Undersized Loft Beam

[Contraflexure74]

Hi. I have an undersized loft beam which was installed in error. Beam is at wallplate level. I need to increase it's strength to get it to work in terms of buckling and deflection. The following thoughts are coming to mind.

A. Take down ceiling, Jack the beam and weld on a T section to increase I value. House is live so trying to avoid site welding.
B. Remove existing beam through roof and replace with bigger beam.
C. Jack beam. Reduce span of beam in question by propping beam off of 1st floor internal wall. The Internal Wall is brick from ground to 1st and stud from 1st to wall plate level. I was going to introduce a steel box section column in the 1st floor stud wall build off of the top brick to prop the undersized beam.

The beam which requires strengthening supports 2 other steel beams at the quarter points of the span. These 2 beams run perpendicular to the beam requiring strengthening.

Any clever ideas welcome.

 

[BridgeSmith]

Can you use the 2 other steel beams to provide bracing? Won't help you with deflection, but may increase bending capacity if it's currently subject to LTB.

If space and access allows, possibly slipping a second beam of the same size next to the undersized beam. That would double the capacity and halve the deflection.

 

[JAE]

I assume your beam is a steel wide flange? You didn't say but welding to it implies steel at least.

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[msquared48]

Can you bolt two smaller depth channels on the lower height of the web to increase your allowable bending, or add an intermediate column?

Mike McCann, PE, SE (WA)

 

[Contraflexure74]

Hotrod 10. The 2 beams sit on the top flange of the undersized beam (not bolted) but on the opposite end they sit on a party wall so not sure you can say Le is reduced. As you say deflection will still be an issue. I like your idea of sliding a second beam in beside the existing. Would you attempt to connect these together or is there any need?

JAE, Yes beam to be strengthened is a steel UC section.

 

[Contraflexure74]

Msquared48 I can only access 1 side of the existing beam as it is tight against a party wall. See point c of my initial post re possible intermediate support.

 

[milkshakelake]

For the intermediate support, you have to evaluate if the magnitude of your loading would crack the brick wall and their foundations. I generally don't trust old brick walls and their foundations, but that's your judgment on this specific brick wall. You can run an axial load and bearing calculation on it. If you can remove a part of it, put in a new post and footing, and fill it back up with brick, that would be better. Or pour concrete around where the steel will sit, dowel it into the brick, and make a little footing for it, like a concrete pier.

About sliding in a second beam, there's no reason to connect the two. Since you're unloading the original beam, you might have an issue where the new one is deflecting under load and the other one isn't, so the floor finish could crack. So use a stiff beam for the new one.

 

[BridgeSmith]

If the 2 beams are supported at the same elevation at the ends and have the same moment of inertia, they should share the load fairly evenly and deflect fairly evenly, as well. If you connect them together, you ensure they deflect the same. Whether there is an advantage to connecting them together for stability depends on whether the individual beams are subject to lateral torsional buckling (LTB). If the reduction in buckling strength is small or the beam capacities aren't reduced below yield, then there is no advantage to connecting them together. Connected together there would likely be no reduction for LTB.

 

[Contraflexure74]

Milkshakelake are you suggesting that the original beam could have already cambered downward as it was undersized? In effect this now could be deemed defunct as it may not be transferring any load once the new beam is installed?

 

[milkshakelake]

Contraflexure74 - I was thinking the opposite, but that's a good point. I was thinking that once you remove the load, the original beam could go back to "zero" camber since the deflection is elastic. I'm honestly not that experienced (9 years) and I'm not sure at what point the deflection becomes inelastic and permanent, so I take back that part of my comment.

I'd put a new support at the middle because it's cheaper than adding a new beam. A new beam would involve shoring the existing floor system, cutting the existing beams, finding a way to get a new beam into that floor, welding the existing beams to the new one, and removing the shoring.

 

[BridgeSmith]

There shouldn't be any permanent camber unless the installed beam has been stressed beyond yielding. If it's still within its elastic range, there's no permanent camber.

If adding the new beam entails what milkshakelake suggests, I'd agree that it's probably not worth it. I assumed it would be an option only if it could be installed from underneath, with one end put in place, the other end brought up to the bearing height with the beam at an angle, and rotated horizontally into place.

 

[CANPRO]

What is the source of load on the undersized beam, is it just from the 2 beams sitting on top? You could break the span of those two beams with a new beam to reduce the load on the existing (if its easier than installing a new beam directly beside existing).

 

[Contraflexure74]

Thanks for all you're comments. The column solution would involve extending 2 levels through 1st floor and ground floor as the ground floor is a suspended timber floor. I would have thought sliding the second beam in through a localised hole on the roof would be less labour intensive.