AS1684 Table 8.18(h) Method A - Installation of HD Rods

[asixth]

Hi team

I had a minor dispute with a builder on-site yesterday regarding the position of HD rods in plywood bracing walls. Reading through AS1684 it says that an M12 rod must be located at either end of a plywood bracing wall, and then shows an indicative illustration of the M12 rod located within the end of the plywood panel.

I saw on-site that the builder had located one of the tie down rods slightly outside the sheathed wall. I mentioned that the rod needs to be within the sheathed section of wall. The response from the builder was that the "rules" are that the rod can be +/- 100mm from the end of the sheathing. I couldn't find anywhere in AS1684 where it says that this is the case.

I would have thought that the HD rod must be located within the panel because the sheathing acts as the structural element. If the rod is located outside the panel then the top plate will need to work in weak axis bending. I know that a 90x45 on it's flat won't be able to transfer a 13kN hold down force.

While we are on the topic of AS1684. What is everyone's opinion on using the standard details and accompanying capacites in AS1684 for buildings other than houses (ie. not Class 1 or 10 buildings as defined in the BCA).

I know when these connections are solved from first principles (AS1720) that the capacity of the connection will be less than what is stated in AS1684. Also AS1720 has greater capacity reduction factors for structurs "other than houses" than it does for "houses".

What are others doing, are they using AS1684 for buildings other than Class 1 or 10 and where should the location of the hold down rod be when using the detail in Table 8.18(h) Method A?

All opinions are welcomed.

 

[OzEng80]

asixth

It would be my preference to locate the HD rod within the sheathed section of the wall but if this was just a 'one off' I would have just let it go. If you have a continuous top plate it is probably seeing very close to the same moment as it would have if the rod was within the sheathed section (ie the 'uplift' in the rod mainly gets into it via weak axis bending in the top plate (via a concentrated load in the end stud) so things really aren't that worse off). Haven't head or the +-100mm rule - but i wouldn't accept a HD rod being greater than 50mm from a stud without checking that it works.

I have used the details and capacities out of AS1684 in other classes of structures.

 

[asixth]

Thanks

How about fixing rafters to the top of bracing walls for load-bearing walls (ie external walls). I have requested that the rafters be fixed to internal bracing walls by detail (e) and (j) in AS1684 (see attachment). Additional blocking pieces haven't been provided for the external walls which are load bearing. The builder says that this is industry practice to do so.

I am having trouble justifying to myself that the ceiling diaphram force can be transferred into these bracing walls. The rafters are fixed to the external walls with 2 skewed nails for stability and GI strap for hold down.

Method (i) in AS1684 says that I can only transfer 1.1kN for rafters which are fixed to the top plate with 2-skewed nails. If I have 2400mm of bracing wall along the external wall and I am relying on the wall to take 15.4kN of bracing force, I require a minimum of 14 rafters each with 2 skew nails to transfer the bracing force.

I guess the question which I am asking is whether blocking pieces are provided for bracing walls which are load-bearing?

Please post your comments, I no longer work with the engineer of this building so I have assumed the position of design engineer and I am frantically teaching myself timber construction practice as I go along.

Regards

 

[OzEng80]

Contradicting statements but, yes blocking should be provided over loadbearing bracing walls (unless the method of fixing can take the load) AND it is industry practice not to do this...
In terms of getting the load into the top plate it is reasonable to assume that rafters beyond the 2.4m wall collect the load (ie the top plate is a (very restrained) compression member with accumulating load each side of the bracing wall). Hip ends obviously contribute greatly to getting the load into this member - so if you have a hipped roof you are probably OK.

I believe the justification for not providing blocking on external bracing walls is based on the presence of both ceiling and roof battens in this location preventing 'roll over of the truss/rafter' (don't think too hard about justifying a load path for the restraining members...). I can't recall whether the 1.1kN load was based on withdrawal of the nails or shear - but if you are willing to digest my previous statement than you can get the full shear capacity out of the nails.

The timber framing that i am typically involved with (I don't do small scale residential) has predominantly 'heavy' connections (brackets etc.) which permits a (fairly dubious) argument for an adequate load path.

I am looking forward to comments from others.

 

[asixth]

I have GI strap for the hold down. The shear of two nails is 1.1kN. If you go through the calcs in AS1720 it is actually 1.2kN (based on ?=0.8, 2.8mm dia nails and joint group JD4).

I am going to tell the builder to put the blocking in, regardless of it being industry practice to leave it out.

 

[asixth]

Thanks for the reply OzEng, it's a shame there wasn't more discussion on this topic. I have a timber frame inspection tomorrow so I will post photos of the framing and see whether the chippy has left out blocking to the external walls.

 

[apsix]

Sorry asixth.

It's those sort of reasons why I don't miss doing domestic construction.

 

[demayeng]

Bit late to this.

M12 rod outside of sheathed section: I would try and justify that Method B is ok. I think you are right that it needs to be within sheathed section but it's probably not worth fighting about.

I use AS1684 for structures other than housing, but I'm usually over for bracing and tiedown so I don't think it is an issue. In my view bracing is less of a concern than tiedown

Rafters: What is the blocking for in this case - are you using it to provide extra fixing to the top plate?
i don't think rollover is the issue here

 

[asixth]

cheers for the input, I let this one go on-site. However in future I will definitely be calling up threaded rod as hold-down from the top plate to the floor slab. I strapped every stud to the top and bottom plate to make them all act as rods and was simply too much labor and the undulations were visible in the plasterboard at strap locations.