WIND LOADS

[sc]

G'day,

I've just come across something that I don't normally have to deal with. I have a client who proposes to construct a building with a trapezoid floor plan (18m long with 17m at the larger end and with the small end 6.50m). The relevant Australian standard deals with rectangular floor plans only for calculation of wind loads.

As I am struggling to recall how we dealt with this at Uni, I am proposing to treat the building as rectangular (18m x 17m). The aim of this being to present a conservative loading case for design.

To add to my concerns one end (6.50m) is completely sheilded by an adjoining building and the opposite end (17m) is 1/4 sheilded. This gives me a variation in wind loading across the roof of the new building. Again I propose to treat the building as unsheilded for a conservative design.

I guess the question is, Is it worth my time researching a detailed answer to have a gain in member size reduction or do I just say hang the expense and be conservative?


regards

sc

 

[g0203447]

Have you tried an estimation of sections used by the conservative and easy-implementing calculation ?

 

[sc]

Sorry folks I've jumped the gun and gone the conservative way now, but I would like to know a more definitive answer for future projects.

For example what is the effect of the varying building depth on wind pressures?
Does internal pressure vary significantly along the length?
Does external pressure vary along the length?

As this particular building has a mono slope roof, what is the effect of the tappering roof height, if any?

I know that I have learnt this at some point during Uni, but unfortunately I no longer have my notes (we lost some boxes in our various moves about).

Regards
sc

 

[JAE]

sc: some thoughts:

1. I would think that simply using a conservative, uniform building width would not excessively add to your wind forces and subsequent design. You probably do not have the fee (hours) to spend micro-analyzing this so as long as you don't overkill your design, simply simplify it and go a bit conservative.

2. In the U.S. we cannot, per code, utilize nearby buildings to shield from wind. The idea is that someday the adjacent structure might be torn down and then leave you with a problem.

3. Interior pressure is independent of shape.

 

[sc]

JAE,

Thanks for the response.

It is interesting to note that the US does not allow for sheltering of buildings.

The sheltering of buildings within the Australian code as far as I can tell is more a recognition that there will always be a building of some description providing some form of partial sheltering. This is meant more for residential or industrialised areas where in the longer term there will more than likely be some form of sheltering.

The reduction factors involved are not great but do offer some reduction of wind loads. The effect of sheltering is a function of adjacent building heights and distance. I guess the view is taken that if a building is removed it will cause a "short" term rise in wind loading, but in the long term it is more likely that a new building will be constructed.

sc

 

[JAE]

sc - The US codes do account for the topography around the site for a number of miles, these are introduced into the calcs in the form of an Exposure catagory (A thru D where D is facing an open flat area).

This does nudge the loads up or down a bit. What the US codes say regarding building shielding is that you cannot go the next step and use a next-door building to negate or specifically reduce wind pressure.