Wind Load Calculator

[Medeek]

I've recently started working on a wind load calculator.

Does anyone know of any free wind load calculators that I can use to check my own program against. I am going to program it for MWFRS using the directional procedure initially.  I will make it a web based program like my other programs and try to employ some SVG that can present the data with some visual aids, something like the image below:

[Medeek]

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.

[Medeek]

Give the gable roof option a whirl, all of the other options are not programmed yet and I may never add them pending further research as to how well I can extrapolate the Envelope Procedure to these configurations.

If there are no overhangs set the overhang values to zero, I will need to add in a footnote about that later just to clarify.

Not a finished product yet, not by a long shot. The current Todo list includes the following:

To Do List:

1) Calculate min. lateral load with roof forces neglected and min. pressures of 8 psf and 16 psf on roof and walls respectively. (psf)
2) Calculate max. uplift and max. horizontal reactions of trusses or rafters assuming 24" o/c and 16" o/c for design of hurricane ties (lbs).
3) Calculate base wind shear in transverse and longitudinal direction (lbs).
4) Calculate shear wall reactions (lbs) and unit shear (plf) assuming only external shear walls (4 walls).
5) Calculate roof diaphragm distributed load (plf) assuming one story building with external shear walls only.
6) Include C & C wind loads for component design.
7) Include Directional Procedure as a comparison to Envelope Procedure or setup separate calculator for this method.
8 ) Complete PDF report output.

http://design.medeek.com/resources/wind/wind_calculator.pl

[Medeek]

Based on the preceding conversation and considering the following load pattern on a gable roof with wind in the transverse direction as shown below:



I then get the following loads applied to the roof diaphragm (plf):



From this we should be able to calculate the reactions on the diaphragm from the exterior shear walls (no interior shearwalls for this derivation).

We will then check the minimum load case where 8 psf and 16 psf are applied to the roof and walls respectively and also the case where the roof loads are neglected and only the wall loads are considered.

In this particular situation the roof zones 2 and 2E are negative so the load case where the roof loads are neglected will govern.

[Medeek]

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:

Transverse:



Longitudinal:



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

[Medeek]

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.

[Medeek]

Going through the process to create this calculator has definitely been an education. It is interesting to note that I 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 (slightly exaggerated but you get my point).

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 do those engineers do?

[Don_P]

I've just done 2 days of training from the AWC's engineers on wind and seismic using the 2012 WFCM, should wrap up tomorrow which is good, 2 hrs windshield time each way. The commentary and workbook (with worked examples... more backchecks) are free downloads here;
http://www.awc.org/standards/wfcm.php

[Medeek]

I've just finished adding the PDF report output to the new Wind Load Calculator (Envelope Procedure ASCE 7-10). Currently it calculates the MWFRS loads, I am thinking about adding in the C&C loads as well just to complete the package. I am still undecided on how best to incorporate the half hip and dutch gable type roofs. Looking at the commentary in the back of the ASCE 7-10 it appears that one can apply this method rationally to the hip type roof which means that roof types that are somewhere in between gable and a full hip should also be safe territory as well. I've had some people (other structural engineers) tell me otherwise but really have received no good reasons why not to.  If a rational approach can be devised that is clear and easily explained then I see no reason not to extrapolate the Envelope Procedure to these two other roof types.

[Medeek]

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.

[Medeek]

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

[Medeek]

C&C Wind Zones for a Gable Roof: