Equivalent udl for purlin design

[asixth]

Hi guys,

I'm in the process of designing the roof for an industrial building. Now, the purlin manufacturer has provided capacity tables based on udl loadings for given spans and bridging configurations. However, as you know, the wind loading that occurs over the roof of the building is not a constant udl but varies.

My question is:

What is a good approach to transform my actual loading to an equivalent udl loading so I can use the capacity tables which have been provided?

 

[frv]

Is it terribly uneconomical to simply design for the worst case?

 

[spats]

Figure out the maximum shear and moment in the purlin, and convert those values to the equivalent uniform load that would cause the same maximum shear (w = 2V/L), or same maximum moment (w = 8M/LxL), and pick the higher of the two.

 

[asixth]

Thanks spats,

I never considered converting to a uniform load based on an equivalent maximum shear. This is something that I will have to investigate.

My current approach is to calculate the maximum moment caused by the actual loading distribution for a simple span, then convert this moment to an equivalent udl.

 

[hokie66]

asixth,

I just divide the roof up into highly loaded, moderately loaded, and lightly loaded sections, then use the capacity tables for each area. The edges and ridges require the most capacity, and as you work to the central sections, less is required.

 

[GalileoG]

Is it typical to do a purlin design based on the maximum pressure (at the corners usually) for the entire roof? say..for a roof plan of 30 x 60 meters. Or is that overkill?

 

[hokie66]

I think that is overkill. I divide it up as noted above.

 

[civeng80]

Hokie66

What your doing is fair enough. But what happens if there is still 2 or 3 different UDLs on the purlin? You think its ok to find the max. moment and then find the equivalent load ? Thats what I have been doing !

Cheers !

 

[hokie66]

I am referring to the Australian wind loading code, which results in discrete steps in the external pressure coefficient depending on how far you are from an eave, ridge, or gable. Therefore, you can use a uniform load on each of the several areas, and load the purlins accordingly. If a given purlin is on the line, I would be conservative and use the higher loading. The only tricky part is with lapped purlins when the break in loading occurs along the way, and then you just have to use engineering judgment and logic.

 

[Tomfh]

asixth,

In Australia our purlin capacity tables contain procedures for converting stepped loads on single purlins into equivalent UDLs. Here is one such document, see page 8:

http://www.stramit.com.au/downloads/PTM_PurlinsGirtsBridging_V0.1.pdf

These procedures can give slightly higher equivalent UDLS than the backcalculating method you are using. This is because these techniques consider pattern loading.

 

[asixth]

Tomfh,

I am also an Australian, it's good to see so many Australian's on this forum.

I haven't look at strammit manual in-depth but have relied on the lysaght manual. The lysaght sales rep sent us plenty of documentation on Supa-Purlins which have folded flanges to give the purlin Supa-capacity (apparently).

The strammit manual has a nice little section on local pressure zones that I am going to have a look at tomorrow morning at work.

Without having read the manual myself, do you know if patterned loading is included for outward loading. I can understand why patterned loading is considered for downwards loading but I don't know whether it should be included for outward loading. I don't think the wind pressures is going to be that discontinuous as to develop patterned loading. If there is wind pressure on one span, then there is going to be wind pressure on the next span, maybe just more or less intense.

 

[csd72]

asixth,

Stramit also do tables that give moment and shear capacity of purlins based on various bridging spacings and moment shapes. You should try and get a copy of these as they are not only useful for this type of situation but are also great for when you have uneven spans(and the standard charts dont apply).

You need to keep in mind that these are not isolated spans, and the end moments at the supports can sometimes be critical.