How do they do it? Why doesn't the wall suffer from weakness?

Discussion in 'Building' started by u587162, 27 Aug 2015.

1. u587162

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This is more for my self interest and understanding construction methods.

If a large RSJ beam / box frame needs to go into a building, say house extension, how is it that a house wall a) does not fall down or b) wall weakness / cracks / structural integrity is not weakened?

I understand that acrow props are used to hold up a wall, but surely when a load bearing wall is removed there is so much downward pressure spread across the broken bond effect of the bricks that even a few millimetre difference will impact the house above?

And how is that when trying to squeeze in the horizontal beam, you can get perfect upward pressure to stop it from falling/cracking, I mean you will never get the exact same upward pressure of the beam prior to the wall opening being made or wall being taken down, unless you're going to tell me that the upward pressure of the beam is greater than the previous downward pressure of the old wall?

I asked this to my builder and he simple said that "they pack it"...which does not do enough to satisfy my curiosity.

The analogy I try to reason with is this: If I push by palm down against another adult's palm to equal out the upward pressure he is pushing against (I know bricks don't push up...but go with it), and that person wants to swap his hand with someone else's, even if they squeeze their hand through, there will be even a 1% drop in height as this happens. How can a wall take this downward pressure as it will never be 100% the same as before even as they try to "pack any millimetre void"?

Please try and explain in simple easy to understand steps for a non construction person!

Thanks.

Last edited: 27 Aug 2015
3. theprinceofdarkness

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When you put your RSJ on the bricks they compress, but by such a small amount that you can't see it. Traditional housing methods actually rely on the weight of the roof to load the walls else they would be wobblely. The packing is to make sure that the weighty bits on top of the RSJ, don't have to drop down before they are supported.
Frank

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4. u587162

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Hi - you've answered my question the other way around. I'm not questioning the compression of the bricks below the RSJ, but rather how is it the old wall above the new RSJ dont come down even slightly by a few mm as the structure is opened and then closed with the insertion of the new beam.

5. noseall

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Because you are constantly applying upward pressure with props and needles. I'm convinced that We actually move everything upwards slightly when we do a beam fit because I'm fanatical about things dropping and am always giving the Acrows one more turn.

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6. OwainDIYer

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Usually the lintel is assumed to only support the triangle of bricks above the opening:

http://www.tyharness.co.uk/bp3/bp3.htm

Therefore the only weight that has to be supported by the props is that triangle of bricks. Everything else just hangs together - at least for as long as it takes to open up the slot for the lintel and push it in. The rest of the work usually follows more leisurely once the lintel is in.

7. u587162

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So in my case we are removing one whole rear section of the ground floor wall (which has a first floor wall above it in situ) and it's connected side wall, to basically make an open plan ground floor and extension. Are you saying apply the same principle except it's a larger triangle Even though it's a whole wall rather than a lintel?

Noseall
That's what I thought (what you wrote) but can the props actually push up a whole heavy wall with roof putting downward pressure on it?

8. ^woody^

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The beam does deflect slightly as load is applied. How much it deflects is part of the calculation so as to ensure that it is within permissible limits.

This does not not apply when new build and the beam is loaded gradually

9. Crystalclear

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I'm sure they do. Very much so.

The 3rd Law (The Law of Action-Reaction)

“For every action force there is an equal and opposite reaction force.”

Anytime an object applies a force to another object, there is an equal and opposite force back on the original object.

If you push on a wall you feel a force against your hand… the wall is pushing back on you with as much force as you apply to it.

The perfectly placed acros are in the middle of this mathematical equation.
If this wasn't happening, your hand would accelerate through the wall.

Standing on the ground, you push on the ground with a force due to gravity, and the ground pushes back on you.

I could go on.... But I think I've made a point?

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11. u587162

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Woody
But thats talking about the beam now which is not what I mean. I'm taking about everything above the beam. How do you stop weakness of the bricks up from coming down even 5mm.

Will a drop of even 5mm affect the structural integrity of the house?

Last edited: 27 Aug 2015
12. ^woody^

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The bricks don't come down 5mm. The props hold them up.

When the props are removed, the beam takes the load and it deflects a little. The small amount is insignificant if the beam has been designed properly.

13. kazuya

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they don't come down even a small amount because the gap above the lintel and below the bricks is packed tight using slate and sand and cement, before the acrows are removed

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14. Crystalclear

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Come on people! What is more important? How does a wall stay up? Or The ban on blow torches when making a creme brûlée ??

15. u587162

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Again, I'm talking about where there is no lintel or where it is a wall above a beam after thrower wall is removed.

How is it possible to pack something so tight , again to prevent even a drop of 1mm.

16. cjard

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If a large RSJ beam / box frame needs to go into a building, say house extension, how is it that a house wall a) does not fall down

Because the beam holds up the triangle of bricks above the opening

or b) wall weakness / cracks / structural integrity is not weakened?

It is weakened but I think you don't appreciate how strong building materials are and how little of their maximum capacity they are subject to in normal operating conditions

I understand that acrow props are used to hold up a wall

No, they only hold up the triangle of bricks above the opening plus anything that might bear on bricks in that triangle like floor joists

, but surely when a load bearing wall is removed there is so much downward pressure

Pressure is force over area, the area is really quite large. A class b engineering brick has a minimum crush rating of 75newtons per millimetre. This means that you'd have to apply more than 161 tons to it before it crushes. A 10 m high wall is going to contain about 154 rows of bricks and assuming 5kg per brick it means the bottom row of bricks has about 3/4 ton sat on each brick (think of a wall one brick wide and 154 bricks high). Do you see how your perspective on what massive pressures must be involved is skew and flawed? A ten metre high building exerts less pressure than your car exerts on the ****ty mild steel jack they give you to change the wheel. The row of bricks at the wall base is carrying 1/320th of what it can stand

So those acrows have to carry a triangle of what.. (2m wide opening, 200mm per brick, 10 bricks, then 9 on the next row thanks to staggered bond, then 8.. Average of 5 bricks per row over ten rows, fifty bricks,) 250kg. You could hold that up with a bit of 4x2 let alone 2steel acrows and a needle

spread across the broken bond effect of the bricks that even a few millimetre difference will impact the house above?

The ground swells more than that when it rains. Buildings are not these huge immovable, inflexible objects you think they are

And how is that when trying to squeeze in the horizontal beam, you can get perfect upward pressure to stop it from falling/cracking,

You can't, but you don't need to. While you had the acrows in your huge weight of 250kg is sat on a few tiny points and those bricks didn't explode with the pressure. Guys on this forum occasionally find lintels put in and then just the front of the gap pointed up, no real support at all, so really the lintel is doing jack all. My point is you don't need perfect upward support. That wall will carry on standing if you just hammered a few bits of slate in next to where the acrows were. By doing a good job of packing with mortar you're enhancing the support but it's really not vital to prevent the wall falling

I mean you will never get the exact same upward pressure of the beam prior to the wall opening being made or wall being taken down,

You don't need to. The wall doesn't stand because the pressure is perfect. The bricks aren't loaded to 99 percent of their max and if so much as one hole is made and the bricks either side are put to 101% and they explode, causing catastrophic failure of the whole wall

unless you're going to tell me that the upward pressure of the beam is greater than the previous downward pressure of the old wall?

If it were the old wall would rise into space. The beam only pushes back as much as it needs to and that's not very much. It's probably not even 250kg for fifty bricks, because the bricks aren't bonded with butter, they don't just slide out the instant their buddies below disappear, otherwise acrows wouldnt work. The mortar in the perps (up down bits between adjacent bricks in a row) packs them together tightly. You take out the bricks below and conceptually they sag a little, they rotate and in doing so get longer, pushing together more tightly. An arch is formed within the material, they support themselves

If you have trouble getting this concept then try it with 3 bricks. Hold them in your hands stacked on end vertically, one hand on top and one on bottom
#
#
#

Rotate them over to horizontal and squeeze your hands together really hard

->###<-

Your hands are the arrows <-

Why doesn't the middle brick fall out? The two outer bricks want to rotate, the one on the right counterclockwise, the one on the left clockwise. If they did the middle brick would fall and then so would they. Your hands prevent this by stopping the bricks rotating. They don't do this by applying a counter rotation. You could stop your bricks from falling by pushing in at the bottom edges only, where the dots are here:

.###.

Why? Because the system will sag a little but the dots are immovable, so very quickly an arch is formed, the middle brick finds itself pinched at the top edges and the lines of force that form the arch run from dots, up to the apostrophes, along and down again

.#'#'#.

This is what the wall does on a microscopic, even atomic scale. It's why you struggle to get a loose brick out from between two other loose bricks. It's why a rawl bolt works, it's why a Stilsson wrench works.. it twists, it jams itself up

I asked this to my builder and he simple said that "they pack it"...which does not do enough to satisfy my curiosity

You won't really get a science answer out of your builder, as he didn't join his trade to debate the finer points of the physical world around us. He just knows instinctively that the materials he uses won't collapse because they are very strong and stuck together. There's a lot more friction and lines of force involved than he cares about

The analogy I try to reason with is this: If I push by palm down against another adult's palm to equal out the upward pressure he is pushing against (I know bricks don't push up...but go with it), and that person wants to swap his hand with someone else's, even if they squeeze their hand through, there will be even a 1% drop in height as this happens. How can a wall take this downward pressure as it will never be 100% the same as before even as they try to "pack any millimetre void"?

As far as analogy goes it's a bit crap, but I'll go with it. Imagine that you and I have very hard inflexible hands that don't deform like other soft n squidgy human hands. When pushing our hands together not every void is filled but it doesn't need to. You accept that our hands are made of stong enough stuff that they do not crush even though only 5% of our skin is touching the other person. You push down, I push up. I'm much stronger than you but very good at matching what you push with. In fact, so good that every time you try to push more or less, I instantly respond with the exact amount of force needed to stop our hands moving anywhere. Yes,I am that good.
Now imagine You are impervious to pain. We have a willing helper come along and drill a hole right through your hand, horizontally. He inserts a metal rod that is so thick your chances of bending it are zero no matter how hard you try. He brings two jacks and winds them up under either end of the rod til the upward force they supply exactly matches the downward force you're supplying. I take my hand away and your hand does not fall, not even a millimetre. I come back with a big girder and put it under your hand. It's not a good fit but I don't care. I drive some wedges between your hand and the girder then remove the rods and the jacks. You're amazed that in the entire process you hand didn't move.

I understand your quandary.. You're wondering how this mere mortal, this lump of slavering meat known as your builder can perform that balancing act so perfectly when driving the wedges that the exact amount of force they supply upwards counters the exact amount of force the wall supplies downwards. He doesn't need to. Because pretty much the universe will do that maths for him, he just uses materials that are capable of pushing back with the requisite force (which is relatively small) even though they probably never supply more than a few percent of their force capacity

Oh, and if anything did move, bend etc, you're talking tiny amounts, and buildings flex more than you think as they heat, cool dampen and dry.

Last edited: 28 Aug 2015
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17. noseall

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Not really no, but it can tighten any 'slack' bed joints.

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