Wind Loading for Buildings with offsets

[Strucguy33]

I am an EIT pretty much and I understand the wind load provisions to a certain extent but have a problem that I am obsessing about. Say I have a building 60ft total in X-direction and 30 ft total in Y-direction and say I add 20' in x-direction and 20' in y-direction pretty much making an L-shaped building. So the perimeter is described using coordinate system as followed: Starting @ 0 go -50' in y direction, +20'x, +20'y, +40'x, +30'Y,
-60'X.
1)My question is and let me mention the roof is a monoslope less than 10 degrees, so there are two different wall heights at the front of the offset and at back of offset depending on slope of roof. When I calculate wind loads on the building do I break the building apart and design the 20'X20' and then the 60'X30' and assume each one has a different mean roof height and L/B ratio. For Example L= 20' and B = 20' for one with say 12' eave height, and L=60' and B = 30' with say 18' eave height. or do I combine them and get one L/B ratio and figure (12+18)/2 for "h". Explain how the shears combine at the point where the two portions of the walls meet.
2)And also If the Roof slope is less than 10 degrees say 7 degrees and considered a flat roof does the wind pressure applied to the surface act vertically over the horizontal roof area or at whatever the roof angle is making X and Y components even though its considered flat.
Hope someone can Help!

 

[ishvaaag]

Wind pressures and suctions are always normal to the (assumed tight enough) surface. Wind friction (when it needs to be accounted for) is usually assumed tangential to the average exposed surface.

Older codes clearly assigned a same overall structural wind pressure for walls forming a concavity against the wind; it is not the shears what are combined, it is the pressure that is assumed common in such concave parts. Suction also took common values over wide areas.

Enter the new codes. They attempt a better representation of the structural effect of wind by breaking -if necessary- the exposed areas in smaller chunks for which the pressures/suctions/frictions are given more accurate representation.

This also means that in some cases one needs to allot the exposed areas in different ways to get the meaningful structural effect for the case we are considering at the moment.

This last paragraph means that most likely for a specific structural intent, say, get the main overall wind loads over a building like this, you need to make severall different assumptions (as you are making) to ensure that your loads, that in the end are but a simplification of those imparted by varying winds, are significative enough to cover the structural effects you are investigating.

 

[a2mfk]

You will always get more and better responses with a sketch. Its way easier than deciphering a coordinate system, as logical as it is we are volunteers with limited time

Wind pressure always acts normal to the roof or wall surface, but a slope less than 10 degrees will hardly make a difference of how you model it.

 

[Strucguy03]

a2mfk, was wanting to know if i have a building that has multiple offsets in it say a 30'X60' attached to a 15'X10' do i separate the two and design them separately or combine them by using the most restrictive of the two same calculated mean roof height etc...
thanks.

 

[JAE]

It depends on whether the two segments are part of the same framing system or if they are separated by a building expansion joint.

If connected, then you deal with the overall building - wind load criteria given in ASCE 7, for example, doesn't provide exact wind pressures for every building configuration - but only fairly simple rectilinear shaped structures.

Your job, should you decide to accept it, is to use the criteria and apply it in the best sense possible, and on the conservative side.

 

[spats]

It should be treated as one building, 60' long with a least width of 30'. Calculate a single average mean roof height, and end zone wall wind loads only occur at the four true outide corners of the building.

Roof wind loads always act perpendicular to the slope, no matter what the angle, and produces a horizontal component that opposes the wall wind for low slopes, so it is usually ignored.

 

[Strucguy03]

THANKS JAE AND SPATS.

 

[MiketheEngineer]

You are "pretty much" an EIT?? Does that mean you almost passed the test??

Just busting your b@!!S a bit.