Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Roof sheets blown off

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Vishwanath,

The fact that everything except the roof panel
remained in place does not necessarily mean that these
elements were adequately designed. If you considered
the load path for wind pressure, the first element to
transmit the pressure is the roof panel. When it
failed the succeeding elements in the load path did
not have a chance to see those loads.

ASCE 7 prescribes wind pressures for three cases:
Enclosed building, partially closed building and open
building. The pressure inside the building for open
building situation can be substantial! There is
positive pressure on the underside of the roof and a
suction on the outside.

The roof panel should also be considered as a
component/cladding element which means, typically,
various segments of the roof experiences various
magnitude of pressures. The corner zone is the worst
loaded which can see as much as 3 to 4 times the
pressure in the middle zones.

>From the description given by your friend, it appears
that:

1. the wind pressure calculation did not include the
possibility of a large opening on one side of the
building

2.the sheet thickness was not adequate

3.the purlins were too far apart.

In all these cases or combinations thereof, the
reaction transfered by the sheet at connection points
were large enough to "shear" away the sheet around the
screws.  

A few years ago, I evaluated a metal building whose
end wall was ripped off. It was a machine shop. During
summer months, the workers would keep a roll-up door
completely open to prevent heat build up inside the
building. On one such day, there was an unusually
stong wind storm (the local meteorological office
reported average wind speed of some 70 MPH). The
workers felt a sudden gust inside, but before they
could do anything, they saw the far end wall ripped
off and the single intermediate vertical column
buckled.

When I ran the calculation, I found that the wind
pressure in an "open" situation, would have resulted
in significant overload for the column. The sheet
itself was adequate but when the column buckled, the
girts deflected sufficiently to destroy the connection
of the sheets to the girts, leading to the tearing
away of the wall sheets. I replaced the single
intermediate column with two equally spaced, W14
sections. That was in 1997.

Rajendran

--- G Vishwanath <gvshwnth(--nospam--at)yahoo.com> wrote:

> 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
> 
> 
> 
> 
> 
> 
> 
> 		
> ____________________________________________________
> Start your day with Yahoo! - make it your home page 
> http://www.yahoo.com/r/hs 
>  
> 
> ******* ****** ******* ******** ******* *******
> ******* ***
> *   Read list FAQ at:
> http://www.seaint.org/list_FAQ.asp
> * 
> *   This email was sent to you via Structural
> Engineers 
> *   Association of Southern California (SEAOSC)
> server. To 
> *   subscribe (no fee) or UnSubscribe, please go to:
> *
> *   http://www.seaint.org/sealist1.asp
> *
> *   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any
> email you 
> *   send to the list is public domain and may be
> re-posted 
> *   without your permission. Make sure you visit our
> web 
> *   site at: http://www.seaint.org 
> ******* ****** ****** ****** ******* ****** ******
> ******** 
> 


__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around 
http://mail.yahoo.com 

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********