Residential Load Bearing Wall Removal (A basic task & possibly an ignorant question.)

[human909]

So I've started to pick up more jobs involving removing load bearing walls in residential (mostly timber) structures... Those who might recognise me from previous posts on Eng-Tips, hopefully recognise that I'm not an ignoramus. Though I would admit that I still have plenty to learn and improve regarding timber and residential construction. This question is almost phrased like a first year engineering student, I'm not that, but I want to keep things simple and basic. So with that out of the way...

What is an appropriate checklist for removing a load bearing wall in a 'generic' residential structure?

My rough checklist:

1. Like anything, chase your loads. Lets assume this is ground floor with a roof or a floor above.
2. Ensure the beam can connect to and pick up relevant roofing or floor members that were previously relying on the wall for support.
3. Calculate the loads and design a beam (timber or steel) of suitable depth to replace the load bearing wall. Ensure that floor and roof loads (if relevant) are captured)
4. Provide additional studs (timber/HSS) to support beam and transfer load to the floor level.
5. Provide suitable load path from the floor level to a footing. This could be a new timber stump, a new concrete spread footing or ground beam, or a small bored pier. (If you are lucky the exiting foundation might be suitable.)

So that is my rough checklist... Am I a long way off track? Anything additional?

Another quick question: for a residential concrete slab of unknown thickness with unknown locations of ground beams, how are people normally gathering suitable information?

 

[GeorgeTheCivilEngineer]

What we call 'buildability' - can the beam be physically installed? Does it need splices? How will the opening be propped? Will propping interfere? Are there significant services that need to be removed? You don't need the full answer to this, but some thought and warning to the Contractor.

Deflection will almost certainly control the design. Where are the brittle finishes? Will there be cumulative deflection to consider?

Consider what lateral stability was provided by the wall (even if it was parasitic rather than intentional) verses a opening. Is a moment frame or box frame required?

And I could add a few more for masonry wall removals, which are much more common in the UK...

Another quick question: for a residential concrete slab of unknown thickness with unknown locations of ground beams, how are people normally gathering suitable information?

Hmmm... trial pit at edge to establish the ground beam dimensions, then drill holes and measure the thicknesses? It may be worth getting a GPR scan though.

 

[human909]

What we call 'buildability' - can the beam be physically installed? Does it need splices? How will the opening be propped? Will propping interfere? Are there significant services that need to be removed? You don't need the full answer to this, but some thought and warning to the Contractor.

Great points. I'm normally on top of this in the areas than I'm more experience in. But this is quite relevant here. I'm going to assume and design assuming no splices. Propping is pretty critical here, I believe the contractor who engaged me will have this covered but I should ensure this is the case. (I've done plenty of propping and temporary works in other circumstances but like I say I'm not a highly experience residential engineer.

Deflection will almost certainly control the design. Where are the brittle finishes? Will there be cumulative deflection to consider?

I think the biggest in the case I have currently in mind is the wall of the upper story above. It isn't hard to design a stiff beam. What is certainly harder is to design it such that you can install suitable such that the deflection after removal of the props doesn't cause issues to the stiff plasterboard/drywall wall above.

And I could add a few more for masonry wall removals, which are much more common in the UK...

I hadn't actually considered that... The job that sparked this question is a double brick exterior. The internal wall could actually be masonry (i assumed as stud wall). Which does makes things more complicated.

Hmmm... trial pit at edge to establish the ground beam dimensions, then drill holes and measure the thicknesses? It may be worth getting a GPR scan though.

Thanks I was thinking all those approaches. Though there are limited budgets to throw everything at the problem. But I'll consider them as appropriate.

 

[TRAK.Structural]

There are some detailing considerations. Flush beam vs. dropped beam. If flush, are you cutting through ceiling joists that act as a tension tie for the roof? Proper detailing for disturbed elements with flush beams can sometimes be hairy.

I almost always put new foundation supports under the stud packs at the beam ends, figuring out the size of the existing foundations can be tough and for residential construction they are almost always tiny.

For slab foundations and LIGHT post loads from the beam ends you could drill a few pilot holes to establish depth and check for point load capacity. It's usually more expense than it's worth to propose GPR and a full structural survey of the slab in the areas of interest unless its high end residential. If there isn't too much interference with existing interior walls/furnishings than its not super hard to cut the slab and install a new spread footing.

 

[Eng16080]

Consider what lateral stability was provided by the wall (even if it was parasitic rather than intentional) verses a opening. Is a moment frame or box frame required?

I'll second this. Overall your checklist seems fine for gravity loads, but make sure you're not neglecting lateral. I've had a few of these projects where I determined that the wall to be removed was acting as a shear wall and a steel moment frame was needed to preserve the lateral strength.

 

[W460x68]

For instances where steel or wood beams will work, I will often spec both to allow the contractor the option to select between a steel beam or an LVL beam. I will make sure to give support post details for both options. Apparently steel beams can be cheaper than multi-ply deep LVL's...

 

[BridgeSmith]

Consider what lateral stability was provided by the wall (even if it was parasitic rather than intentional) verses a opening. Is a moment frame or box frame required?

If mitigation of the lateral support lost is required, it may be possible to increase the shear capacity of adjacent wall sections, in lieu of designing and constructing a moment frame.

 

[human909]

Thanks all. And thanks for reiterating lateral. It was in the back of my mind when I started that checklist, but I forgot to include it.

There are some detailing considerations. Flush beam vs. dropped beam. If flush, are you cutting through ceiling joists that act as a tension tie for the roof? Proper detailing for disturbed elements with flush beams can sometimes be hairy.

A good point. In the particular case that prompted this question it is a floor beam. But some of your advice still applies with slightly different challenges. An inspection camera (which I don't currently have but are cheap) would be useful here if wall/ceiling can't be accessed to view the existing beam layout.

I almost always put new foundation supports under the stud packs at the beam ends, figuring out the size of the existing foundations can be tough and for residential construction they are almost always tiny.

That is my feeling and what I have done previously. I might have a concrete ground beam here in this circumstance, but still the loads will be quite concentrated so I'll be wary.

For slab foundations and LIGHT post loads from the beam ends you could drill a few pilot holes to establish depth and check for point load capacity.It's usually more expense than it's worth to propose GPR and a full structural survey of the slab in the areas of interest unless its high end residential.

I was considering the pilot hole approach, I've done this before on other slabs.

For instances where steel or wood beams will work, I will often spec both to allow the contractor the option to select between a steel beam or an LVL beam. I will make sure to give support post details for both options. Apparently steel beams can be cheaper than multi-ply deep LVL's...

For new builds I have a tendency to choose steel, but I'd normally offer both unless it is a moment frame. In this case I know the contractor (already engaged) would prefer a deep LVL if it can be done.

 

[milkshakelake]

I think everyone got it covered. One tiny consideration is lateral bracing of your new columns. I don't like to rely purely on the diaphragm. If you're using wood columns, I think some bridging would do the trick.

It's a bit more tricky with steel columns. First of all, do you even have to brace them? I found that AHJ's are lax in this regard. But if you decide you do, how far do you take it? 2% of the axial load? It can add up to quite a lot for a little house. Personally, I neglect it because it's been a non-issue. But some bridging helps me sleep at night.

 

[Greenalleycat]

So I've started to pick up more jobs involving removing load bearing walls in residential (mostly timber) structures... Those who might recognise me from previous posts on Eng-Tips, hopefully recognise that I'm not an ignoramus. Though I would admit that I still have plenty to learn and improve regarding timber and residential construction. This question is almost phrased like a first year engineering student, I'm not that, but I want to keep things simple and basic. So with that out of the way...

What is an appropriate checklist for removing a load bearing wall in a 'generic' residential structure?

My rough checklist:

1. Like anything, chase your loads. Lets assume this is ground floor with a roof or a floor above.
2. Ensure the beam can connect to and pick up relevant roofing or floor members that were previously relying on the wall for support.
3. Calculate the loads and design a beam (timber or steel) of suitable depth to replace the load bearing wall. Ensure that floor and roof loads (if relevant) are captured)
4. Provide additional studs (timber/HSS) to support beam and transfer load to the floor level.
5. Provide suitable load path from the floor level to a footing. This could be a new timber stump, a new concrete spread footing or ground beam, or a small bored pier. (If you are lucky the exiting foundation might be suitable.)

So that is my rough checklist... Am I a long way off track? Anything additional?

Another quick question: for a residential concrete slab of unknown thickness with unknown locations of ground beams, how are people normally gathering suitable information?

We do this a lot, you've got the main items covered

Other things to consider - will the beam be below the ceiling (usually much easier to install but less desirable to clients) or in the roof space (a PITA but looks better)
Be careful with beams in the roof space as you often get very tight to the outside edge - I have had 290mm beams that would need to be chamfered to 80mm to fit under the eaves
Technically this may calc out but I hate the way it looks and will push for a steel beam with an engineered chamfer that is far more reliable

Often I am vague on fixings to existing structure and note that it is to be confirmed on site by the engineer and builder
Otherwise you have to get up into the roof and figure out how everything is framed as best as you can
Hopefully they are dropping the ceiling at the same time so you can have a proper discussion with your builder about reconnecting everything to the new beam

Consider bracing of the structure as well - walls serve more purposes than just vertical load bearing!
You may need to offset with new bracing elsewhere in the building

RE Foundations: the onus is on you as the engineer to figure it out, so you have to get a rational basis if there aren't drawings (and even if there are, be careful with trusting them!)
Usually digging next to foundations will help figure out founding depths
Don't forget to do your shallow geotechnical testing to give you ground bearing parameters to check/design to
I wouldn't trust a slab only as I would assume that they (at best) stripped a little grass off the top and threw down their hardfill - bearing is likely to be dubious