Quick Help

[G Vishwanath]

I am away from my office and am requesting some quick help from any one of you who is seated right in front of his computer and with ready access to  Manuals and / or software.     A concentrated load of 4000 pounds is to be lifted using an overhead steel beam  about 8" deep and about 5" wide (I am presuming this is a  W8x18 beam)  The load may be at midspan.   I know this beam will not be safe. Quick back of the envelope calcs tell me the BM is 25 kip feet. I don't have the properties of the section at hand. The bending stress may be less than 0.6 x 50ksi But top flange flange stability will be the issue here as the beam is not held laterally any where within the span.   Questions: 1)If not 4000 pounds, how much can this beam lift safely? 2)What minimum size of beam is needed to lift this safely. Deflection is not an issue. The load is temporary.   3)If we use two such beams running parallel to one another and separated by about 3'  for the entire span and link them at 3  intermediate points with cross beams of 3' span each how much can be lifted safely assuming  a) the load is shared by the beams and b) assuming the load is not shared by the two beams but suspended from the bottom flange of one of two beams only with the other beam and cross beams merely used to offer lateral support to the top flange of the beam that takes the load?   If any of you can send me an answer in the next twelve hours from now, I would appreciate the help. It's past 10 pm in India now  I am off to bed. I need to answer this first thing in the morning and I still wont have access to my regular computer and my software and tables and manuals.   Thanks in advance.   Regards Vish

[Harold Sprague]

Your questions require more info.

 

1)If not 4000 pounds, how much can this beam lift safely?

What is the span?  Back calculating your bending moment it appears you have a 25 ft span.  Correct me if I am wrong. 

Braced (how often) or unbraced?

A W 8x18 is the smallest 8" beam with a 5" flange.  The ASD moment is 26.5 kft and the LRFD moment is 39.9 kft.  (Msub rx)  Lsub r is 13.5 ft.  Using ASD you are OK, if you have a brace at about 13.5 ft max. 

 

The other questions are contingent on the span and bracing answers.

 

2)What minimum size of beam is needed to lift this safely. Deflection is not an issue.

The load is temporary.

 

3)If we use two such beams running parallel to one another and separated by about 3'  for the entire span and link them at 3  intermediate points with cross beams of 3' span each how much can be lifted safely assuming 

a) the load is shared by the beams and

b) assuming the load is not shared by the two beams but suspended from the bottom flange of one of two beams only with the other beam and cross beams merely used to offer lateral support to the top flange of the beam that takes the load?

Regards, Harold Sprague


 

---

Date: Thu, 19 Nov 2009 08:33:19 -0800
From: gvsh...@yahoo.com
Subject: Quick Help
To: sea...@seaint.org

---

Hotmail: Trusted email with Microsoft's powerful SPAM protection. Sign up now.

[G Vishwanath]

Thanks Mark and Harold for your quick response.   Yes,I inadvertently omitted to mention the span. You both correctly guessed it as 25 feet from the BM value.   Threre is no horizontal brace to restrict the unsupported length of the compression flange.I  I later got the opportunity to do the calcs.   W8x18 is confirmed as being hopelessly inadequate. However if the beam is somehow laterally held at mid third points , it will pass muster. I checked this out later.   Without lateral support the smallest beam that I could suggest was W12x30.   This is in response to a quick query from the field. They have a W8x18 ready which they can support on walls and they need to lift 4000 pounds just once and they don't want to spend too much money.   I have noted Mark Johnson's point also alerting me to the need to prevent rotation at the ends of beam resting on the wall. I am advising the field staff accordingly.   Thanks once again. Regards Vish

[Rhkr...@aol.com]

In a message dated 11/20/09 5:29:31 AM, gvsh...@yahoo.com writes:

This is in response to a quick query from the field. They have a W8x18 ready which they can support on walls and they need to lift 4000 pounds just once and they don't want to spend too much money.

AND they (presumably) don't want to kill anyone in the process.

Ralph

[Harold Sprague]

If you have the W shapes in the field, you might want to consider welding a channel cap the same way you would if you were designing the support for a crane.

Regards, Harold Sprague


 

---

Date: Fri, 20 Nov 2009 05:28:50 -0800
From: gvsh...@yahoo.com
Subject: Re: Quick Help
To: sprag...@hotmail.com; mark...@yahoo.com
CC: sea...@seaint.org

[Gary L. Hodgson and Assoc.]

If the load is applied to the bottom flange only, the beam is unlikely to buckle sideways.  However as the building code (here in Canada) does not differentiate between points of load application, I would be obliged to brace the beam laterally, based on Limit States Design.  One brace in the centre will work if using 50ksi steel.  However, using A36 steel, I would need 2 brace points.  This last one was done rather quickly using old fashioned ASD, so don't hold me to it.
Gary

******* ****** ******* ******** ******* ******* ******* *** * 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 seai...@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 ******* ****** ****** ****** ******* ****** ****** ********

[David Topete]

I don't believe the point of load application matters as much as the stresses in the beam.  if you sit a post on a beam or hang a load from the bottom flange, the top flange will still be in compression while the bottom flange is in tension.  therefore lateral bracing points are still to be added to avoid the buckling.  unless I am missing something in the discussion...

--
David Topete, SE

[Gary L. Hodgson and Assoc.]

Actually, some research has been done on this, but I cannot remember the names of the authors except Nethercote and Galambos.  If you apply a load to the top flange, i.e. above the neutral axis, the load will magnify any top flange lateral buckling due to any eccentricities (P x e_sub_x). On the other hand, a load applied to the bottom flange( or below the neutral axis) tends to pull the beam back into toward its vertical axis.  If you wish I can send you a copy of a page from Galambos that gives K factors for various beam support situations depending on where the load is applied.
Regards, Gary