Cut Bracing Between Floorboard Joists

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Hi,

I am in the middle of a project to install an inceiling projector screen in our downstairs living room. I have started taking the boards up (in the room above) as cleanly as I can and have come into contact with crossed bracing and a more sturdier wooden beam which is reinforced with a bar (see the image below).

ekPgCbP.jpg


The bigger beam doesn't run across the joists, however the metal bar does as shown below.

bFCI97S.jpg


Now the tricky part. I need to cut out an area in the beam (and remove the bracing's further down) otherwise the inceiling projector won't have enough room to be mounted flush to the ceiling below. The depth required is 17cm according to the projector specs.

From the board below to the top of the beam there is a total height of 20cm, assume some 2cm or so thickness in the boards too then the plaster so maybe total of 23cm?

The following image shows what I am trying to achieve.

SpiWLGf.jpg


I guess I probably need to see a builder about this? Or is there a way to achieve what I am after? My guess is cutting the beam by over 2/3's it's height is most definitely not a good idea.

Would like some suggestions/direction if possible.

1. Can I remove the x bracing further down
2. Can I cut into the larger beam from below with the metal bar

What are my options otherwise?

Thanks.
 
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fungus1487, good evening.

Some assumptions? hope I am correct?

The wall on the right of the images you have posted is an external gable wall?? if so then.

1/. The metal strap you can see is a "Lateral Restraint" which is a strap of metal, with a right angled bend in it, that is sited within the cavity of the gable wall, if you Google "BAT Lateral Restraint Straps" you will see what I am getting at. Their purpose in life is to give [as described] Lateral Restraint is to assist in holding or restraining the gable wall in place. If you consider the gable wall it is a very thin but very, very high structure, in modern property two stories at say 2.4.m plus about 3.0.m in the loft so the gable wall is 7.8.m high? in effect too high to stop the very, very thin wall buckling, by that most walls of this height will at some time buckle / bend outwards. To stop this and give the "Lateral Support" these straps are built in, but to adequately restrain the gable wall the load MUST be spread over several of the first floor joists.

So far so good, [I hope?]

Similar straps will be present in your attic, where identical straps are built into the cavity, then as above the long part of the strap is fixed to the rafters in the attic, with noggins [Dwangs in Scotland] so the imposed wind load on the gable is spread over several rafters, wind suction in a high wind is very high.

Both of the above restraint systems are generally designed by a Structural Engineer and indicated on the plans for the property.

Having bored everyone as above, what can you do?

I would suggest

1/. Do NOT cut the metal strap shown on your images!
2/. Is it possible to move the device to get away from the gable Lateral Restraint?
3/. If you MUST? cut the noggin that the metal strap is attached to then I think that you must reinforce the remaining area of the noggin, using say metal on both sides of the cut noggin, or? simply put in two additional noggins, each side of the cut noggin.

The reason for the noggin that the metal bar is resting on is that it stops the crushing effect of the load acting on two or three or more joists, the noggin acts in unison with the first floor joists, the noggins making for a very stable anchor for the strap?

Hope all above makes sense?

Ken
 
fungus1487, good evening.

Some assumptions? hope I am correct?

The wall on the right of the images you have posted is an external gable wall?? if so then.

1/. The metal strap you can see is a "Lateral Restraint" which is a strap of metal, with a right angled bend in it, that is sited within the cavity of the gable wall, if you Google "BAT Lateral Restraint Straps" you will see what I am getting at. Their purpose in life is to give [as described] Lateral Restraint is to assist in holding or restraining the gable wall in place. If you consider the gable wall it is a very thin but very, very high structure, in modern property two stories at say 2.4.m plus about 3.0.m in the loft so the gable wall is 7.8.m high? in effect too high to stop the very, very thin wall buckling, by that most walls of this height will at some time buckle / bend outwards. To stop this and give the "Lateral Support" these straps are built in, but to adequately restrain the gable wall the load MUST be spread over several of the first floor joists.

So far so good, [I hope?]

Similar straps will be present in your attic, where identical straps are built into the cavity, then as above the long part of the strap is fixed to the rafters in the attic, with noggins [Dwangs in Scotland] so the imposed wind load on the gable is spread over several rafters, wind suction in a high wind is very high.

Both of the above restraint systems are generally designed by a Structural Engineer and indicated on the plans for the property.

Having bored everyone as above, what can you do?

I would suggest

1/. Do NOT cut the metal strap shown on your images!
2/. Is it possible to move the device to get away from the gable Lateral Restraint?
3/. If you MUST? cut the noggin that the metal strap is attached to then I think that you must reinforce the remaining area of the noggin, using say metal on both sides of the cut noggin, or? simply put in two additional noggins, each side of the cut noggin.

The reason for the noggin that the metal bar is resting on is that it stops the crushing effect of the load acting on two or three or more joists, the noggin acts in unison with the first floor joists, the noggins making for a very stable anchor for the strap?

Hope all above makes sense?

Ken


Thanks for the response, it has helped a lot.

In answer to your questions

1. I won't cut the strap :)

2. Sadly I can't move the device to either side of the gable lateral restraint, it needs to drop centrally in the room below and I would need to shift it all at least 18 inches to the left.

3. I am in the position where I need to install 2 braces of wood between the joists anyway either side of the screen to be secured into (I don't like the idea of it just hanging from the ceiling board). I could increase the size of these (to the same size as the existing noggins) they would be 7 foot apart, roughly 3.5 foot either side of the gable lateral restraint.

I probably need someone qualified to look at this I guess? Could you recommend perhaps a trade that would be able to help? Am I looking for a general "builder" or more specifically an engineer or carpenter?

Thanks very much for your response.
 
As per Ken, the steel strap is to provide lateral restraint to the wall. The noggins are there to stiffen the strap when the wind load is against the wall, ie to
stop the thin strap from buckling (if it buckled, it wouldn't transfer any load to the next joist).
Why not consider taking out the blocking piece and re-installing it at 90 degrees, ie flat, and re-nail the strap to it.
 
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As per Ken, the steel strap is to provide lateral restraint to the wall. The noggins are there to stiffen the strap when the wind load is against the wall, ie to
stop the thin strap from buckling (if it buckled, it wouldn't transfer any load to the next joist).
Why not consider taking out the blocking piece and re-installing it at 90 degrees, ie flat, and re-nail the strap to it.

Now that is using your brain!

You might just have won an award with that suggestion.

I assume that whilst I remove the noggin and installed it at 90 degrees there is little risk in damaging the strap?
 
A friend suggested installing 2 noggins at either end of the device and the straps are cheap so I could go for broke and install 2 more straps in the process.
 
The point of installation the metal restraint strap atop the solid strutting (NOT noggins BTW) that the straps are to provide restraint AND the strutting is to stiffen the joists so that they don't twist under load. If the strutting is turned flat you will lose the stiffening effect and you might as well not have them!
 
The point of installation the metal restraint strap atop the solid strutting (NOT noggins BTW) that the straps are to provide restraint AND the strutting is to stiffen the joists so that they don't twist under load. If the strutting is turned flat you will lose the stiffening effect and you might as well not have them!

I have seen them called nogging, blocking and now solid strutting :p

That makes sense about them twisting though. Although I imagine by adding two more either end would limit this twisting effect somewhat?
 
The point of installation the metal restraint strap atop the solid strutting (NOT noggins BTW) that the straps are to provide restraint AND the strutting is to stiffen the joists so that they don't twist under load. If the strutting is turned flat you will lose the stiffening effect and you might as well not have them!

The horizontal compressive force under wind load on the wall will be along the strap itself. The solid blocking piece below the steel strap is solely to avoid buckling of the strap when under compression; Approved Doc. A allows the blocking piece to be down to ½ the depth of the joist, which of itself would not be wholly adequate to stop the joists twisting.
The device which stops the joists twisting by holding them vertical is the X bracing further along.
OP will be fine if he turns it flat.
 
The point of installation the metal restraint strap atop the solid strutting (NOT noggins BTW) that the straps are to provide restraint AND the strutting is to stiffen the joists so that they don't twist under load. If the strutting is turned flat you will lose the stiffening effect and you might as well not have them!

The horizontal compressive force under wind load on the wall will be along the strap itself. The solid blocking piece below the steel strap is solely to avoid buckling of the strap when under compression; Approved Doc. A allows the blocking piece to be down to ½ the depth of the joist, which of itself would not be wholly adequate to stop the joists twisting.
The device which stops the joists twisting by holding them vertical is the X bracing further along.
OP will be fine if he turns it flat.

Would you recommend installing nogging either end whilst I remove the piece attached to the streel strap and reposition it? I can't imagine there will be any excessive force but suppose you can never be too safe.
 
Personally I wouldn't even bother adding noggins either end while you undertake any alteration.
These things are only for times when there are maximum wind loads (eg in a gale); they're not precisely engineered - more like just rule-of-thumb
approach. There will be similar straps in your roof/ceiling.
 
The point of installation the metal restraint strap atop the solid strutting (NOT noggins BTW) that the straps are to provide restraint AND the strutting is to stiffen the joists so that they don't twist under load. If the strutting is turned flat you will lose the stiffening effect and you might as well not have them!

The horizontal compressive force under wind load on the wall will be along the strap itself. The solid blocking piece below the steel strap is solely to avoid buckling of the strap when under compression; Approved Doc. A allows the blocking piece to be down to ½ the depth of the joist, which of itself would not be wholly adequate to stop the joists twisting.
The device which stops the joists twisting by holding them vertical is the X bracing further along.
OP will be fine if he turns it flat.
Not according to the structural engineers I've worked with. To be effective solid strutting needs to be at least 70% of the height of the joists and preferably of similar cross section - but hey I and the rest of the trade have obviously been doing it wrong for at least 150 years......
 
I wonder if you are confusing solid strutting with bracing?

The most effective way of preventing joists from twisting - and also maintaining the floor as a rigid plate - is the traditional cross-bracing. The reason for this is that, as the joists shrink sightly in depth after completion of the building, the cross-bracing automatically tightens up against the side of the joist because it is being forced into a slightly smaller gap (the modern tin things that Simpson do are useless for this).

Although used more often today, solid blocking is not really as effective for floor stiffness because when the joists shrink, a small gap opens up between the joist-side and the blocking piece. This means that it is only the nails that are transmitting the force because the blocking piece is then not directly bearing on the side of the joist.

The blocking under the steel strap is only to prevent buckling of the strap itself when subject to compression and has little to do with maintaining rigidity of the floor.

But hey, I've only been doing this since 1851.....maybe the SEs you've been dealing with need to go back to college:D
 
I wonder if you are confusing solid strutting with bracing?

The most effective way of preventing joists from twisting - and also maintaining the floor as a rigid plate - is the traditional cross-bracing. The reason for this is that, as the joists shrink sightly in depth after completion of the building, the cross-bracing automatically tightens up against the side of the joist because it is being forced into a slightly smaller gap (the modern tin things that Simpson do are useless for this).

Although used more often today, solid blocking is not really as effective for floor stiffness because when the joists shrink, a small gap opens up between the joist-side and the blocking piece. This means that it is only the nails that are transmitting the force because the blocking piece is then not directly bearing on the side of the joist.

The blocking under the steel strap is only to prevent buckling of the strap itself when subject to compression and has little to do with maintaining rigidity of the floor.

But hey, I've only been doing this since 1851.....maybe the SEs you've been dealing with need to go back to college:D

Now now guys this isn't a big dick competition :D

Thanks for all the help so far though. My ceiling hasn't fallen through yet!
 
fungus1487, hi again.

Interesting post

One final thing [from me at least]

I assume the projector produces heat? if so, where is the heat dissipated? does the projector self ventilate into the room it is serving or? where?

Ken.
 

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