Hello,
Whilst plumbing in the bath, I noticed how many holes had been cut in the joists for various pipes. Are there any rules about how many holes, what sizes, and where you can cut through joists? I would imagine as few and as small as possible!
Thanks.
well yes and no.
The center of a joist carries very little load, and if you look at e.g. the construction of beams where wight really matters - like aircraft - you will find there is not much between top and bottom members at all.
The stresses are all carried on the top and bottom of a beam, the function of the bits in between is to preserve the distance between the top and bottom to prevent Euler buckling.
However its a real hassle to make a beam like that, so you average journeyman builder shoves in a lump of 6x3 or whatever and doesn't bother his head with the efficiency or otherwise of the design.
This can safely have loads of bits removed from its center, and, if it has a decently attached chipboard or plywood type floor on top, can safely be notched on top as well.
Not to mention that the resulting removal of weight makes the ceiling below less likely to crack ;-)
Peter
Is this not the rule for notching: you can notch to a max of 1/8 of the depth of the joist and then only over the range of 0.1-0.4 of its length? i.e. not near the middle and not at the ends
Robert
"Peter Ashby" wrote
This may be true in older properties where timber sizes were more generous. However, as each building member gets designed more and more close to its working limit (as is the modern way), it is likely that removing material from any part of the joist cross section will reduce its inertia sufficiently to allow more bending and more cracks to the ceiling below.
Phil
I suppose it would be a balance between propensity to bend and the loss of weight going the other way. I suspect that if anyone could figure out an economical way to make a 6X3 joist into an I bar while keeping intact the pieces thus removed it would have been done by now.
I suspect the cheapness of the metal equivalents has put paid to that.
Peter
Inertia? what has that do do with static bending loads? Mind yoou, the rest of what you say bears no relationship to any engineering I can think of either ;-)
well yes and no. That the 'idiots guide' being as how the (bending) stresses on a beam are generally largest in the middle.
However the reality of it does mean that with a well constructed stressed floor in top, you don't have to be as conservative.
But drilling through the middle is always preferable if you can.
Actually there are beams now made from box section ply. very strong, very light, and minimal wood used.
I suspect basically wood is cheap, working it is not, and so shoving in highly non-ideal structures is just what everyone does.
The use of Warren braced roof trusses - very low wood content, strong, but a total pain to clamber around in, shows you that in a factory floor situation, where machines are doing the work, material will be pared to the minimum.
Yes, a couple of years ago I took one of the offspring to a local orthodontist. It was an old building that had been opened up. They had an exposed wooden box beam that had been made a very nice feature.
Peter
The second moment of area of a geometric shape (sometimes AKA moment of inertia) is the displacement of material about the centroid of the section. This has EVERYTHING to do with structural engineering and the ability of a spanning member to withstand bending loads. Take a beam of rectangular section, then redistribute the same area of material into an I shape. The amount of material hasn't changed but the distance of the material from the centroid and its ability to withstand bending has. This is what gives each section its individual characteristics.
Now consider the I section in its usual environment, used horizontally looking like the letter "I" when viewed on end. The top and bottom horizontal sections of the I shape are called flanges and are relatively thick (again to push the material away from the centroid of the section). The centroid will be at the mid-height of the I shape assuming that it has not been modified (when considering displacement relative to the normal bending x-x axis). The vertical leg of the I is relatively thin and tends to be considered as resisting direct shear loads only. The principle caveat when using slender I shapes is to watch for lateral stability (the tendency to buckle sideways). This is usually accommodated by either cross bracing, or the diaphragm type stiffness imparted by flooring.
To complete the picture, hybrid beams are made called "Litzka" beams. These are standard steel UBs with the centre web profile cut and inserts added to make the "I" shape even taller. Again these improve span capacity but at the expense of an even more slender profile.
Hope that this provides a useful explanation
Phil
But it is nothing to do with inertia. In all my engineering degree I never heard it called that. Inertias is strictly a dyanmic quality.
it also takes compressive loads to prevent...
...aka "Euler buckling".
Or by tying in a stressed skin floor of ceiling or both.
Maybe terminology has changed. It was certainly called that in my engineering degree.
The formula for bending is
M = -EIy" . What do you call I in that case?
(Not that I remember that off the top of my head, but the Structural Engineer's Pocket Book is a useful reference - and also uses the word inertia).
It is confusing terminology, because 'I' here is the 2nd moment of area, and really nothing to do with inertia - it just happens to also be the moment of intertia / mass, assuming a homogenous body).
The message from Ben Blaukopf contains these words:
My college days are too far in the past to remember anything specific but I have retained a text book I used during the first year at college
- Applied Mechanics by PD Collins, published 1960.
That has an interesting take on the subject:
"Mass Moment of Inertia
In ... on the theory of bending and ... on dynamics we encountered integrals of the form S ay^2 and S my^2.
The first of these integrals represents the second moment of area for a plane surface whilst the second represents the mass moment of inertia of a given body.
In view of the similarity of both these integrals it is the common practice to use the symbol I to represent both of them. For this reason the terms second moment of area and moment of inertia are often employed for the same integral S ay^2. This is particularly so in the bending of beams.
In order to avoid confusion it is important that the integral S my^2 be called the Mass Moment of Inertia and not just the moment of inertia, even though the context may clarify the meaning."
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