Trusses against brick wall

[MortarThePoint]

15 hours ago, Gus Potter said:

Hi MortarThePoint.

 

The ledger option can work well but as Peter points out you get this twisting effect.

 

Often you get stuck with ledgers as you stuggle to get the fixing capacity. Many fixings are rated on, not least, being fixed away from the mortar bed etc. If you have perforated bricks say the capacity of the fixings really drops off. Expanding type anchors don't work well in perforated brick and there is little test data available.

 

I have attached a very quick sketch showing another way of doing this where you extend the bottom chord and pocket the wall. You can add glued hardwood packers to level up. Here you get a good solid bearing on the masonry and by using the dwangs / noggings you can add extra horizontal tying of the trusses into the existing wall.

 

You may need to beef up the bottom chord a little as it is acting in shear rather than bearing.

 

 

 

 

 

 

Thank, prettier drawings by hand than my computer ones, but my hand drawing skills are bettered by my 5yo.

 

The left hand mounting load of each truss is around 490N (49kgf). There are 7 trusses so that totals 3.5kN (350kgf). If I add a tank (dotted red) to this roof space as I am considering, it would up the total load by about 65% to 5.8kN (580kgf). If using a ledger board and lag bolts at average 300mm centres that would equate to around 16 bolts so 360N (36kgf) per lag bolt. Does that sound scary?

 

Truss mount load on left calc:

• Flat roof part: Fo = 1.719m*0.6m*13kg/m2*10N/kg = 134N
•                          Mo = (1.481m+0.5*1.719m)*134N = 314Nm
• Tiled roof part: Fb = 2.854m*0.6m*78kg/m2*10N/kg = 1336N
•                            Mb = 0.5*(1.481m - 0.450m)-1336N = 689Nm
• Truss selfweight: Fp = 22Kg*10N/kg = 220N
•                                Mp = (1.481m+0.5*1.719m)*220N = 515Nm (over estimate)
• Left hand mount: Mr = -(Mo+Mb+Mp) = -1518Nm
•                                Fr = Mr/3.1m = 490N (49kgf)

Edited April 4, 2021 by MortarThePoint

[PeterW]

Your loadings aren’t consistent though and you’ll get rotation about a point, which by the nature of the design is somewhere left of centre so is an issue. 
 

Also, what lateral restraint has the SEng specified as you should also be using cranked straps I expect to hold this all the the f

left wall. These are acting as ceiling binders by the looks of it. 
 

 

[MortarThePoint]

58 minutes ago, PeterW said:

Your loadings aren’t consistent though and you’ll get rotation about a point, which by the nature of the design is somewhere left of centre so is an issue. 

 

Yes the bottom chord forms the ceiling for below.

 

I calculated the load on the left hand mount (490N = 49kgf) by balancing the moments about the right hand mount. Balancing the overall forces gives the load on the right hand mount (wallplate) as 314N + 689N + 515N - 490N = 1028N = 103kgf (or about 1400N if I add a tank). Intuitively this makes sense as the largest force involved is that of the tiled roof part.

 

58 minutes ago, PeterW said:

Also, what lateral restraint has the SEng specified as you should also be using cranked straps I expect to hold this all the the f

left wall.

 

I have only involved the truss designer on this, not the SEng. The wall is 4.8m long cavity wall and continues up a storey. As far as I can tell, such a restraint straps serves to limit the possibility of the of the joist and the wall separating as shown by my red arrows on the image at the very bottom of this post. The lag bolts would resist a ledger board pulling away from the wall and then the joist would have to be strapped to the ledger board.

https://www.labc.co.uk/news/how-get-it-right-supporting-joists-external-walls

 

  

 

Edited April 4, 2021 by MortarThePoint

[MortarThePoint]

[PeterW]

Ok so I would be asking a structural engineer and BCO to comment and confirm what is needed as currently I think you’re missing some important elements. 

[MortarThePoint]

1 minute ago, PeterW said:

Ok so I would be asking a structural engineer and BCO to comment and confirm what is needed as currently I think you’re missing some important elements. 

 

OK, thanks because I do want to get it right

[MortarThePoint]

Unless I can face fix the restraint straps or use a ledger board and notch it to accept the strap between it and the masonry I'm in for a world of pain. The modern straps are quite thin so perhaps I can just pinch it between the ledger and the masonry.

 

 

I can't spy any restraint straps on this setup:

 

http://www.fourwalls-uk.com/blog/category/extension/page/3/

[PeterW]

Ok so they are floor joists and will be the lateral restraint. You’re looking at ceiling binders, what is holding the top of that right hand wall and stopping it being pushed away from the main building ..?? 

[MortarThePoint]

24 minutes ago, PeterW said:

Ok so they are floor joists and will be the lateral restraint. You’re looking at ceiling binders, what is holding the top of that right hand wall and stopping it being pushed away from the main building ..?? 

 

Yes, I can appreciate it needs something. I expect it will be:

Left wall --> ledger board --> truss bottom chord --> right wall

So I'd need to add straps at both ends of that chain

 

[DragsterDriver]

Usually the bottom chord wouldn’t have any extra length to ‘chop in’, and fitting a plate to the wall would change the span

 

I would 100% cut brick hangers in.

[MortarThePoint]

6 minutes ago, DragsterDriver said:

Usually the bottom chord wouldn’t have any extra length to ‘chop in’, and fitting a plate to the wall would change the span

 

I would 100% cut brick hangers in.

 

I discussed it all with the truss designer and he has shifted the left hand vertical to allow the truss bottom chord to be trimmed to suit a ledger board.

Edited April 5, 2021 by MortarThePoint

[Gus Potter]

A few points worth a look.

 

@MortarThePoint For a short cut you can ask the truss fabricator for the reactions at each end of the truss. Or you can go back and check your calcs. If you want you can show them all but it looks like you may not have included a roof access / snow load / wind load in your provisional calculations.

 

Often once the truss manufacturer knows they are getting the order they wil give you the support reactions. Make sure you know if these are factored or unfactored reactions (with or without safety factors) as this is important when selecting a hanger  / choosing the right fixing if using a ledger.

 

Often you find with something like this is all is going great until you try and select a hanger / ledger fixing. Many fixings are rated on being installed in uncracked concrete. Blocks are different and aerated concrete blocks are worse. You find that you can't get the shear  / tensile capacity of the fixings to support the loads.

 

Also tie straps are meant to be built into the blockwork as it is going up, their rating is based on this. Face fixing straps requires a good look, not least as they bend, rawl plugs don't have the tie capacity that a built in strap has.

 

In summary, ask what kind of blocks do you have and how can you fix, tie to them. You may find that the truss with the horn removes a lot of issues.

 

If you are in any doubt them please consult and SE.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[MortarThePoint]

3 minutes ago, Gus Potter said:

A few points worth a look.

 

@MortarThePoint For a short cut you can ask the truss fabricator for the reactions at each end of the truss. Or you can go back and check your calcs. If you want you can show them all but it looks like you may not have included a roof access / snow load / wind load in your provisional calculations.

 

Often once the truss manufacturer knows they are getting the order they wil give you the support reactions. Make sure you know if these are factored or unfactored reactions (with or without safety factors) as this is important when selecting a hanger  / choosing the right fixing if using a ledger.

 

Often you find with something like this is all is going great until you try and select a hanger / ledger fixing. Many fixings are rated on being installed in uncracked concrete. Blocks are different and aerated concrete blocks are worse. You find that you can't get the shear  / tensile capacity of the fixings to support the loads.

 

Also tie straps are meant to be built into the blockwork as it is going up, their rating is based on this. Face fixing straps requires a good look, not least as they bend, rawl plugs don't have the tie capacity that a built in strap has.

 

In summary, ask what kind of blocks do you have and how can you fix, tie to them. You may find that the truss with the horn removes a lot of issues.

 

If you are in any doubt them please consult and SE.

 

Thanks @Gus Potter You're right I've only included a subset of the loads so it needs to have more consideration. I will ask the designer for the loads which he has been happy to provide before for some other trusses.

 

In the meantime all I have is the hangers he has specified for use with a timber mounting, KH-75 and KH-38 rated to SWL of 12.48kN which is way over anything likely.

 

For masonry hangers the area is a bit complicated by a transition from blockwork (7.3N I think, definitely no weaker) to brickwork. The bricks are thankfully frogged and not perforated and have strength of >=25N/mm2. The transition is a horizontal one at 2700mm above DPC and the top of wallplate is 2610mm which will also be bottom of bottom chord. A 225mm high masonry hanger would therefore go above the second course of bricks. That is all at odds some what with the sketchy guidance from the warranty documents. I could probably get the thumbs up from the likes of Simpson StringTie though. I also think a ledger board is an easier install for my set of skills. I don't fancy fitting a restrained masonry hanger as that would require removing the brick it sits on so as to get the strap hooked over.

 

A 200mm ledger board could be mounted to have bolts ~50mm below the top of the highest block course (2650) and ~middle of the first brick course (2740).

 

42 minutes ago, Gus Potter said:

Also tie straps are meant to be built into the blockwork as it is going up, their rating is based on this. Face fixing straps requires a good look, not least as they bend, rawl plugs don't have the tie capacity that a built in strap has.

 

With the strap tucked behind the ledger board it should be capable of a far load.

 

 

[Gus Potter]

Hi MortarThePoint.

 

Admire your tenacity.

 

Yes the Cullen KH38 has a characteristic capacity of 12.48 kN, SWL is a bit less..for all make sure you don't get your safety factors mixed up. Also note that the capacity is based on the hanger wrapping over the ledger. Now cutting to the chase.

 

Mortar.. ask the truss designer if they can recommend a fixing for the ledger, the ledger depth etc.  I think you may find they run a mile.

 

Now, a resin fixing into a standard brick  / 7.0 N block has a SWL tension / shear capacity of about 0.8 -1.0 kN with a fair wind. An expanding anchor.. well most are rated based on being installed in concrete for structural applications. To even get this you need to get really lucky in that your ledger has to hit the sweet spot on the coursing (fixings need to be near the centre of the bricks, blocks) and also you need to make sure that the fixings are not to close to the edge of the timber. You have more chance on winning the lottery! Also take say the Hilti fixing data, many of the declared fixing strengths are based on a EU masonry unit size, not what we have in the UK.

 

If you can have a look again at the loads you need to support. Then look at the interface that transfers the load to the wall. That is often the key weak spot. Once you have investigate this then it may be worth having a look again at the simple stupid horn concept.

 

 

 

 

 

 

 

 

 

[Gus Potter]

As an after thought.

 

If you look carefully at the stated Cullen hanger capacity it's base on a timber grade of TR26 / C27. Now if you want to go the route of the ledger then the TR26 is the same grade of timber that most truss fabricators use. So when you order your trusses get the fabricator to put some loose TR26 timbers at the length you need on the wagon or go and pick them up early. Now you have got all your ducks in a row.  All the materials / timber grades are now compatible with the stated connection capacities.

 

 

[MortarThePoint]

12 minutes ago, Gus Potter said:

As an after thought.

 

If you look carefully at the stated Cullen hanger capacity it's base on a timber grade of TR26 / C27. Now if you want to go the route of the ledger then the TR26 is the same grade of timber that most truss fabricators use. So when you order your trusses get the fabricator to put some loose TR26 timbers at the length you need on the wagon or go and pick them up early. Now you have got all your ducks in a row.  All the materials / timber grades are now compatible with the stated connection capacities.

 

I asked them to include a 5.4m length of 47x222 with that in mind. I'm also getting some 47x147 and 35x147 as I had paid before then dropping an exterior area that I want to do in treated timber that they don't do.

 

Do you think 47mm is too thick for the ledger board, do you think 35 would be better?

[Gus Potter]

Hi Mortar,

 

No 47mm is better for the ledger. If you have the wrap over type hanger then you want to make sure that you still get the edge distance on the nails on the top wrap over. Also, if you are using a  ledger arrangenment then you want to make the ledger as torsionally stiff as you can (twisting) as this helps spread localised load over more fixings. However, there is no free lunch as the thicker you make the ledger the more torsion you get as the end of the truss is sitting further from the wall.

 

If I was doing this analysis I would look at a 47mm thick ledger to start with. For practical design I would always look at what I'm trying to connect to, age of building, type of masonry, then do a fag packet calculation on the truss reactions, then look at what could work technically (especially the masonry fixings), then look at what is practical and economic for a local contractor / DIY self builder to execute.

 

Much can depend on how straight and plumb the existing walls are for example.

 

 

[MortarThePoint]

Well I have had some numbers back from the truss designer:

• 2-ply truss 6.3kN (factored)
• 1-ply truss 4.4kN (factored)

? Must be all that snow on those two days a year we get here. That makes for a total load on a ledger board of 35kN which falls into my scary category.

 

I phoned Rawlplug asking about options. They were very helpful and have since done some looking. "the concrete screw gives quite good loadings 2.3kN recommended load if you need higher loadings then you need to look at Resins". That's not a scary number of screws 35/2.3 = 15, so would be one every 300mm.

 

He shared a datasheet which included this:

I guess he is using the 8mm tension figure of 2.3kN, the shear figure is actually slightly higher.

 

His email had a potential killer sign off though "There is no load data for concrete screws and bricks but we have used concrete screws into bricks and they work well the only issue is all bricks vary and it would mean doing on site pull out tests to prove the brick is suitable." My bricks are 25N/mm2 but I don't have the equipment to do a pull out test.

 

I hate having to talk to by Structural Engineer. Of all the people so far, he has been the most difficult to deal with as there's always a vibe of "why are you bothering me with this".