What purpose does this buttress serve?

[cjard]

I'm converting a commercial building into residential. Part of the plan involves creating an opening 4000mm wide in a side wall, directly underneath a high up semi circular window that is also 4000mm at its widest point.

Here's a panoramic shot of the entire building - please note that this building is square, it's just the fishbowl effect that makes it look like the side walls (with the semi circle windows) are not parallel

Under the windows are brick buttresses that project into the building space by about 100mm, here's a slightly less blurry pic:

See full size image

As noted, I want to form a new opening under the semicircle windows in the front part of the building, which would involve . I sent pictures to one SE and asked for a quote to calc the steel beams I'd need as lintels and he said I couldn't form an opening there as it would necessitate removing the buttress, which would destabilize the wall and it would risk collapse.

The buttress is approximately 5m high, 500mm wide and 100mm into the room. The wall construction is engineering brick inner leaf, stone outer. The mortar is lime and the "cavity" is filled with a mix of lime and stone rubble. Every 4th brick course is a header course that laps onto occasional longer stones projecting into the "cavity" from the outside, so the wall seems quite well keyed together. Overall the wall is 450mm thick, 550mm at the buttress. At the highest point, above the window, the wall is 10m high. The ground is boulder clay. On the outside of the wall, coincidental with where the steel columns supporting the roof enter the wall is a larger buttress - the wall at this point is closer to 1 metre thick when the inner and outer buttresses are considered. The building was built in 1926

Now, the SE didn't really go into detail as to why loss of the buttress would be a disaster for the wall, so would some kind soul here be able to provide supplementary info? Do these buttresses under the semi circle windows really have such a huge effect on the stability of the wall?

Regards
CJ

 

[JAE]

Yes - they provide some measure of stability but nothing is impossible here. Supplemental reinforcement of the wall using added steel shapes can "frame" the wall and create load paths that would make up for the loss of the buttress.

It would appear that the corbeled sill under the arched window serves as a stiffening element across the window which allows the adjacent side walls, and the center buttress, to take out any lateral wind forces.

The challenges would include:
1. How to connect supplemental structural shapes to the brick to engage all wythes of the brick.
2. How to connect the base of the shapes to the floor or foundation to take out the loads.
3. Would added structural shapes be in the way of the space usage inside the building.
4. Would the shapes be too ugly for the usage (hey - I used to aspire to being an architect so I do have some small level of aesthetic sensitivity).
5. How much would all this supplemental framing cost relative to your budget.



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[JedClampett]

Who knows? I wouldn't want to seal a drawing and be wrong about it.
Normally there would be a support (heavier built up section) on either side of the window. Apparently the original designer made a different choice with one buttress centered under the window.
If you absolutely have to put an opening there, tear down the wall to the roof line and rebuild it.

 

[cjard]

Thanks JAE..

An additional question, then: the structure going inside this building is a pair of timber frame houses, but the design of them is such that quite a bit of steelwork is being employed, together with an additional set of reinforced foundations set into the existing slab. Is it conceptually possible that the steelwork (or possibly additions to it) for the new structure could provide the equivalent support that the buttress is currently providing? From a maths and geometry viewpoint, surely a small buttress such as this can only have a relatively minor stiffening effect - could that same effect be achieved by connecting the wall to an extensive network of beams and columns behind the wall?

Jed - How does rebuilding the wall help?

 

[JAE]

Yes possible.

Just keep in mind what the buttress does for the wall in terms of strength and stiffness. If you remove the buttress, then what you replace it with should mimic the buttress effect.

The key I think is to really understand the wall system and how it provides a load path for vertical gravity loads as well as lateral wind/seismic loads.

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[boo1]

ask the original SE

 

[cjard]

@jae - see that's probably the entire problem - i'm not sure what a 100mm pier does for a wall that is already 450mm thick. Local building codes say that to be effective against racking forces on a freestanding wall, the pier should effectively triple the wall thickness; this wall isn't freestanding (i can't find anything about use of piers on such a wall) and this pier takes a 450mm wall to 550mm.
Is the intention of the pier to help the wall resist wind loads by preventing it from buckling along its horizontal centreline? Surely though, the most effective position for such a restraint would be on the outside, because to fold the wall inwards would induce compressive forces on the outer skin and tensile on the inner. These bricks are held together with lime mortar, and this inner leaf hence appears to have little in the way of tensile strength.
If the intention of the pier is to help resist racking, why is it so narrow relative to the thickness of the rest of the wall?

@boo1 - as the building is just under 90 years old, I'd say it's a fair bet that the original SE is no longer with us.. :/

 

[JAE]

cjard - wind blows many directions - the wall will experience inward directed wind and outward directed wind suction.

Usually pilasters (pier) either simply increase stiffness and strength or they provide a ledge for some kind of beam support.

Without analyzing the wall I'm not sure I can speculate on what that pier/pilaster actually does for the wall.
If your first SE didn't want to mess with it (uncertain/unsure/not talented enough?) perhaps another SE might have better insight.





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[boo1]

original SE you talked too.

 

[JAE]

boo1, the "original" SE was just an engineer that the OP initially approached and has no other association with the project....at least that is the way I read it.

There's nothing wrong with approaching another SE for a second opinion as the first SE appears to have declined to help on the project.


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[cjard]

Thanks JAE; if you were local I'd have invited you round - alas I think you're USA, and this project is UK but your insight has been massively helpful!

 

[JAE]

If you can afford to fly me over there (all expenses paid) I'd be glad to help!



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