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hsrai ...
Joined: 26 Jan 2003 Posts: 155 Location: Ludhiana, Punjab, India

Posted: Tue Oct 12, 2004 11:22 am Post 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... Guest

Posted: Tue Oct 12, 2004 11:51 am Post subject: Angle of dispertion: Web Cripling/Buckling 


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... Guest

Posted: Tue Oct 12, 2004 12:47 pm Post subject: Angle of dispertion: Web Cripling/Buckling 


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 SEFI Member
Joined: 26 Jan 2003 Posts: 10

Posted: Wed Oct 13, 2004 5:00 am Post subject: Angle of dispertion: Web Cripling/Buckling 


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 ...
Joined: 26 Jan 2003 Posts: 155 Location: Ludhiana, Punjab, India

Posted: Fri Oct 15, 2004 5:41 am Post subject: Angle of dispertion: Web Cripling/Buckling 


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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