Wind load distribution on a sawtooth steel roof

Question

I have a steel structure (see picture below) and its roof is a type of sawtooth roof. The plain/flat part is from glass and the inclined surfaces are some kind of pergolas, but I prefer them to be from steel sheet. (Pergolas are not good for areas with snow.)

My question is, what is the wind load distribution?

In my opinion there will be no wind in the direction of 0 degrees (because there are existing objects which prevent forming of the wind), only in the direction of 90 degrees. Also I'm not sure that the object will be fully closed (with facade walls) or partly open. I'm aware that this is important (the surrounding), so we can make solutions according to the two situations.

Answer 1

A foreword

There are two issues that you need to consider :

• standards and norms (legislation for your specific country)
• what really happens with wind.

When you are building you should consider both, but depending on the structure you might focus more on one of the two.

standards and norms

Each country has its own standards and norms for buildings. For example in europe you can have a look at Eurocode EN1991-1-4 (Action no structrues - general actions Part 1-4) and the annex particular to the country you intend to raise the building.

In particular section 7.2 (pressure coefficients) is the most relevant to your problem (although you might need to do some work because the arrangement is not common).

What you need to be careful in this scenario, is that you need to consider all possible directions of the wind, especially the directions with the worst scenarios.

What really happens with wind

1. The first thing that sprung to mind is that your drawing has the 0 degrees perpendicular to the building.

In real life, the wind usually has a strong preference with respect to direction (where I am from for example strong winds are usually from north-west and north and one month in the year from the south. If I were erecting a structure, I would take that directionality into account.

2. Its not only the downward pressure that you should worry about (not as common but still important if your area has that type of weather)

Usually the downward pressure is more important. However, especially with seesaw roofs (not only them actually), the upward pressure (or more specifically the pressure difference between the top and bottom of a roof) can be very important. Particularly if you are planning on wooden structures. Again, this becomes even more so important when you are building in areas with hurricanes.

your specific example

According to Eurocode, the wind from the 0 degrees will definitely have some downward pressure. This is because the wind will create vortices over the pitch roof, and the turbulent flow with create a pressure difference, which will create a force.

Additionally, the 90 degrees wind direction as you guessed will also have a wind pressure.

The exact wind pressure values and/or the procedure is a matter of an experienced engineer that will see the requirements for the local building codes. IMHO it's not a possible to provide an estimate without inspecting the actual blueprints/location.

Answer 2

Below are contours of wind pressure on the mono-slope roof and sawtooth roof. Note the direction of the wind is from left to right, and the high edge of the roof is on the left.

In the US, $C_p$ is the external pressure coefficient. Please check your design code for the equivalent coefficient and how to apply it.

Here is a research paper for your use. http://davidoprevatt.com/wp-content/uploads/2010/09/bo-cui-2007-wind-effects-on-monsloped-and-sawtooth-roofs.pdf

Pressure diagram of a mono-slope roof (per ASCE7):

Downward pressure on lower structure:

Wind can blow in any direction, the influence area and distance of a wind stream and the intensity at the contact surfaces, except experiment through the wind tunnel study, are of anybody's guesses. This sketch is provided for your further thinking. (Note the height of the wall and its distance from the building will influence the wind flow and intensity to a great extend. You could be right that the effects other than the 90 degrees case are negligible. But we have no way to prove it.)