Load Bearing Clay Brick Wall

[asixth]

Hi guys,

I need to provide a concrete stair that is adjacent to a historic building to provide new access. The building itself is classified a "historic landmark" so it cannot be touched.

The architect has the idea of using 110mm (4.5") wide loadbearing clay brick to support this stair so it has the same appearance as the landmark building which was built before 1900.

I do not know much about masonry design in general but I have done some numbers which show the clay brick wall has capacity for 93kN (9.3 tonnes / 20kipf) and the ultimate load from my analysis is 110kN (11 tonnes / 25kipf).

The wall dimensions are 2000 (7') wide and 3010 (10') high.

My questions are as follows:

1. What should I be looking at when designing loadbearing clay brick walls? I think a slenderness ratio has to be satisfied.

2. How can I strengthed this wall so it can support the load required? Can I provide reinforcement of any nature.

Just by looking at the section my natural instict is that the wall doesn't look right. I would rather provide a concrete masonry unit wall with a non-loadbearing clay brick vaneer.

All comments will be appreciated.

 

[hokie66]

asixth,

Go with your instinct. You can build the wall in brick, but not single skin. Check the robustness requirements of AS3700.

 

[csd72]

Could you provide brick piers to reduce the slenderness?

 

[kslee1000]

I am in-line with Hokie. CMU with vaneer is better/simpler solution.

 

[asixth]

Just an add-on question, my calcs show that the brickwall can support 93kN (20kipf or 9.3 tonnes). This seems like alot of load for that wall to support, is there any typically detailing available for brickwalls supporting concrete slabs?

 

[hokie66]

asixth,

You really can't connect the 110 single skin of brick to the slab, as there is no place to put reinforcing. My advice would be either to use a double brick wall, 230 thick; or otherwise use a 350 x 350 square pier at each end. In the piers, you can build in a vertical bar.

Tell the architect that it is not structurally honest to use a single skin bearing wall in a historic building, as they would never have used just a single skin in those days.

 

[oxbridge]

I think you'll get lateral forces at the top of the wall due to the geometry of the stair. To deal with this often the top of the wall is taken to be a roller allowing the staircase in tension to restrain the load (providing you can transfer the load at the other end). In practice, I'm not convined you can get it to behave as a roller though. I've seen engineers specify slip membranes to creat a roller effect but not sure how successful these are.

 

[hokie66]

I don't think that lateral force or movement of the landing is an issue. The thickened section at the bottom will be restrained by the slab, and the stair itself is quite stiff.

 

[oxbridge]

I think it will come down to the stiffness of the joint between the landing and stair. I would do a quick model and take the wall support as a roller and see what lateral movoment you get. then if it's less than the allowable deflection at the top of a wall (about h/300) then you're ok

 

[asixth]

oxbridge,

I remember making a crude model, mainly so I could estimate the deflection and moments for the stairs. I know I didn't make an accurate assumption of the brickwork stiffness and my model was getting 12mm deflection (0.5") at the top of wall. I have not modelled any moment transfer into the wall so it esentially acts as a roller support.

hokie,

It is attaching to the side of the old grandstand at the exhibition grounds. The inside skin will not be visible from any direction and I have to completely isolate it from the existing stand, I just want to make it appear as one structure.

 

[apsix]

If you want to match the existing brickwork it is unlikely that stretcher bond will be accurate.
It will possibly be English or Colonial bond (see

http://www.behelp.com/route50/architecture/brick_terms.htm).

Unless stability is a particular problem an unreinforced 230mm thick wall should be fine. As per above, 110mm thick is too slender.

 

[asixth]

OK, I have provided a load bearing CMU wall and made the brick masonry balustrade twin leaf.

My question is:

Is this an appropriate solution for the architectural requirements I was briefed?

That being, a clay brick masonry appearance that cannot tie in with the existing structure.

I have heard stories of engineers providing a 100mm (4") cavity between brick leafs, providing steel reinforcing and then core-filling to get more capacity out of the brick walls.

 

[hokie66]

Well, my opinion on the bearing wall remains the same as I stated above. There is no reason why it cannot be double brick, so a 230 brick wall. No real reason to introduce blocks, although it will work that way.

The cheek wall (the one beside the flight) can be non load bearing, but it has to be braced at the flight. As well, the balustrade itself cannot be unreinforced brickwork. Unreinforced brick cantilevers should be avoided. What happens to the balustrade on the landing?

It is no story, you can reinforce a brick masonry wall in the manner you described. You need some ties across the cavity.

 

[asixth]

The sketch is not 100% correct, there will be no block masonry on the landing, it is being replaced by a steel balustrade.

When you mention 230 brickwall, that is two skins of 4.5" brick (110 brick), this only leaves room for a 10mm cavity, I would image that the cavity would need to be reinforced with a minimum of N12-400 each way if core-filled.

Do you know of any clay brick institute websites on the web that have some good reference material. I found the clay brick paver institute of Australia to be lacking detail?

Is it possible to make the inside wall loadbearing and constructable at the same time. I image you can downturn the stair slab onto the wall to match the stretcher bond however, the wall that would then sit onto the stairs would need to be cut to suit the stairs.

 

[hokie66]

The 230 wall I mentioned was just the bearing wall under the landing, and I don't think that wall needs to be reinforced. The wall that extends above the flight would need to be reinforced, and yes, that would require a thicker wall, say 350. Not a very sensible way to build a balustrade, but the bricks can be cut to suit. Doesn't the old grandstand have metal railings?

I will try to find some reference material for you.

 

[BAretired]

You could use a pair of steel columns cross braced together for lateral stability. These could be clad with brick on each side, possibly notching the bricks at the columns to reduce overall wall thickness.

Or you could build three walls under the stair such that the concrete is cantilevered, say 600 mm on three sides. This would permit a reduction of about 3" in slab thickness. The steel railings would then be fastened to the concrete.


BA

 

[asixth]

I found a picture on google and have highlighted the area where we are providing new access stairs. There is a steel balustrade that runs right around the stand. A section of this handrail will need to be removed to allow our stairs to come in. The stand at the bottom of the picture has already been demolished.

 

[BAretired]

There are many ways you could support the stair. Hokie66 has suggested some. There are other possibilities, but a 4.5" thick load bearing clay brick wall is not one of them.


BA

 

[apsix]

For the balustrade loads that you are required to design to I would think about an RC wall (150? thick) cantilevering off the stair, with a brick leaf to the outside face, continuing up from the brick below.

A point to watch if supporting the stair on mixed block/brick. Concrete shrinkage and brick growth could result in the brick carrying all the load, it's best if there is asoft joint at the top of the brick.

 

[BAretired]

asixth,

Here is another idea (see attached).


BA