## How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

## How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

(OP)

The following question regarding wind load combinations has come up a couple of times before on this forum, but I haven't found a complete answer. Reference the figure below.

Question = How do you handle the summation of loads on the roof?

A quick recap: Previous posts point out that all combinations of wind loads from different directions must be considered, resulting in several separate wind load cases. That makes sense, and treatment of the horizontal forces on the walls seems straight forward. Previous posts have also pointed out that the wind loads on the roof (for a given wind direction) typically have a positive and negative load case, with differing pressures on the windward and leeward half of the roof. What isn't clear to me is how you handle the roof summation for a given pair of wind directions and say the "net pressure pressure" case from each direction. Do you break there roof into sections (quarters) and sum the pressure in each section?

Question = How do you handle the summation of loads on the roof?

A quick recap: Previous posts point out that all combinations of wind loads from different directions must be considered, resulting in several separate wind load cases. That makes sense, and treatment of the horizontal forces on the walls seems straight forward. Previous posts have also pointed out that the wind loads on the roof (for a given wind direction) typically have a positive and negative load case, with differing pressures on the windward and leeward half of the roof. What isn't clear to me is how you handle the roof summation for a given pair of wind directions and say the "net pressure pressure" case from each direction. Do you break there roof into sections (quarters) and sum the pressure in each section?

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

1) Where both, opposite side shear demands are CCW, they need to be added.

2) Where both, opposite side shear demands are CW, they need to be added.

3) Where one shear demand is CW and the other is CCW, they can be subtracted. This simply won't be a governing condition.

I find it helpful to do little differential sheathing panel element diagrams like those shown below.

In the interest of truthiness, I've not yet seen an engineer go to the level of detail in diaphragm design that we're discussing here.

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

I have to be honest, this is my first time through the code and its not immediately obvious that this figure is even talking about roof loads.

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

I don't

thinkso but it's hard for me to tell as I'm not sure that I understand the approach that you're suggesting. With the vertical lateral force resisting elements (bracing) placed at the perimeter of a rectangular shaped building, we typically treat the diaphragm as a simple spanning beam in each of the two directions.I recommend developing a particular load case and bracing scheme that you'd like to study and then posting that here. That way, we can work through the example together and, hopefully, iron out any questions that you have.

It appears to be talking about wind loads applied to the walls of a building. Of course, at the upper level of the building, it is the roof deck that laterally supports the tops of the walls. So, in that sense, the figure is talking about uniformly applied wall loads (PSF) that will be aggregated into lateral line loads (PLF) at the perimeter of the deck.

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

These cases are intended to determine the load to be resisted by the main wind force resisting system - shear walls, braced panels, braced frames, moment frames, etc. and diaphragms - so we don't care about the VERTICAL force in this case. The description of Case 1 and Case 2 states "design wind pressure acting on the PROJECTED area perpendicular to each principal axis of the structure". Case 3 and 4 then reference 1 and 2, so the same concept applies. So using Figure 27.3-1 (C.p), we see that you would apply the horizontal component of the windward and leeward pressures with the windward and leeward wall pressures.

If it were a flat roof, there would be no contribution. If you have parapets or mechanical equipment, you'd need to include those in the appropriate directions.

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

I guess that I should be more specific on what I'm trying to do. I'm designing a pergola for my back yard (pictured below). I'm a structural analyst in the aerospace industry, so my analysis method is one that is familiar to my day job... but I am not trained in civil/structural building design. I built a 3D finite element model of the structure (every nook and cranny modelled) and am trying to get wind pressure loads to apply to it, for use with an allowable stress design approach. I've used both the flat-monoslope wind calcs (theta = 0) and pitched roof wind calcs (theta = 7 deg) from ASCE 7-16 Chapter 27 to estimate loads since my roof is curved but nearly flat. Horizontal loads are the minimum 16lb/ft^2 design load from ASCE 7-16 27.1.5 applied to the frontal area of the pergola. I was planning to apply the wind loads to the model roof and "wall" frontal area simultaneously with dead and live loads (per the load combinations in ASCE 7-16 24.1) in several separate load cases. Now I'm asking how to combine wind load from different directions on the model.

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

So... I'm back to wondering how to define separate wind load cases on the model.

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

The pergola is approximately 21' x 15', which is just big enough for my local town to require a building permit and structural report.

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

Is it really a roof if the diaphragm is made out of perforated metal?

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

I'm not a structural engineer looking at buildings every day so I don't tend to think in terms of diaphragms, shear walls and the like. To me it is just another structure that needs to take some loads. I'm happy to learn what I can though about your end of the industry, and appreciate your comments. I've read through several of your codes for this project (ASCE 7, IBC, AISC 360, ACI 318). My Ph.D. and work experience is more along the lines of a general purpose structural analyst (race cars, speedboats, airplanes, missiles, factory cranes and stands, etc.)

The tubular steel frame for the pergola is quite stout. The beefy size is more aesthetic than structurally necessary. Its gotta be pretty since it is for my wife I've already done some preliminary analysis and I can toss just about any combination of the ASCE 7 loads at it and have plenty of margin. My question about the roof wind combinations comes because I don't want to turn in something that's out of keeping with your best practices.

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

You don't state where you live but as this is a small residential project the reviewer will likely not even be an engineer. If it is an engineer he'll likely be more interested in making sure your detailing isn't crazy and that you have some reasonable footings and anchorage/embed.

What do I know though I don't design racecars...

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

So, are you thinking that Case 3, on my structure, would ignore roof loading (or a least the vertical component of the roof load)?

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

If you are talking about dead load then using ASD load factors you would use 0.6 time the resisting dead load for uplift and overturning(0.6D+0.6W).

If by roof loading you mean the wind load on the roof, Cn from Fig 27.3-4 is your net pressure coefficient which includes contributions from the top and bottom surfaces of the roof. This is all to be applied to the lateral force resisting system components (in your case it appears to be a series of cantilever columns). I would also ignore diaphragm action from the panels and check the edge beam weak axis for the OOP wind loads combined with full dead load (D+0.6W Chapter 2).

For components such as the panel attachments this would be designed for C&C wind loads from Chapter 30.

This is an open structure correct? No walls. If so then the only wind load is due to the roof. No structural engineer I know would calc the wind load on those columns.

## RE: How to Combine Wind Loads On Roof? (ASCE 7-16, Directional Method)

For vertical pressures, I only consider the higher value of the two orthogonal directions (case 1a and 1b).

For horizontal pressures (walls, parapet, or component due to roof slope), I do consider directional cases 1-4 as appropriate.

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just call me Lo.