Application of Wind Forces at Top of Wall Form

[Trillers]

I am designing a wall form system for a low wall (less than 8'). I applied the wind force at the top of the form, but the reviewer indicated this force should be applied as if the wind force is triangularly distributed.

The references I have seen all indicate a minimum wind load "applied at the top" so I assume all wind loads should be applied at the top of the form.

What am I missing?

 

[Trillers]

I should clarify - in stating the wind force should be applied as if triangularly distributed this would place the resultant at 1/3 below the top of form, not the top of form where I believe it should be.

Thanks.

 

[WinelandV]

Can you draw out a free body diagram of your situation?

Why do you think the resultant of the wind force should be at the top of the form?

Is this inside or outside? What references are you using?

Please note that is a "v" (as in Violin) not a "y".

 

[BridgeSmith]

Wind load is typically applied as a uniform load, so the centroid would be at H/2. The triangular distribution would be conservative in most cases (unless there are upstream obstacles that change the flow pattern). Applying the total wind on the wall as a point load at the top would be conservative in every scenario I can conceive of. However, if it works and doesn't change how the forms have to be assembled and supported, then there shouldn't be a problem with doing it that way.

Simple and conservative assumptions are often used where they don't change the outcome. If your point load at the top assumption requires extra bracing, etc. then it would be prudent to consider the reviewer's assessment; otherwise, make a note that your loading assumption is conservative, and move on.

 

[phamENG]

Wind loading is pretty complex - there are simplified methods that apply it at the center of the wall, and more complex methods that look at more realistic distributions. None of them apply the loading to the top of the wall for out of plane loading. In plane (shear walls), yes. But that's another story.

The triangular distribution is a decent approximation depending on the height of the wall. Anything up to about 15feet will be a relatively uniform pressure...not exactly true but friction with the ground and other surrounding terrain features cause enough disturbances to make this a realistic enough assumption (essentially a boundary layer if you're familiar with fluid flow in pipes). Above that and we start acknowledging the fact that the higher you go the faster the wind blows and you get an exponential growth of wind pressure. For low and mid-rise buildings, assuming a triangular distribution in this area is reasonable.

 

[Trillers]

Both ASCE and ACI state that a minimum wind load should be applied at the top of a vertical wall or column form if I’m reading correctly.

It doesn’t seem practical to apply the wind loading at mid-depth if we’re required to add a minimum wind load ar the top of the form wall.

I guess then the question is which would govern. Possibly which of the two results in a higher bracing requirement?

Or would it be a combination of both?

 

[phamENG]

Can you actually cite the provision calling for this? Nobody else here seems to know what you're referring to.

 

[Trillers]

I was looking at ASCE/SEI 37-14, 6.2.3 Accelerated Wind region which states that at "top edges of enclosures, pressures shall be assumed to act upward as well as horizontally". This citation might not apply to the formwork we are designing.

The other reference is ACI 347-04, 2.2.3.2 - which stats that "Bracing for wall forms should be designed for a horizontal load of at least 100 lb/linear feet (1.5 kN/m) of wall length applied at the top."

 

[phamENG]

Thanks. I don't have a copy of aci 347, but I'll check asce 37 when I get to the office tomorrow. I'm not familiar with that provision so I'll have to do a little research.

 

[BridgeSmith]

Your second reference I would take to indicate a 100 plf load applied horizontally at the top of the brace, not the top of the wall. Also, is there any indication that the force is a wind load? We have similar provisions in AASHTO, for cases where the wind load is small, so there's some bracing to handle loads from construction activities.

 

[Lomarandil]

Trillers, I read 37-6.2.3 to be the analogous provision to building overhangs with wind. The bottom face of an overhang is designed for the same wind pressure (upward) as the wall below sees (horizontally)

And I agree with BridgeSmith that 347 is specifyig a minimum load to provide some level of resilence against unforseen loading, not an additional wind load. (Also, it doesnt take much concrete form pressure to match that minimum. I would only expect it to apply for slab edge forms)