tube bending moments

dangeruss

dangeruss

Well-known member
Joined
May 12, 2010
Messages
1,303
Reaction score
0
Location
Rutland
Hi
Could anyone involved with scaffold design give me the formula for working out the bending moment of a normal galv tube.

A friend did give me the formula but I think I have forgotten it or written it down wrong

I need to find out the load before bending of a tube supported at 6m centres ( 20ft tube fixed either end)

Does load x tube length divided by four ring any bells ? or am I about to risk certain death !
 
Maybe this fella knows dangeruss


[ame=http://www.youtube.com/watch?v=m5CgIAsFNM8]SCAFFOLDER bending a 5 ft scaffolding tube - YouTube[/ame]
 
For a CPL (centre point load) to calculate the bending moment it's ( P x L ) / 4

For UDL (uniformly distributed load) to calculate the bending moment it's ( W x L ) / 8

P is the centre point load
W is the UDL load
L is the span

Be careful though over 6m there will be a significant bending moment just from the tubes own self weight and deflection over that size span will be a issue.

The above calcs give you the actual bending moment...the allowable bending moment is more complex but take 1 kNm as a rule of thumb.

---------- Post added at 09:10 PM ---------- Previous post was at 08:40 PM ----------

If you have an iPad look up ibeams on the App Store. It's a neat (and free) little app for calculating moments in different beams.
 
dangeruss said:
Hi
Could anyone involved with scaffold design give me the formula for working out the bending moment of a normal galv tube.

A friend did give me the formula but I think I have forgotten it or written it down wrong

I need to find out the load before bending of a tube supported at 6m centres ( 20ft tube fixed either end)

Does load x tube length divided by four ring any bells ? or am I about to risk certain death !
Click to expand...

As AxSD has indicated the Moment of Resistance is around 1.0kNm.
Working Backwards from the MR to find the allowable load to reach your bending moment not the same as finding the allowable load on a span.

However you may find you will reach your maximum allowable deflection long before you reach your maximum allowable bending.

Again as AxSD has indicated the self weight of the tube alone would give you around a 26mm sag (deflection) in your tube the allowable being around 22
 
Just be careful on your understanding of what UDL is...and you don't want to combine axial load and bending either or that'll really do you!
Also, have been using a lot of that 3.2mm high yield stuff lately - its stronger and lighter - a winner all round!

I imagine it just costs £1M more.

IDH.
 
Russ.... you should ave just called me back mate!!
 
You must log in or register to reply here.
Top Bottom