Cutting Members Under Load 1

[TLHS]

I'm looking at a situation where someone is cutting back a beam upper flange. New plates are being welded perpendicular to the web a few inches down to replace the flange and then the upper flange is being removed for a distance. The math all makes sense when you're looking at the final in service loads, but I'm having a hard time deciding what happens if they don't shore this thing and then cut that flange. All the load will shift to the new top flange and the original top flange will ever so slightly open up, but what does that energy release look like. It's effectively a prestress being released.

My gut feeling is that removing any reasonable live load is probably enough for something like this.

I've notched flanges under load but I don't think I've ever seen this exact situation where you're going to cut through and release that flange stress.

Potentially the thing to do here is to see what the change in deflection/rotation is under dead load and get a feel for the magnitude of the movement that will happen.

 

[SWComposites]

if there is a doubt, shore it.

 

[Enable]

Depends how they cut it. Torching it quickly will be vastly different than taking a long time with a reciprocating saw (more time for stress to redistribute).

Looking only at the section and not ancillary stuff (e.g. masonry wall being supported) you could consider the transfer as an impact load and amplify the forces in your new plates / weldments accordingly. It would probably be prudent to do at least some amplification for this reason anyways.

 

[WestLevel]

Well I would say you need to be careful of cutting any section in tension. It can violently fracture before you cut through it and it likely will not give warning.

 

[milkshakelake]

If you cut the flange starting from the ends, there's less moment there (assuming pin-pinned, positive moment), which means less tension. It's not like cutting a cable, where tension is constant throughout the thing. So the risk increases as you get towards the center, but you give the beam enough time to settle as you're doing it.

 

[WestLevel]

Just consider that if you alter a beam in service the loading may be shed elsewhere if the stiffness is changed. So consider the system as a whole also and how the construction sequence may affect it.

Easiest way is to just shore it.

If your client cant shore it you can design something to destress the section during the cut, but that is usually very expensive.

 

[Tomfh]

It's more akin to cutting a bonded tendon, as opposed to cutting a free tendon. There's only so much energy stored in the little bit you're cutting.