posi joist design problems - intersection with lintels...

[sean1933]

I am at the stage where we are ready to get the joists ordered and I've been struggling to deal with a few issues that have given me a headache.

Our house is rather complicated with a lot of of glass including floor to ceiling windows on both floors. Construction is brick and block 200mm cavity.

 

I initially wanted to go down the route of timber plates and resin bolts to help with air tightness but the structural engineer for various reasons wasn't keen. I have therefore resorted to a combination of built in, and top cord hung onto various steels (some without a timber plate). Obviously I want a flat floor and various steels being up to 4mm out of size caused a few headaches, but nothing a steel packer plate at the bearing ends wont fix. There is one problem however that I'm struggling to overcome and my head hurts, any solutions welcome!

 

Joists (posi or easi joist) are 254mm

Ceiling height being 2.4m.

In a couple of areas of the house we have floor to ceiling windows both above and below the Birley HD190 cavity lintels - I believe this rules out hangers due to no downward load above.

The frustration I have is that where the joists are built in they obviously go from bearing on a block, to bearing on the lintel (which is some 5mm thick + mortar bed = 10+mm)... thus meaning they are some 10mm out of level the joists next to them. Solutions I have considered:

 

- Joists with trimmable ends. Wolf say this is length ways and the bottom bearer cannot be trimmed down more than 6mm - this mitigates a bit but doesn't fully fix the problem. 

- Pack out the bottom of every block bearing joist with packers - bit bodge like....and laborious.

- Accept a slightly uneven floor and hope it isn't noticeable or if it is fix with a thin latex fibre screed - as above...

- I did consider switching to top cord hung joists at these points (or maybe the entire floor) but comparing old technical documents from Wolf vs the newer version, this option isn't included so I can only assume it is discouraged. I have asked Wolf, but no reply yet. I seem to recall reading something similar on here but cannot find it for the life of me...

 

Any solutions greatly requested!

 

Edited January 14, 2021 by sean1933

[PeterW]

Are the steels in ..??

 

Get the fabricator to put 100x10mm plates at 300mm intervals along the steel and drill them and the web at 16mm. Fit a timber beam along the whole of the face of the wall and the steel, bolting it to the steel and resin bolting it to the wall. 
 

Now use timber to timber hangers onto the wall plate.  

[sean1933]

Hi Peter. Sorry I should have highlighted that the lintels in issue are not steels but the box standard cavity type lintels - birtley hdx 150 type (albeit 200 cavity).

[PeterW]

How are you using lintels with no blockwork above .? Or am I misunderstanding ..?

[sean1933]

joists and block infills would be located where the block is shown in the above diagram, with 1 further brick above to make up the 300mm spacing between floors. Hopefully the below helps:

 

 

 

Edited January 14, 2021 by sean1933

[PeterW]

How wide is that ..? Can’t you land joists either side ..?

[Dave Jones]

.

Edited January 14, 2021 by Dave Jones

[sean1933]

4 minutes ago, PeterW said:

How wide is that ..? Can’t you land joists either side ..?

Its 1200mm with joist span being 4m to spine wall, currently at 400c's. This problem also happens at 2 other locations seen below - one being a door opening 2250mm wide...

[Dave Jones]

Are you saying the blocks either side of the lintel are lower than the lintle?

 

Have a photo of the problem ?

[sean1933]

1 minute ago, Dave Jones said:

Are you saying the blocks either side of the lintel are lower than the lintle?

 

Have a photo of the problem ?

 

Hi Dave. Yes thats what I'm saying pretty much. Joists and the lintel are both bearing at 2.4m high. Over the window joists will be bearing directly on the lintel which is bearing on the blockwork... so thickness of lintel and pug between block and brick adds 10mm to bearing point so these joists sit 10mm higher.

 

I also considered I could take the problematic lintels back off, cut block down underneath by 10mm then replace so bearing point is flush... but there is face brickwork on the big door opening so may look a bit weird plus is this the best option as it would mean all the other windows (where joists run parallel) would be 10mm taller.

[Dave Jones]

cant you get the brickes raise the muck bed of that row of blocks all the way round to sit flush with the lintels ?

[sean1933]

7 minutes ago, Dave Jones said:

cant you get the brickes raise the muck bed of that row of blocks all the way round to sit flush with the lintels ?

I could have but the blocks are already down and ready for joists (Hasn't been a major priority as only 2 week lead time on joists and a big section of the house yet to be build is single story vaulted), not to mention it wouldnt work on the external wall for the doors as face brickwork so fat joint would look awful.

 

It's frustrating that Posi joist say I can only trim 6mm off the bottom cord, if it was 10mm all would be fine. I struggle to understand this logic as top cord would still have full thickness and full bearing.... and you can obviously spec top cord hung !?!

 

[Dave Jones]

Why have they mucked under the lintels anyway ? 

 

Every new build has these lintels and dont have wonkey floors what your brickies doing different ?

[Dave Jones]

the outside skin is not at issue the pozi sit on the internal skin so a think muck joint inside wouldnt matter.

[sean1933]

Difference on our build is that windows are floor to ceiling. Most lintels are below floor ceiling height so joists sit on the blockwork above lintel/window opening top.

Reality is the SE should probably have spec'd steels here so we could use hangers, but we wont go into that!

 

6 minutes ago, Dave Jones said:

Why have they mucked under the lintels anyway ? 

 

Every new build has these lintels and dont have wonkey floors what your brickies doing different ?

its only a very slight skim, not full bed. I am told put under steels and lintels to help with full/smooth bearing (due to brick/block inconsistencies). But combination of this skim and lintel thickness/ridges adds up to 8-10mm.

Edited January 14, 2021 by sean1933

[Dave Jones]

Ah ok got ya.

 

talk to your pozi supplier and see if they can do you a ladder arrangement. pozi either side of lintel and ladder (short pozi on joist hangers) the wide gap. Same principal used for stairs openings.

[sean1933]

thanks Dave - I can see that working for the small window but not for the  big 2.4m door otherwise it would be taking a huge amount of load on 2 joists (inc a bathroom!). I'll speak to the supplier and suggest it as an option.

 

Anyone else got any ideas, or have experience of trimming posi joists?

[Dave Jones]

8 hours ago, sean1933 said:

thanks Dave - I can see that working for the small window but not for the  big 2.4m door otherwise it would be taking a huge amount of load on 2 joists (inc a bathroom!). I'll speak to the supplier and suggest it as an option.

 

Anyone else got any ideas, or have experience of trimming posi joists?

 

They will just double them up either side, talk to them. We had a 1600 spacing where the stairs came up and they just just joist hangered all the short ones off a cross beam. Pozi's are amazing strong for puny bits of 3x2.

[Dave Jones]

you could of course just packer every pozi, goto your local metal bashing place and get legths of 100mm wide steel the same depth as your trying to pack and sit them all on that ?

[mvincentd]

16 hours ago, sean1933 said:

I believe this rules out hangers due to no downward load above.

Maybe...add a small box section steel above the lintel similar to the blue one shown in pic below....hang off that.  Might need bespoke hangers, or fab' hangers into the box section even!

That one pictured is a 1.8m wide window below, green roof above, and the pozi's over the window also support a 240kg skylight....the SE didn't spec the box section, it was at my insistance.

 

G20789AC Posi Flat Roof (Higher Roof) Layout 31_01_2018.pdf

[Gus Potter]

14 hours ago, mvincentd said:

Maybe...add a small box section steel above the lintel similar to the blue one shown in pic below....hang off that.  Might need bespoke hangers, or fab' hangers into the box section even!

That one pictured is a 1.8m wide window below, green roof above, and the pozi's over the window also support a 240kg skylight....the SE didn't spec the box section, it was at my insistance.

 

G20789AC Posi Flat Roof (Higher Roof) Layout 31_01_2018.pdf 213.22 kB · 2 downloads

Hello all.

 

A few thoughts.

 

Before you start changing levels, methods of hanging joists, packing things etc it is really important to get a handle on the basics.

 

Take a simple beam subject to vertical load. Often to perform it has to be kept vertical, not twist, or move sideways. We call this lateral and torsional restraint. Quite easily, if you muck up the restraints you can reduce the capacity of a beam / wall so much so that it will fail, sometimes even before the building is erected. I have seen this happen.

 

Often a beam is designed where say floor joists are framing in above and provide this "lateral restraint". There are general rules that say for example that a mid floor joist can provide lateral restraint to a lintel or beam. But if you start to introduce  packers and so on it raises some questions as you can introduce an extra slip plane, the assumed friction can be lost, thus the effective restraint is lost and this is not safe.

 

I have quoted mvincentd.. it's a good photo. 

 

 mv.. mentions a green roof.. which is heavy, hence probably the heavy twin engineered joists. Here you can see double joists framing in on the inner leaf of the lintel. The lintel is a cold formed steel lintel, say by "Catnic" or "Key Stone".

 

For all. Please bear in mind the following:

 

Catnic for example quote the capacity of their lintels base on a uniformly distributed load. That means that roughly the load is applied fairly evenly along the length of the lintel. Just as importantly they also specify what is called a load ratio. To allow the lintel to meet the stated capacity both the inner and outer flange loads have to fall within certain limits. In other words you can't put all the load on the inner flange and expect to get away with it as the lintel starts to twist too much. Having a cursory glance at the photo it seems that we have two double joists on the inner flange and very little load on the outer flange. All is not lost however if you want to go off piste and the load ratio is exceeded. You can contact the manufacture and ask them to check that what you propose is ok. The overall load may be less, while the load ratio may exceed the brochure limits they can check it will still be adequate.

 

Please be aware of the consequences when changing the support details. Remember that the engineered joist manufacturer will often take no responsibility for anything other than their product.. be careful what you do.

 

It's also worth thinking about how the lintel is supported at the ends. You can see from the photo that the double joists are more towards one side, thus the load at the right hand support will be more than the left. A consequence of this is that the lintel is subject to more of a shearing force at one end than the other. So again, it's worth checking this with the manufacturer.

 

If you have an uneven load on the supports which the photo shows this is a flag for any checker.

 

It's an aside, but you can see the wavy scratch marks on the block. Often folk think this is to provide a key for the plaster, but it also identifies the manufacturer of the block and the block strength. mv's photo is great. You can see the clear wavy marks, looks like there are four per block and the wave has a "defined shape". Thermalite/ Hanson have same photo and this looks like their High strength 7 block. So if you are doing a refurb garage conversion this info can be really helpfull when you maybe want to rest a beam / padstone on this kind of wall but don't know what strength it is.

 

in summary please think carefully before you make what appear to be innocuous changes, or "take advice from the brickie". It only takes a few minutes to phone the Architect,  SE or if you can't get hold of any of the former post here to get some possible pointers. Rest assured from their (SE etc) point of view having the Client calling them with the odd query is no problem as can often head off serious time consuming / problematic issues later.

 

 

 

 

 

 

 

 

 

 

[mvincentd]

7 hours ago, Gus Potter said:

Catnic for example quote the capacity of their lintels base on a uniformly distributed load. That means that roughly the load is applied fairly evenly along the length of the lintel. Just as importantly they also specify what is called a load ratio. To allow the lintel to meet the stated capacity both the inner and outer flange loads have to fall within certain limits. In other words you can't put all the load on the inner flange and expect to get away with it as the lintel starts to twist too much. Having a cursory glance at the photo it seems that we have two double joists on the inner flange and very little load on the outer flange. All is not lost however if you want to go off piste and the load ratio is exceeded. You can contact the manufacture and ask them to check that what you propose is ok. The overall load may be less, while the load ratio may exceed the brochure limits they can check it will still be adequate.

All very interesting, particularly this bit.

Not to hijack the main purpose of @sean1933's thread i'm interested in this because you're right about my load diffential between inner and outer leafs there.  Outer is the pretty negligible 1 block height, whereas inner carries the roof and all that entails.

So my Pozi's were specced and supplied by Crendon working off drawings from my SE which identified that Catnic lintel.  I raised a slight concern over it with my SE....he responded;

"....capacity of the catnic lintels is actually within the required loadings, so theoretically the lintels should be fine.  That said, if they do feel flimsy as per your description on the phone then perhaps we should take note and include say a 100x100x5 angle as a protective measure, almost as a bit of insurance...With lintels we do not often get feedback from site, they are usually just constructed by the relevant contractor and that is it, but since you raised this concern it makes sense in our eyes, to eradicate any doubts, especially if you have sourced some additional steelwork already, to include an additional angle where the joists span onto the lintels."

No mention/discussion of 'load ratio' and no references to it in Catnic product details/specs, so I wonder if contractors ever give it consideration....or ever need to, perhaps the standard tolerances of standard lintels is wide enough that it would be a bespoke steel rather than a standard lintel being specced when load ratio is a factor?

Overall i'm unsure if my SE was just humouring my paranoia, or was overly casual in his original spec', or genuinely acknowledging in-practice observations deserve a weight of consideration against 'the maths'.

In summary i'd agree @Gus Potter not to change SE details without checking with them....but do check, don't dismiss bricky opinions or your own instincts, and don't entirely be slavish to the presented maths.  I'd also absolutely go through all pozijoist specs carefully as I suspect the estimators at those organisations likely churn out quotes avoiding direct dialogue with SE's on fine detail....put your pozi quote in front of your SE.

 

(just for the eagle eyed....yes the steel isn't 100x100x5.....my fabricator had some 80x40x5 and the SE approved that!)

[Mr Punter]

Hi @sean1933.

 

Aside from the 10mm lump which may be caused by the mortar bed and lintel, I would want written confirmation from the lintel supplier / engineer that it is acceptable to land the joists on the lintel as proposed.

 

Instead you may need to use the special masonry restraint hangers on the block/brickwork above to reduce the point loads.  This would also resolve any issues with joist levels.

[Gus Potter]

Hi Sean.

 

@sean1933  "The frustration I have is that where the joists are built in they obviously go from bearing on a block, to bearing on the lintel (which is some 5mm thick + mortar bed = 10+mm)... thus meaning they are some 10mm out of level the joists next to them. Solutions I have considered: .."

 

It's wise to look carefully (as you are doing) at the implications of making what appear to be small design changes.

 

@mvincentd " No mention/discussion of 'load ratio' and no references to it in Catnic product details/specs, so I wonder if contractors ever give it consideration..."

Yes some contractors do. This info can be a little harder to find as sometimes it is not presented in the load tables. However, in the Catnic brochure this info is presented at the end, under the heading "Technical Information"

 

Here is a link to the Catnic downloads  https://catnic.com/downloads

Follow the lintel products selector for the pdf brochure and the load ratio info is round about page 64.

 

@Gus Potter  or "take advice from the brickie". Apologies as this is a bit of a sweeping statement, should have qualified it a bit at the time of writing. Yes it can be helpful to take on board advice from experienced brickies etc. But always check with the designer as they are best placed to assess the structural implications of any design changes in terms of say global building stability.

 

If you are proposing to alter the support conditions of say floor joists always check as the floors are often used to provide lateral stability to the walls. Also, look carefully if you are using hangers / brackets /resin fixings as some are able to carry different amounts of axial load (tension / compression) in the joist as well the vertical loads. The tension / axial load arises when say for example the wind is sucking the walls out and you are relying on the hanger (other type of connection) to tie the wall back into the floor.

 

 

 

[Temp]

On 14/01/2021 at 18:13, sean1933 said:

Pack out the bottom of every block bearing joist with packers - bit bodge like....and laborious.

 

Not that bad. Mass produce a load at once out of WBP?