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Angle of dispertion: Web Cripling/Buckling

 
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hsrai
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Joined: 26 Jan 2003
Posts: 151
Location: Ludhiana, Punjab, India

PostPosted: Tue Oct 12, 2004 11:22 am    Post subject: Angle of dispertion: Web Cripling/Buckling Reply with quote

˜In design of steel beam, we take 30 degree angle of dispertion of web
cripling, while it is 45 degree for web buckling.

Why is it so?

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Robert.Nicolson at Sub...
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PostPosted: Tue Oct 12, 2004 11:51 am    Post subject: Angle of dispertion: Web Cripling/Buckling Reply with quote

Dear Mr Rai, I don't know if there is any real experimental results to
back this up.
I think it is more to do with a reasonable approach.

1) For buckling, at centre of the web, 45 degrees seems a sensible
dispersion.

2) For bearing / ( crippling ), at the top of the web, 45 degrees would
give a very short length for a point length, which would be
unreasonable, whereas a 30 degree dispersion would give 1.73 times this
dispersion length, which seems more reasonable.

Remember that in either case these dispersions are very conservative,
and therefore even if you adhere to them you are more than safe.

Thanks, Rob Nicolson.

-----Original Message-----
Message From  hsrai@gmx.net [mailto:hsrai@gmx.net]
Sent: 12 October 2004 17:52
To: Robert Nicolson
Subject: Angle of dispertion: Web Cripling/Buckling


In design of steel beam, we take 30 degree angle of dispertion of web
cripling, while it is 45 degree for web buckling.

Why is it so?

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Robert.Nicolson at Sub...
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PostPosted: Tue Oct 12, 2004 12:47 pm    Post subject: Angle of dispertion: Web Cripling/Buckling Reply with quote

Dear Mr Rai, I don't know if there is any real experimental results to
back this up. I think it is more to do with a reasonable approach.

1) For buckling, at centre of the web, 45 degrees seems a sensible
dispersion.

2) For bearing / ( crippling ), at the top of the web, 45 degrees would
give a very short length for a point load, which would be unreasonable,
whereas a 30 degree dispersion would give 1.73 times this dispersion
length, which seems more reasonable.

Remember that in either case these dispersions are very conservative,
and therefore even if you adhere to them you are more than safe.

Thanks, Rob Nicolson.

-----Original Message-----
Message From  hsrai@gmx.net [mailto:hsrai@gmx.net]
Sent: 12 October 2004 17:52
To: Robert Nicolson
Subject: Angle of dispertion: Web Cripling/Buckling


In design of steel beam, we take 30 degree angle of dispertion of web
cripling, while it is 45 degree for web buckling.

Why is it so?

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pvyas
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Joined: 26 Jan 2003
Posts: 10

PostPosted: Wed Oct 13, 2004 5:00 am    Post subject: Angle of dispertion: Web Cripling/Buckling Reply with quote

Dear Mr Rai,
I will try to explain difference between two phinomina as under.


Web Crippling:- This is also termed as direct compression failour meanse if
the bearing area (the area through which direct compression force in torms
of reaction or point load transfer trhough it) is not adiquate then there
will be chanses of crushing of web at junction of flange and web (at root of
fillet). And this will be controlled by controlling of actual bearing stress
well wethin the permissible stress limit and it is 0.75fy. So code assume
this point load transfer to web by having dispersion angle 30 degree. And
thus the bearing area calculated like a+2hxcot30degree. (where a=width of
bearing plate and h=distence of root fillet from extrem fiber)


Web Buckling:- Web buckling may occur due to

1) Diagonal compression due to shear
2) Longitudnal compression due to bending
3) Vertical compression due to consentrated load
4) Any combination for above

1) Diagonal compression due to shear:- It is found that critical shear
stress will be equel to shear yield stress 0.4fy when ratio dw/tw equel
1344/rootfy, and that is 85 for fy=250MPA.Where dw=clear depth of web and tw
is thickness of web
2) Longitudnal compression due to bending:- Similarly it is seen that
critical longitudnal compressive stress from the neutral axis has its
maximum value approximately equal to the yield bending stress for hw/tw
equel to 3200/rootfy and that is 200 for fy=250MPA. (Similaly like plate
girder say vertical stiffner require for a d/t ratio 85 which will take care
of buckling of web against diagonal compression due to shear and horizontal
stiffner require for a web d/t ratio 200 which will take care of buckling of
web against Longitudnal compression due to pure bending.)
3)Under vertical compression:- The tendency of web to buckle under direct
reaction or point load.

In all the three phinomina listed above after some d/t ratio tendency of web
is same like column (after some slanderness ratio limit of wed it is
behaving like compression member column)
Now the difficulty arises about slenderness ratio of web column (what should
be the effective length of web column.)  Code spacifies Effective
slenderness ratio of webcolumn = (dw*root3)/tw.
Second difficulty arises with regard to the length of web that really
buckles under the action of concentrated load.
And code spacify effective width (length of web that really buckles) of web,
B=D/2+Length of stiff portion of bearing (where D=Overall depth of member)


Conclusion: As web crippling is the bearing failour and wed buckling is a
instability failour of a member.(tendency of web to buckle under comp load
not allowing to utilise its permissible yield stress. Or before it reaches
to permissible yield stress it will have instability failour.) Hope it will
clear the idea. If i am wrong pl correct me. And if some one want to add
more pl go ahed.


Regards,


Parth Vyas.(Sr. Engineer civil/structural)
Tecnimont ICB PVT LTD, MALAD, MUMBAI.



-----Original Message-----
Message From  hsrai@gmx.net [mailto:hsrai@gmx.net]
Sent: Tuesday, October 12, 2004 10:22 PM
To: Vyas Madhusudan Parth (Mumbai - Civil)
Subject: Angle of dispertion: Web Cripling/Buckling


In design of steel beam, we take 30 degree angle of dispertion of web
cripling, while it is 45 degree for web buckling.

Why is it so?

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hsrai
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Joined: 26 Jan 2003
Posts: 151
Location: Ludhiana, Punjab, India

PostPosted: Fri Oct 15, 2004 5:41 am    Post subject: Angle of dispertion: Web Cripling/Buckling Reply with quote

Wednesday at 10:30am -0000 P.Vyas@ticb.com wrote:

Quote:
I will try to explain difference between two phinomina as under.
Web Crippling:- This is also termed as direct compression failour

<snip>

Thank you Robert Nicolson and Vyas for your responses. This group is
really doing good service for the Structural Engineering Community.

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