no roof structure, I think the staging still sees uplift as the floor structure will see it.

I agree with the tank scenario for the wind application, just verify the slope of the roof and use the worst case.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ron]

Similar to hokie66 and msquared48, it would work like a parapet; however, you also have an overhang condition that you do not have with the tank. With an 8-foot width, you can get parapet action...if it were closer to the tank, the roof surface would probably mitigate most of the parapet action.

 

[Maturin]

Is a parapet applicable given that the staging does not extend above the roof line and the wall is 16' deep?

My reference is the NBCC (National Buiding Code of Canada) and it doesn't discuss parapets directly. What pressure coefficients (+) and (-) typically act on a parapet?

 

[BAretired]

NBC defines Category 3 Buildings as those with large openings through which gusts are transmitted to the interior. You have large openings, i.e. the roof, so you should probably use Cpi = -0.7 to +0.7.

Figure I-1 is a Flow Chart for calculating wind load and effects on buildings. Proceeding down the left hand side,you have Velocity Pressure q₅₀ for your area.

Structural components and cladding...yes
Static Procedure...yes
Ce (exposure factor)...open terrain?
H < 20m? No...Cg = 2.5
Cp = +1.0 to -1.7 (Fig. I-24)
Cgi = 2.0, Cpi = +/-0.7

It is not very precise, but it's the best you can do.


BA

 

[Ron]

The fact that you have no roof and you have a distance from the cladding to the tank of 8 feet, you have enough to develop a slipstream across the top of the cladding, turbulence on the backside, and a dip in the slipstream...all of which define the loading on a parapet.

 

[a2mfk]

Hmmm.... After reading everyone's suggestion, if it were my name on the calcs I would:
-treat the floor like a roof overhang
-wall as the worse case of a parapet or wall cladding

A few extra PSF to be safe is worth it, I cannot see how your structural design will have a large effect on the cost of this entire operation and the temporary structure. A little bit more framing and bolting to make sure this temporary structure at a NUKE plant stays in place and is safe is worth it.

 

[csd72]

Just because there is no roof does not mean that there is no internal pressure. In fact it is just the opposite, with no roof this beacomes a large dominant opening and the internal pressure would approach that which would be the average roof pressure.

There will also be additional uplift on the underside of the structure as it the wind that hits the front wall will then deflect in all directions.

In this situation I would apply the following factors:

For the front wall I would apply the positive external front face pressure plus an internal pressure equal to the external pressure in the middle of the roof (as it is averaged around the circumference).

For the floor I would apply the same uplift pressure plus a force on the underside same as that for an elevated structure.