Retrofitting restraint straps to joists in existing 9inch brick wall

[structural2000]

I am a structural engineer looking to retrofit some new joists with restraint straps in an Existing 9 inch solid wall. The joists will span onto the face of a new wall plate bolted to the face of the existing wall at 400mm c/c. Always like to hear builders’ opinions on these things, as I like to make sure I’m specifying details that actually work and are easily buildable!

What are my options for retrofitting restraint straps? I have considered three options so far but not sure which is best and perhaps there are others:

- Provide restraint straps which are turned down and screwed to the back of the new wall plate (not sure if this is easily done). I would do this with engineered restraint straps to keep them as thin as possible.
- Take out a couple bricks locally to get the restraint strap into the wall turned up and brick back up. This seems reasonable to me, but again I don’t have to build the thing. This doesn’t seem perfect though as the straps can’t be screwed in place, but I suppose the straps could be plugged and screwed into the outer course (I.e. face fix and then brick up).
- Face fix the restraint straps to the walls, plugged and screwed. I have had builders suggest this before, however, I believe this is not typically recommended by NHBC and after running some quick numbers, plugged screws give pretty minimal pull out capacity so I’m not sure I could justify this theoretically.

Interested to get some opinions from people who have done this before.

 

[tony1851]

Have a look at this:

Experimental pull-out tests and design indications for strength anchors installed in masonry walls - Materials and Structures

This study deals with the identification of the mechanical behavior of chemical anchors embedded in masonry walls. 108 pull-out tests are carried out in five types of masonry walls built with clay brick or vertically perforated units with cement mortar. Different parameters are taken into...

link.springer.com

It's Italian research done about 5 years ago (in English) into direct pull-out tests of resin-fixed bolts into brickwork of various types (resin-fixed are far better that plastic-plugged screws).
I've used this several times for gallows brackets when inspectors say "what about the upper bolts pulling out because of the offset load?"
(If the link doesn't work, pm me and I'll email you the pdf).

 

[structural2000]

Thanks Tony, I agree that resin bolt fixings provide much greater resistance, but I am not aware of any restraint strap products that can be face fixed using resin bolts? Is this something you have seen before?

 

[Ron0z]

As an engineer, you may be interested in the eng-tips site.

 

[^woody^]

A situational sketch would help.
I'm a little confused with the terminology of "spanning on to the face" (is that face fix or bearing on top?
Wall plate (top of wall) or ledger plate (on face of wall)?
If the plate is bolted, isn't that doing the restraining?
Or are these lateral restraints perpendicular to the joists?

The problem with straps, is the relatively small fixing or restraining area, which is reliant on the masonry unit in that area being firm and itself restrained by the wall/units around it.

Face fixing with deep resin studs or expansive bolts into secure brick/block would normally be OK

Inserting the strap into the wall and hooking it would normally be best as long as the work involved does not loosen the area the strap will then hook to - but it's not impossible to form a reactively small hole to manoeuvre the strap in

For poor masonry, a fitting a wider plate or thick piece of ply to the wall to cover a wider area, and then to strap fixed to that can be done, but I'm not sure if that fits in with situation proposed. This can be useful for gable strapping.

 

[structural2000]

Thanks for your comments.

To clarify, yes it is technically a ledger plate bolted to the face of the wall with joist hangers. The lateral restraints are parallel to joists.

I agree that the ledger plate will probably restrain the wall, however, I need to be able to academically justify this and it requires the joist hangers being able to transfer tension into the joists and manufacturers only give a shear capacity for the hangers.

Sounds like cutting out a couple of bricks and then bricking in the restraint straps could be the way to go.