IBC 1609.1.2 Application

[AlohaBob]

The load prescription I question is the 10 psf prescribed minimum down wind load for roof components. Shouldn't this also apply to MWFRS? If yes, how should it be combined as an additional wind loadcase? As a combined adding effective wind load with other wind loads, or a distinct seperate combination?

It doesn't appear to be adopted by the premanufactured building design standards.

 

[ERV]

I believe what you are referring to is the minimum loading of 10 psf for components and cladding. Components and claddings are NOT part of the MWFRS. Components and claddings loads are transferred to MWFRS.

If the component is, for instance roof sheathing, the loading of the roof sheathing is transferred to the rafters, purlins, etc. (negative or positive). If the component is siding, then the load of the siding is transferred to wall frame system (negative or positive).

Pre-manufactured buildings are generally designed (for wind) using the simplified method such as in ASCE 7-98 or 7-02.

Try this link and down load this program:

http://www.standardsdesign.com/WLS/2002/2002.htm

It would be helpful to get ASCE 7-98 or 7-02, Minimum Design Loads for Buildings and Other Structures,

http://www.pubs.asce.org/BOOKdisplay.cgi?9990609

 

[AlohaBob]

The Code does prescribe the 10 psf as a minimum horizontal load for MWFRS. It seems to me the probability of a down load 10 psf wind would be a common occurence regardless of other load cases and directions. The application to component load hence is proper. It would appear however that for the overall building frame design loading the probability of this load in some defined or undefined combination is neglected. Perhaps the thinking is that other load cases and factors adequately prescribe this effect. I see no mention in the IBC commentary.

 

[ERV]

AlohaBob,

1609.1.2 "Minmum Wind Loads" (IBC 2000), Simply states that the minimum wind load on the MWFRS shall not be less than 10 psf multiplied by the area of the building or structure normal to the direction of the wind. This would include the 10psf transferred by the components and claddings.

Note: the surface area of the component (such as siding) is the area normal to the wind, multiplied by 10 psf, this is your total load.

Of course wind analysis is not that simple.

1609.1.2 Is basically the introdution of Section 1609, Wind Loads.

 

[AlohaBob]

In your opinion, and for the sake of clarity, you interpret IBC 1619.1.2 to prescribe "10 psf wind down" as an additional distinct minimum load case to be applied in combination with all other prescribed loads and combinations for the design of the MWFRS as well as to the individual component/claddings design loads?

I have two differing opinions now which both seem valid. They both agree however the code appears ambigious or inconsistent on this point.

Issues associated with the interpretation include validation of direct physical observation and importantly, economics. E.G. The load consideration would seem to make sense, however if it's not specifically required, and if Joe S. doesn't design with it...

Thanks for your response.

 

[JAE]

This is how I interpret 1609.1.2:

1. Calculate your MWFRS loads as you normally would. This is based on ASCE 7 and for MWFRS includes wall and roof loads. These loads produce axial, shear and moments in your building elements. This is W which is included in the load combinations specified in Chapter 16.

2. 1609.1.2 then says that, despite what you get in item 1 above, you still need to calculate axial, shear and moments for elements using a 10 psf LATERAL load on the projected area. No vertical loads area specified here. If this 10 psf does not produce higher axial, shear and moments, then OK...otherwise this is a contributing load case that must be considered. It is a load case W to be included in the load combinations of Chapter 16.

3. For components and cladding, you would calculate the required wind pressures for your various elements of your structure and compare these pressures with +/- 10 psf. In this case, 10 psf is simply a minimum pressure - so use the ASCE 7 C&C pressures or 10 psf whichever is greater.

 

[ERV]

AlohaBob,

Let me try this again! 1609.1.2 MINIMUM WIND LOADS. This section sates:

The minimum wind load is 10 psf, not added to anything, it is just a minimum of 10 psf. If you have an external pressure of 13 psf and an internal pressure of 3.2 psf, your net pressure on the wall is 9.8 psf. YOU MUST USE 10 PSF because this is the minimum.

Maybe you are getting confused with the statement "In the calculation of design wind loads for components and claddings for buildings, the algebraic sum of the pressures acting on opposite faces shall be taken into accoount".

This statement simply means that if you have wind pushing on one side and you have internal pressure resisting it, subtract the resistance from the pressure this gives net pressure(as in the example above).

There is no "down wind load" mentioned in the code (either ASCE 7-98 which the IBC is based on, or in the IBC 2000 or 2003).

I think the code writers try to make these codes complicated (and they are not), leading to confusion.