Raw plug spec for wallplate strap fixing.

[Gus Potter]

Hello Epsilon and Peter and all.

 

Here is a bit of an over view, my stab at things, no spell checking, for a bit of fun and so on.

 

Say you have a sail on a yacht. On one side the wind blows against it (pressure) and the wind has to move around this blockage. On the other side of the sail there is a negative pressure before the air flow returns / stabalises to normal further down stream. This causes a suction force on the back of the sail. The two forces combine to create an overall wind force and as the sail is at an angle to the wind it drives the boat as you get a roughly perpendicular component to these forces. .. or you can capsize it... hence the lift jacket. The same happens to a house.  I'll leave this for now but the wind forces on a roof (not just the walls) can move the house from side to side horizontally too and that is where we look at building stability.

 

But for the sailors here, you'll notice that at the edge of the sail there are some really high forces, hence why often the sail rips. Here the wind vortices are significant, forces can be two possibly three and a bit times more. The same principles applies to a roof.

 

However, unlike a boat a house is static, it has a roof and as the wind moves about in direction it can cause both a downward force and an uplift force on the windward side ( depending on the roof angle and obstructions like chimneys)  and an uplift force on the leeward side. A roof is a bit like a tilted sail on it's side?

 

To turn to the holding down straps. you need to start at the top surface of a roof. At the edges say and round chimneys you have high wind forces. That is why you often see tiles sucked off here, hence the tile fixing specification. Once you go through the depth of the roof you have battens / wind bracing / sarking  and so on. These all spread / shead the localised loads.

 

Now you have the roof timbers etc. these too load share / spread the load and this load ends up at the wall head where the roof rests. You also have an effect called " non simultaneous action.. wow! this means (good for you)  that the whole of the worst wind does not act on the roof at the same time, and this depends on the size of the roof and orientation.

 

Most roofs are designed to be stiff, like floors. They (roofs) act like deep beams and tie all the walls together. In other words the holding down straps take a more general load rather than a concentrated load that occurs over small areas of the roof. So there is an element of Engineering judgement applied.

 

This needs a leap of faith here. We know roofs lift off so you need to hold them down. We need to connect the roof to something heavy, or to a timber frame that will be connected to something else that is heavy or stiff and we can use holding down straps to do this. Follow the load path. The first connection is between the straps and the roof timbers say. The next between the strap and the masonry, which is the point here.

 

But.. before you get to the strap fixings, it's worth havng a look at some of the generally accepted norms for say 1960's standard housing. There were no holding down straps. The wall plate was just nailed essentially down into the brick. There are some quirky bits in the design codes that allow you to to take into to account a masonry height (for wind uplift) that can contribute to a counter weight to hold things down, although the mortar in in some respects acts in tension which is often not recommended when designing masonry for other purposes.

 

But now we often have lighter roofs, lightweight blocks and timber frames.

 

With timber frame life is fairly simple, you make the truss clips do the work and transfer the wind uplift down the studs and couple this with the sheeting and spread the load further into the structure.

 

Previously for standard housing a brick cavity wall was common. The walls are often too cold nowadays. Here, you can calculate the weight of three courses of masonry / or more ~ 3 vertical feet and design for that. Actually, I don't think anyone bothered! they just knew it would be ok for standard housing.

 

But for lightweight blocks the simplest way (if you have a concern) is to buy a twisted strap. The top of the twist connects to the truss say. You have two choices here and this is about the cost of procurement. You can site bend a 5mm thick the strap at the bottom, drill out a bit of mortar bed and turn the leg back into the light weight block buy 80mm , takes a couple of minutes. Fix this to the block with 5 x 80mm screws and plugs, 2 fixings per block vertically, these are to hold the strap to the wall so you don't need to calculate the shear capacity of these fixings. They also build in reduncancy / robustness.

 

Or you can you do the full calc route.. which will require a lot of effort. Or you can just just nail / use any plugs you want and not turn them back into the lightweight block. Some inspectors just like to see the straps and don't look at the fixing type and compatibility with the masonry etc. For me I would do it the right way and turn the strap back into the masonry if light weight block. It's not that much work, a few quid.

 

Lastly, well nearly, as a caveat.. what you want to look out for is things like canopy roofs and say wrap around bifolds on corners of buildings, anything that looks a bit out of the normal.

 

Oz .. think your job is safe.

 

Holding downstraps are hard to get your head around as the manufacture's only give you part of the data. Often it's easier to skin the cat in a different way and look forward to spending the time and money else where.

 

Peter if you want some pointers on how to calculate wind loads then..

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Edited October 26, 2020 by Gus Potter
life jacket..

[ToughButterCup]

Hmmm, so that Mr Epsilon's question about the humble strap

proves anything but boring.  I knew it ! 

[joe90]

6 hours ago, Gus Potter said:

But for lightweight blocks the simplest way (if you have a concern) is to buy a twisted strap.

Thats wot I said and what I have done, common sense in my opinion.

[epsilonGreedy]

@Gus Potterthank you for your extensive reply.

 

Distilling your post down to the essentials my takeaway is that local vortices above parts of a roof are the most likely source of roof wind damage and in this case the damage will be to the cover. Looking more broadly at wind stresses on the whole roof structure, a large proportion of the roof should not be subject to simultaneous uplift strong enough to cause trusses to tear off their wall plate clips.

Quote

 

Say you have a sail on a yacht. On one side the wind blows against it (pressure) and the wind has to move around this blockage. On the other side of the sail there is a negative pressure before the air flow returns / stabalises to normal further down stream. This causes a suction force on the back of the sail. The two forces combine to create an overall wind force and as the sail is at an angle to the wind it drives the boat as you get a roughly perpendicular component to these forces. .. or you can capsize it... hence the lift jacket. The same happens to a house.  I'll leave this for now but the wind forces on a roof (not just the walls) can move the house from side to side horizontally too and that is where we look at building stability.

 

But for the sailors here, you'll notice that at the edge of the sail there are some really high forces, hence why often the sail rips. Here the wind vortices are significant, forces can be two possibly three and a bit times more. The same principles applies to a roof.

 

 

True in part, however a correctly trimmed sail should have a laminar airflow like an aircraft wing. The winning edge on a racing yacht is obtained through encouraging just a hint of a vortex along the leading vertical edge of a sail but not enough to develop a large volume of turbulence behind the sail. I can see some similarity between the cross section of a low pitch roof, yacht sail and aircraft wing. Unfortunately my house will not come equipped with a mainsheet that can be released during a storm force gust to spill the wind hence my interest.

 

The power in a sail is ultimately derived from slowing down the airflow and extracting kinetic energy from the moving airflow. Going back to O level physics and E = Mass * Velocity squared * 0.5 then at 80 mph the forces get disconcerting i.e. a routine 40 mph gale force gust has 4 times less energy than a winter storm 80mph gust.

 

In 80 mph a yacht will be running under a bear mast at a surprising speed whereas 40 mph is just a nasty gale where some sail is left up for control. Getting back to my 30 degree roof after some further reading I understand that 10 to 15 degrees is the pitch most likely to start behaving like an aircraft wing with associated lift. 30 degrees is still a concern. 

Edited October 27, 2020 by epsilonGreedy

[joe90]

22 minutes ago, epsilonGreedy said:

30 degrees is still a concern. 

Mine is 30’ and we get some real gales down here being near the Atlantic and it ain’t budged , I think your over thinking this ?.

Edited October 27, 2020 by joe90

[Gus Potter]

2 hours ago, epsilonGreedy said:

@Gus Potterthank you for your extensive reply.

 

Distilling your post down to the essentials my takeaway is that local vortices above parts of a roof are the most likely source of roof wind damage and in this case the damage will be to the cover. Looking more broadly at wind stresses on the whole roof structure, a large proportion of the roof should not be subject to simultaneous uplift strong enough to cause trusses to tear off their wall plate clips.

 

True in part, however a correctly trimmed sail should have a laminar airflow like an aircraft wing. The winning edge on a racing yacht is obtained through encouraging just a hint of a vortex along the leading vertical edge of a sail but not enough to develop a large volume of turbulence behind the sail. I can see some similarity between the cross section of a low pitch roof, yacht sail and aircraft wing. Unfortunately my house will not come equipped with a mainsheet that can be released during a storm force gust to spill the wind hence my interest.

 

The power in a sail is ultimately derived from slowing down the airflow and extracting kinetic energy from the moving airflow. Going back to O level physics and E = Mass * Velocity squared * 0.5 then at 80 mph the forces get disconcerting i.e. a routine 40 mph gale force gust has 4 times less energy than a winter storm 80mph gust.

 

In 80 mph a yacht will be running under a bear mast at a surprising speed whereas 40 mph is just a nasty gale where some sail is left up for control. Getting back to my 30 degree roof after some further reading I understand that 10 to 15 degrees is the pitch most likely to start behaving like an aircraft wing with associated lift. 30 degrees is still a concern. 

Eplison.

 

That's interesting about how a sail works in more detail, learn something new all the time. Your spot on about how the velocity of the wind leads to an exponential increase in the wind force.

 

The wind acting on a house is pretty turbulant, it has small vortices that can act over a small area causing significant uplift and also larger areas of lower pressure / suction that can easily lift a whole roof off. In summary the key is to fix your tiles ect as per the manufacture's guidance, follow good practice in terms of fixing the timbers ect of the roof together and again follow good practice re truss clips, holding down straps etc.

 

Sometimes you need to dig a bit deeper. If you build at the top of a cliff, or on a large hillside say the wind can be more severe so deploy common sense as Joe90 says. Ask the local folk. Some farmers may say in passing.. "that is a windy bit of the hill"..or " the stock avoid that bit in the winter", no barns built there as the last one blew down... Also, if you are in built up areas you can get a bit of wind funneling if you have tall buildings round about.

 

There is lots of information about the wind etc available on the internet but there comes a point where once you have done your due dilligence then maybe as others have noted it's time to move on as you do want to finish the job at some point.

 

 

[CrispDust]

Sorry to resurrect an oldish thread.

Presumably said fixing screw or nails for the vertical restraint straps should be fixed into the block, not the mortar between the blocks?Thanks.

[markc]

1 hour ago, CrispDust said:

Sorry to resurrect an oldish thread.

Presumably said fixing screw or nails for the vertical restraint straps should be fixed into the block, not the mortar between the blocks?Thanks.

Correct, fixing into mortar is a last resort.

[epsilonGreedy]

On 22/11/2021 at 07:01, CrispDust said:

Sorry to resurrect an oldish thread.

Presumably said fixing screw or nails for the vertical restraint straps should be fixed into the block, not the mortar between the blocks?Thanks.

 

Defo the blocks, on a full size roof the straps should reach down across 4 blocks with a screw/nail in each. I suppose in the case of your single garage the forces will be lower.

 

Having fixed many wall plate straps since starting this thread I suggest fixing with wall plugs particularly in the case of softer blocks under 10kg in weight (not to be confused with the compression rating typically expressed as something like 7N. @joe90offered an excellent tip in this thread which was to use a slightly undersized drill bit when drilling into soft lightweight blocks.

 

In the case of my hipped roof I adjusted the regulation 2m strap spacing to ensure I had a strap on each wall closer to the hip rafter corners.