Wind Pressure Coefficients 2

[structek22]

Hi all,

I'm designing to the Australian/NZ wind code 1170 (which I hear is similar to the American code).

I have a question about wind pressure coefficients.

For a typical structure say, you work out a coefficient of +0.7 on the windward wall, -0.5 on the leeward wall, -0.5 or 0 on the windward roof and -0.5 on the leeward roof.

Could some one explain to me how these are combined? i.e. for the walls, is my combined coefficient +0.7 + (-0.5) = 0.2?

Also could someone explain how the internal pressures come into play - i.e. how are they combined to the roof/wall coefficients?

Thanks in advance.

 

[BusyBerlin]

It depends on what you're designing.

If you're designing the building's lateral system (the wall as you describe) then yes, you would combine windward and leeward coefficients. Depending on your layout you may also need to consider an end-moment acting on the top of this wall due to the uneven roof pressures. There is no need to use internal pressure coefficients when designing the main structure - the internal pressure should be exerted equally in all directions inside the building and cancel itself out when acting on the building structure.

Internal pressure coefficients are used for the design of cladding or other "local" elements, typically elements that don't have both windward and leeward faces. For example, if designing cladding on the windward side of the building, the net effect of wind on the cladding is the windward pressure plus the internal pressure (the internal pressure "replaces" the leeward pressure on this local element).

I've always found it helpful to think of elements as always being subject to two pressures (in typical cases): a) external + external (on opposite side) or b) external + internal.

 

[StrEng007]

A great way to think of external/internal pressure is:
Imagine wind flowing over a structure, let's called it a regular box shaped building that is enclosed w/low pitch gable roof for this visual description. As the wind flows, from "left" to "right", the external pressure on the building's windward face (the wall on the left side) is positive. That is, the pressure is acting "towards" the windward face. Wind flowing over the top of the roof will experience some turbulent wind flow that leads to separation, causing negative external pressure on the roof. Here the pressure is acting away from the top surfaces of the windward and leeward sides of the roof. As the wind continues to travel and tumble over the rest of the structure, it reaches the right side of the building and continues to flow from left to right. Here the wind is pulling away from the leeward face and the external pressure on the leeward wall is negative. Those are all the forces that are occurring on the external side of the structure.

However, the structure will experience air leakage through any voids/gaps or openings in the exterior envelope of the building. This leakage will cause internal pressure within the building and it can be either positive or negative. Positive internal pressure occurs when leakage allows air to enter the windward side of the structure, as a result of the external wind pushing towards the left side of the building discussed above. This causes the interior sides of the structure to feel an expansion similar to a balloon being blown up. All the interior surfaces feel positive pressure. Negative internal pressure occurs when voids/gaps or openings are present on the leeward side of the structure. As wind pulls away, it creates a negative internal pressure as leakage of air is drawn out from the building. This causes the interior sides of the structure to feel a contraction like air being sucked out of an empty plastic bottle.

When designing for components and cladding, building elements must consider the combined effect of the external and internal pressure. For instance, if you're calculating the maximum positive pressure for a window on the windward face of a building, you'd consider combining the +external pressure with the -internal pressure. In a free body diagram, both of these force vectors would be pointing to the right. But wind can flow in all directions, so your job is to consider all the combinations of external/internal pressures.

I'm leaving out a bit of detail on coefficients and how they are determined. I hope this gives you a basic visual idea of how the pressures work so you can further explore the calculations and get more technical with it.

 

[prasadrmb]

You need to combine the wind pressure coefficients when loads are calculated. However, you can not always combine the wind pressure coefficient obtained for windward and lever sides. For example, when you are calculating wind pressure in a building, we have to consider the internal pressure.

If we plan to apply loading separately for windward and leaverd sides desperately, we calculate wind loading separately by combining each pressure coefficient with internal pressure coefficient.

Internal pressure coefficient may vary. Generally it has fix values. It could be positive or negative. Consider calculation of wind loading on windward side. Say external pressure coefficient is +0.7 and internal pressure coefficient +0.3 and -0.2 (depend on the code; some codes says to consider the effect of both).

Then wind pressure coefficient will be as follows.
0.7 - (-0.3) = 1.0 or
0.7 - 0.2 = 0.7

Wind pressure can be calculated considering the both. Similarly for the other side pressure coefficient can be calculated.
Say levered pressure coefficient is -0.5

-0.5 - (-0.3 = -0.2
-0.5 - 0.2 = -0.7

Now we can apply loading after calculating pressure.
Case 01
Pressure coefficient Windward 1.0
Pressure coefficient Leaved -0.2

Case 02
Pressure coefficient Windward 0.7
Pressure coefficient Leaved -0.7

Similarly, depending on the win pressure coefficient, we can calculate the wind loading.

Structural Engineering

http://structuraldesignbs.blogspot.com/

http://eurocode-2.blogspot.com/

 

[hokie66]

structek22,

I don't think your statement is correct that the Australian and US wind loading codes are similar. Maybe the results are similar, but there are a lot of variations in the way they are presented.

In your example, the positive coefficients are toward the given surface, and the negative coefficients are suction, so for a frame design, the combined coefficient for the windward and leeward walls is 1.2, not 0.2.

 

[Agent666]

In terms of the sign convention, in the AS/NZS its generally taken as follows for a building structure

external pressures
towards a surface = +ve
away from a surface = -ve

internal pressures
away from a surface = -ve
towards a surface = +ve

therefore +ve external pressure is in the same global direction as -ve internal pressure on a wall or roof surface.

in your example the windward and leeward wall external pressures are therefore additive (i.e +0.7-(-0.5)=+1.2 net coefficient).

 

[structek22]

Thanks for your help.

I'm currently doing a bracing check on the walls (using plasterboard).

I understand my wall combined pressure works out to be 1.2

However, my upwind roof pressure is -0.5 and 0 and my downwind roof pressure is -0.5. I don't quite understand what the worst case combination would be and whether you would calculate this pressure on both upwind and downwind roof area or just one of them.

Any ideas?

 

[Agent666]

Load combinations are in AS/NZS1170.0.

Typically you will look at 1.2g & wind (down case)
And 0.9g & wind (up case)