Need a book? Engineering books recommendations...

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

Re: Steel beam--change of depth

[Subject Prev][Subject Next][Thread Prev][Thread Next]
> From: Michael Hemstad <mhemstad(--nospam--at)mbjeng.com>

> I have a project with existing steel roof girders (W27).  Due to vertical=

>  turning the W27's into heavy W14's.  As originally intended, this was to=

>  bottom flange.  My question is, what should be this overlap distance?

>  stress).  Deflection is acceptable; shear stresses are low.  Can anyone=
>  direct me to a reference discussing the required overlap?  With 12 inches=
>  clear between the new and existing bottom flanges, I currently have=
>  settled on 24 inches of overlap as providing a gradual enough transition=
>  that I hope to avoid a vertical stiffener.  Does anyone know of an=
>  accepted technique to actually calculate this length?

No references come to mind. Search research publications from ASCE or
AISC or similar.

There are more variables to consider than just overlap distance to
transfer the bottom flange forces.

If this is a continuous beam, you may have some bottom flange stability
issues if there is ever compression in the bottom flange.

I agree with your, and others, assessment that 2x the difference is a
conservative, low effort overlap to develop a) strong-axis section
transition and b) shear lag from stiffener to lower flange.

Another consideration is the size of stiffener longitudinal weld that is
required. Longer with single-side, single-pass, top fillet vs. short
with double-side fillets. At 24" this may not be an issue on the radar.

I disagree that you can avoid vertical stiffeners IF the inner web
extension stiffeners are parallel to the lower flange. However, there is
an option that resolves bottom flange stability and shear lag ...

> From: "Kestner, James W." <jkestner(--nospam--at)somervilleinc.com>

> Another approach could be to connect the two flanges at approx. 30 =
> degrees with full pen welds and provide vertical stiffeners at each end =
> where they connect with the horizontal flanges.
> =20
> Jim K.

Taper the inner flange extension stiffeners from the inner flange to the
outer flange at 2:1. Same stiffeners, same length, added welds at one
end.

You can do this whether you cut the web square at the transition or you
could cut the web to match the taper and use a single piece flange
replacement (more cutting, reduced welds?). Full strength welds at each
end (no calculating).

You could check the transition forces to determine if you require
minimal stiffeners or you could simply put some in. Without the
stiffeners you will probably need to check the web-flange weld.

Regards
Paul
-- 
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

******* ****** ******* ******** ******* ******* ******* ***
*   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 
******* ****** ****** ****** ******* ****** ****** ********