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RE: Roof sheets blown off

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I have also seen where metal buildings rack slightly under wind loads,
causing elongation of the screw holes over time.

Bob Garner, S.E.

-----Original Message-----
From: G Vishwanath [mailto:gvshwnth(--nospam--at)yahoo.com]
Sent: Thursday, July 28, 2005 8:15 AM
To: seaint(--nospam--at)seaint.org; steel-detail(--nospam--at)yahoogroups.com
Subject: Roof sheets blown off

List,

A young engineer who worked with me before in my
previous Govt Owned Consultancy organisation sent me
the following mail from Sudan where he is working now.

I have also appended my reply.
I welcome any additional thoughts/opinions/or useful
information if you wish to share.

Regards
Vish
==========


> Dear Mr. Vishwanath,
> 
> How are u and family? How is your work?
> I am fine in Sudan.  Work is also going on well.
> Life in Sudan is little boring . as there is no
> entertainment.
> 
> We have our factory of  40m wide  x 60m long   x 10m
> eaves height( one central col.) - a closed shed
> all-round , to which we made an extension of 40m x
> 24x10m ( 4 bays of 6.0m)  to be used as fabrication
> yard , with the front gable side kept completely
> open, for easy movement of forklifts / materials.The
> longitudinal sidesof the ext. had a wall cladding
> upto 6.0m height.( Bottom 4.0m kept open). The roof
> was sandwich panel - profile 38/200 - PPGI 0.41mm
> TCT with insulation 35mm thk. backed with al. foil.
> In recent times.the heavy wind blew off the roof  in
> second bay of extension  , keeping the Self tapping
> screws intact in position on purlins. On
> observation, the Sheets had torn off completely
> around screws , making bigger holes of 25mm dia or
> so.
> 
> Most of them were of the opinion that, the front
> side which is kept open has resulted in the suction
> pressure, thereby ripping of the sheets.  
> 
> Could this be the main reason? Can we not design
> building with one gable side open and one side
> closed completely?
> 
> I am of the opinion that the sheet thickness- 0.41mm
> could be one of the main reason.
> 
> what are your views? Please advise.
> 
>
============

Dear ******,

Thank you for keeping in touch.
I agree with your opinion.
0.41 mm thickness for the roof sheet is too thin.
In India I have always specified 0.91 mm for the roof
and 0.63 mm for the sides.

The fact that the screws and the rest of the steel
structures  are still in place, shows that the screws,
purlins, rafter, and main building columns have
successfully withstood the wind forces.

Two possibilities are:

1)The spacing  of the screws was too much.
Hence the uplift load per screw was too much.

2)They tightened the screws excessively and that cut
into the steel sheet and damaged it. There is hardly
any "flesh" in a thin steel sheet of just 0.41  mm
thickness and there could have been local damage at
the screwing points so that the sheet simply tore off
at these points. The fact that not all the sheets tore
off lends credence to this suspicion of mine.

The building being left open at the ends should not be
the issue here.
It is a functional requirement.
Of course upward forces on the sheets would have been
more.
The suction forces on the top of the sheet and the
internal upward pressure would both be complementing
each other but the designer must have taken this into
account.

This is of course my opinion.
I will discuss this with others and let you know if I
am able to learn more on this.

Do keep in touch with an occasional mail.
Is Sudan hot?
Bangalore is cool (25 degrees celsius). I use a light
blanket at night.

Regards
G Vishwanath







	
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