Allowable vertical deflection of blockwork wall panels 5

[damo74]

Hi all.

I'm designing a single storey basement car park, supporting 2 storey blockwork houses on top. I'm using a transfer slab to support the dwellings, which is supported on reinforced concrete columns and retaining walls. I've designed my slab within acceptable deflection limits (L/500 to B.S 8110). However, I'm concerned about the blockwork over it cracking, because of the slab deflection. I've read that blockwork panels crack at much lower differential settlement values (L/2500).

1) Can anybody throw some light as to what limits blockwork panels crack at?

2) Or any measures I can take to prevent it doing so?

3) If I used columns and beams instead of a transfer slab, to support my blockwork dwelling walls, the code allows the above limit of L/500 for members supporting brittle materials. Surely this would also be well over the acceptable limit stated above (L/2500). However, this form of construction is used extensively throughout Ireland and I'm sure everywhere else. Any advice?

 

[Wopshistos]

Not convinced that you have a real problem here.
If you were designing a steel beam to support the wall L/360 would be acceptable as a calculated deflection but this is assumed to be only the live load fraction.
In domestic construcion live loads are small relative to the structural mass and, in fact, block walls contribute to stiffness.
Timing is all important in your calculation as most deflections occur during construction. Walls only tend to crack under larger differential movements & Dead load effects are not necessarily cumilative in that case.
Precautionary bed joint reinforcement is advised if you are still hesitant.

 

[damo74]

Wopshistos,

Thanks for the advice.

I agree with your point about replacing the slab with beams and calculating the anticipated deflections (it was my logic also!)and I was going to specify bed reinforcement.

However, I should point out that my calculated deflections are expected non-linear deflections taken from a finite element analysis program, so they take into account creep. Consequently, my deflections will increase by about 10mm as time passes after construction ceases. My typical block wall panel is 6500mm long. This gives me a span/deflection ratio of 650. This is my main cause for concern. Any further advice?

 

[UcfSE]

In the US our reference code ACI 530 limits us to the lesser of L/600 or 0.3 inches. L/2500 seems a bit excessive. Some engineers drop the 0.3-inch requirement if the masonry is reinforced but always design to L/600 minimum.

 

[haynewp]

I am afraid your deflection of L/650 for a 2500 mm block wall will be enough to crack it.

 

[damo74]

haynewp,

The length of my wall is 6500mm and my limiting deflection ratio is L/500. This gives me 13mm. Do you feel this is too much?

 

[haynewp]

That's over 1/2" which is higher than I would allow (I think in inches).

I would look at if there are there any control joints in the span, or making the wall act compositely with a concrete support beam.

 

[pba]

A take it the walls are load bearing masonry?

If this was a normal house built off strip footings in the ground the geotechnical report work give you an allowable bearing pressure based on a total settlement (usually 25mm) and a presumed differential settlement of about half this.

Strip footings are usually mass concrete and don't 'soften' the differential settlement in the same way as a continuously reinforced slab.

However there are not any live load deflections on strip footings. These are what could lead to cracking in the masonry and these should also be calculated.

Bed joint reinforcement is a good idea and should be placed in each block course immediately above the slab and at alternate courses at higher levels. You need the expanded metal lath type to control cracking rather than the solid wire 'ladders'. Expamet and BRC are possible suppliers and will give you advice.

For the record unless I'm pressured to do anything else I check both beams and slabs for L/250 total load deflection. L/360 imposed load deflection and L/500 total load deflection for members supporting masonry...

 

[damo74]

pba,

Yes the walls aare loadbearing. I was using the same logic as yourself. I've already been in contact with BRC. Thanks for teh advice.