cmu wall overhang 1

[Whyask]

I am the EOR on a residential house in Florida. The mason fixed an out of line monolithic slab by overhanging the cmu up to 1" off the edge of the slab. This allowed him to keep the walls straight and plumb. The building department has stopped construction pending a fix from me.

I have not been able to find the acceptable overhang tolerance in ACI530-05 or the Fl Buiding Code.

What is allowed and what is a typical fix short of adding to the foundation?

Thanks

 

[rowingengineer]

is the wall fully cored filled?

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[Whyask]

No it is a partially grouted load bearing wall, 10' tall with 8" cmu.

 

[JAE]

Not sure about Florida code rules but here's a few thoughts:
1. You are the EOR on a house. Most houses are designed and built to empirical design rules. When an engineer is involved, the engineer can and should use engineering principles to check problem areas. If the checks work out, you should be able to sign off on them.

2. The eccentricity in the wall bearing (i.e. the slab doesn't fully cover the width of the wall) creates some checks that I would do..namely -
a) Check compressive bearing on the masonry since only a part of the block transfers load to the concrete slab.

b) Check the wall with the additional bending due to the eccentricity of the support.

c) Check lateral and longitudinal shear between the wall and the concrete - verifying that the proper shear transfer is provided.

d) Check bearing on the concrete - although I would doubt that this would control.

A possible fix might be to add steel plate brackets, epoxy doweled into the slab edge and bracketing the wall sides to ensure adequate connection between the two.

 

[asixth]

What load is bearing on the offset cmu wall over? Is it a timber framed roof or another concrete slab? Would the design have complied if 6" cmu were used instead of 8".

 

[dik]

I don't know about the codes in effect in your area, but the earlier masonry codes (prescribed methods) permitted a max overhang of 1" to act as a bit of a 'drip' at grade beams.

Dik

 

[Whyask]

Thanks JAE for the detailed checks.

Asixth - The wall is supporting a pre engineered shingle roof truss system with varying spans less than 30'. I suspect the wall will pass as 6" cmu and I will take a look at it that way first.

I spoke with the mason today and he claims in the past another engineer has directed him to fill the bottom 2' of the wall solid with grout in all areas with an overhang greater than 3/4".

I still need to find the tolerance allowed in a code book just so I can accurately reference it in a repair letter and make sure my repair or non repair is consistent with the code.

 

[ron9876]

I haven't seen this covered in the FBC.

 

[concretemasonry]

The 1" overhang or "corbelling" (a traditional term) usually requires a solid unit, but a partially filled 8" unit may fit within the paramters, especially since the first course is usually laid in a full mortar bed and there is partial grouting and reinforcement.

The key is to allow the loads to re-disribute themselves in the critical area. The solid unit requirement background of the corbelling limit is based on the fact that the traditional brick is only 2 1/4" high and not 8" high, so a solidly filled first course is more than adequate and a partially filled unit should also qualify.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[msquared48]

There should be a continuous 8" minimum deep bond beam at the overhang, and the CMU wall thickness should be around 1.25", less than the 1" minimum overhang, so there is some bearing at the outside wall without the grout, but a significant reduction nevertheless.

Other than the extra eccentricity to check, I do not see a major problem here.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask