Timber fixing 1

[Aleeeex]

I want to install a timber trimmer flush with the face of an existing external timber wall. Should I size the trimmer based on the spacing 900mm if I fix the trimmer at 900mm spacing? for example, as continuous beam 900mm each span?

Would you recommend using screws or bolts for fixing? If the shear force is 2kN and the tensile force is 1kN. The timber trimmer is intended to support floor joists for a new extension.

Please let me know if I am missing something to consider?

 

[driftLimiter]

This is done all the time. Things to consider are flexure of the trimmer (ledger) between fastener points, fastener strength (going through sheathing?) required joist bearing etc. However as it appears to be a load bearing element, I would say that this is a life safety connection and should be designed by a qualified professional. There have been many a failure, lawsuits, and even deaths due to bad ledger connection details.

 

[Aleeeex]

In your opinion would you use Bolts e.g M16 at 600mm spacing, rather than screws?

The joists are going to be fixed on joist hangers which are supported by the trimmer. The bolts should be connected to existing trimmers going through the sheathing at 600mm spacing.

Based on my calcs, we have 0.5kN/m dead load and 1.5kN/m Imposed load. The new trimmer is 50 x 150mm. So the shear total load is 2kN/m times the bolts spacing 600mm which is 2kN/m x 0.6m = 1.2kN shear.

Also, working out the tensile force assuming the load applied at 50mm from the new trimmer 2kN/m x 0.05m divided by half the depth of the trimmer 150mm/2. This results in 1.33kN.

The shear capacity of M16 bolts to connect the trimmer 47mm with the existing 38mm thick would 2.569kN which is greater 1.2kN.

I am not sure about the bolt tensile capacity surely cant use 70.3kN.

 

[phamENG]

Bolts are nice, but may be hard to install depending on what's going on on the other side of the wall/beam. Here in the States we have manufacturers who have designed specialty screws specifically for this application. I'd be surprised if Simpson or an equivalent isn't operating in your market.

 

[Aleeeex]

Thank you phamEng,

I spoke with Simpson earlier and they would recommend SDW type structural screws.

https://www.strongtie.co.uk/en-UK/products/structural-wood-screw-sdw#2

In their link I can only see Characteristic tensile capacity 21.4kN but not sure about the shear capacity?

I am not that keen to use screws, however, if I have to I would use SDW type structural screws e.g SDW22438-R50E at 300mm spacing.

Do you think that is sufficient?

 

[phamENG]

Wow. That's a terrible website and pretty sparse technical information. Here's an example of what they give us to work with: Simpson SDWS Screws

I really can't say if it's sufficient or not. I'm not sure if they are different from our SDW screws, and I'm not sure about the size of the addition your designing. But if you're not comfortable with screws, go with bolts.

 

[phamENG]

Unless I'm missing something, tension demand on the fasteners will be negligible. If you're worried, though, I'd look at a) the tensile capacity of the bolt, b) the bearing capacity of the washer(s) on the wood, c) the torsional capacity of the beam, etc. etc.

 

[Aleeeex]

This connection is a little awkward. I should also check the lateral load carrying and splitting capacity. Connecting a 60 x 200mm trimmer with M16 at 600mm spacing to existing 2x38x225 joists will fail in the shear plane.

I am not sure using M20 is a good idea. Has anyone came across these sort of connections before? I've previously made quite few connections between timber trimmer and existing block wall, but never with existing timber.

Please any suggestions.

 

[driftLimiter]

I almost always use the SDWS screws. I'm not sure how you expect to to close bolts. you could try lag screws?

 

[hokie66]

The existing wall is said to be external. Is the new floor to be internal or external? If external, you need to rethink.

 

[Aleeeex]

Yes it is external floor and I considered service class 3 in my design. I am not convinced with screws, it just about working using bolts. I only managed to work it out using M16 bolts in 2 rows, so that is the only way it will work. However, I am not sure how the contractor is going to close the bolts. Any thoughts?

 

[driftLimiter]

If your detail is accurate and they used a double rim with a band joist then the access to the bolts is pretty obstructed. I think if your feeling off about screws you aught to look more into the standard practice. Screws have good performance for this application which is backed up by testing.

Here is an example [link LINK]https://www.deckbuilderoutlet.com/simpsonledger.pdf[/url]

I suppose if your adamant about the bolts you could bore out that band joist to make an access hole at your bolt locations. Big enough to fit a decent plate washer, nut and driver into. Then some chap has to stand outside with a spanner and hold it while another closes it from inside?

 

[phamENG]

Then some chap has to stand outside with a spanner and hold it while another closes it from inside?

Aren't you from Nevada?

OP - can you expand your detail a little? I'm not really sure what's going on. Is that a beam in the wall? A header over a window? What's on the other side? Open space, a floor system? Unless we know a bit more about the rest of the circumstances and potential restrictions, span of the new floor, proposed loading, etc, we won't be able to make any constructive contributions.

 

[driftLimiter]

@phamENG yes I am. I guess the OP's metric mumbo jumbo triggered my euro lingo, I promise I won't start using periods instead of commas to indicate 1000's .

OP could always ask builder what their preferred method of attachment is. Seems to me something from the outside would be much preferred by like Pham says its hard to tell from this detail alone.

 

[Aleeeex]

I hope these sketches help you understand things better.

 

[Said the Sky]

whats below your proposed ledger? it seems something is there or could be added to go down to the concrete foundation. Can you add a blocking in there from the interior? then you can use screws and still sleep at night.

 

[hokie66]

How do you make that detail waterproof?

 

[DaveHolder]

Bolts should be used instead of screws, in my opinion. Also, request waterproofing details from the architect.

 

[Aleeeex]

Any more feedback would be greatly appreciated, guys.

 

[phamENG]

I'd be using heavy duty screws on this all day long. I have one architect I work with that refuses to allow bolts - he thinks the waterproofing details work better with screws. That's his game, so I won't argue. But screw installation is faster and easier, and there's significant testing that shows that they are reliable when installed correctly. Considering you probably can't install bolts correctly here, you need to either pick a fastener that can be installed correctly or expand your scope of work to dismantling more of the building to make bolts work.