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.