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I've started working on a new wind load calculator:

Notice the complexity when overhangs are added to the roof. These load diagrams show the Envelope Procedure for a Gable Roof:

Load Case A:





Load Case B:



If torsional load cases are required then another Load Case A and B are required for the torsional load case.

Interesting to note that for a gable roof only the wind forces on the walls contribute any lateral forces to the roof diaphragm and then to the shear walls for the longitudinal direction (wind parallel to ridge).

The breakdown of the forces would then be:





One could argue that there is a certain amount of wind force on the edge of overhangs but I'm thinking its insignificant.

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Shearwall reactions are now complete, assuming of course that only the exterior walls are acting as shear walls. This assumption is pretty useless for building with dimensions exceeding a certain limit since they will probably involve steel moment frames or multiple internal shear walls. I'm wondering if it might be useful to add some sort of advanced option which lets one specify an internal shearwall spacing or even a internal shearwall layout. The programming would get pretty complex.

I also added the ability to calculate truss/rafter uplift and horizontal load. Not entirely sure how useful this is either but I've personally ran into it enough times so I thought it might be of some benefit. The funny thing is after writing the code and creating the image for this section (3) I happened to look at a truss manufacturer's output for a garage I was designing a while back. I quickly noticed that the horizontal reactions and uplift were listed on the document, so if you've already taken your design to get the trusses quoted you probably don't need this information calculated. Just out of curiosity I used the same parameters as the truss manufacturer used for their wind loads and after adjusting for the TC and BC dead loads both my horizontal reactions and uplift were within 0.5 lbs of their values. Nothing like a third party check nailing it so perfectly, that is why I love this stuff.


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Marc that is a very good question. Unfortunately, I did not develop the Envelope Procedure so I can't give you a detailed answer as to the "why" of your question. However, it is not a separate load case. Images #1 and #2 represent the same load case "A" but with a slightly different load patterns depending on the geometry of the building and the pitch of the roof. I refer you to Note 8 of Figure 28.4-1 of the ASCE 7-10. This note explain how and when to apply this alternate configuration of load case "A". As mrj6550 previously stated the details of how to correctly apply the wind loading from the ASCE 7-10 is rather complicated and can be a bit confusing, hence the need to program a calculator like this that hopefully takes care of most of the details for you.

Even myself, an engineer, can sometimes miss all of the possible load cases and minimum requirements. That is why in my calculator for the lateral loads I have included tables with side by side comparisons. I can now analyze a simple gable structure for wind in about 2 minutes which once took me a couple hours of manual calculations.

I like to call it code "bloat". If you compare the current ASCE 7-10 and IBC with its predecessors back about 20 years ago you will notice that the level of complexity has greatly increased. Almost to the point that an engineer has a hard time getting a feel for the numbers. Most engineering today is efficiently done with programs such as this that take the actual engineering calcs out of the hands of the engineer and effectively make him a technician.

An excellent example of where this all leads to is the mpc truss industry. Yes, there are still truss engineers but by and large most truss packages are generated by technicians using software from companies like Mitek. The engineering is all automated and performed by the software. I've seen stamped truss documents from some truss manufacturers but I often wonder what exactly those engineers do?

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  • 5 weeks later...

Compare the pressure profiles for a hip with the gable roof(Load Case A):



The calculations to resolve the pressures on the roof surfaces into lateral forces for the hip roof is going to get a little more involved.

This is my best guess at the pressure distributions for Dutch Gable, Half Hip and Flat Roof types using the Envelope Procedure:




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