Load Capacity - Stand Made of Cast Iron

Yes. If the table is placed on soil with a low load bearing capacity. See prior thread.

Are the legs hollow? I don't know the numbers, but cast iron is probably good for 20ksi compressive (?). If all four legs touch the ground, and you put the known weight in the middle, it will be evenly distributed. If the ground is uneven, and only three legs touch, place the load above the centroid of the resulting triangle.

legs are not hollow, and it is placed on a hard even surface .. wat will be the permissible stress on the legs ??

I was just guessing (20ksi) that cast iron wouldn't be as strong as steel. If it is ASTM40 grey cast iron, the compressive strength is min. 120,000 psi (= 120ksi). You'll have to determine the total cross-sectional area of the four legs..., and then maybe assess the bending stress in the tabletop.

You must also take into account the length of the legs (assuming they're equal lengths) and apply Euler's Formula utilizing the "slenderness" of the legs. Check the CR4 blogs or Google to obtain the formula or use the Engineer's Toolbox software package found in this Forum.

To be safe I'd use an allowable Yield Stress (Fy) = 18.0 Ksi Maximum.

By using the slenderness ratio you can then calculate the Allowable Compressive Strength (Fc) by applying the Euler Formula.

Once Fc is found (in Ksi or PSI) multiply it times the cross sectional area of each leg (in square inches) and again by 4 legs. That'll give you the allowable (maximum or permissible) load that the table can safely support.

A somewhat interesting historical exhibition of the somewhat unique compressive strength as well as general durability of cast iron is viewable at http://www.civil-war-photos.com/Misc/imagepages/Bridge%20across%20Rock%20Creek,%20on%20Line%20of%20Penn.%20Ave.,%20carrying%20Washington%20Aqueduct..html and other related sites. After construction before the Civil War, I believe these twin 48" cast iron pipelines, in the form of an arch with over 200 feet clear span, for about fifty or sixty years basically carried the dead load/weight of the bridge structure, the water of the Washington Aqueduct, along with that of the crossing rail and other traffic of a little ol' street called "Pennsylvania Avenue". I believe the pipes incidentally are still in service, though now basically encased in a larger reinforced concrete bridge. I believe the Capitol Dome has likewise been supported by cast iron for a very long time. Modern ductile iron pipe and similar cylinders, with a compressive strength some less than gray cast iron but incidentally/some uniquely still ~10-15% greater than its 42,000 psi yield strength, is also now used in some modern columnar applications around the world such as trenchless "push-me" pipes, piles and poles etc.

Strangely a 4 legged stiff table with a centrally placed load carries the load on two diagonally opposed legs (assuming rigid legs and hard surface on which the table is placed). The other two legs only serve to stabilise the load. It has to be assumed that the legs are exactly equal in length and that the surface is perfectly flat for all 4 legs to carry the load.

A 3 legged table with a centrally placed load will share the load between the three legs.

My chair with 4 legs is sitting quite nicely on its 4 legs due to the flexibility of the seat joining the 4 legs. But my table is a bit wobbly as it is a bit old and has lost a little of its squareness but somehow retained its stiffness.

So it's all down to geometry, stiffness, tolerances and - as has been mentioned before - buckling of the legs and of course the strength of the material.

While I think the point is well taken, and most certainly for a perfectly aligned/ centered "point" load, I think it is possible more than two legs could near immediately carry some load depending on a practical distributions of real loads, and maybe also in the case of any sufficiently high point load that challenges the strength or deformation of any one leg [also, I meant to write in my prior post that the compressive yield strength of ductile iron is 10-15% higher than the tensile yield -- these properties also affect bending strengths via shifts of neutral axis etc.]

Dear Friend,

You have not informed

1) The height of the stand and distance between the legs, the cross section of the leg and its span.

2) The distance between the legs, whether it is tapering at the top

3) The braces if any and its spacing in the legs, whether in all sides or only 2 sides opposite to each other side.

This is very essential to decide the stability of the stand, buckling effect, to decide whether the legs act as a STRUT or a COLUMN . etc.

If the length of the leg is more than 20 times of the cross section of individual leg ( I consider cross section is equal for all the four legs) then it is a column, and If it is less than 20 times, it is strut. The calculation of buckling for strut and column is different.

More than the compressive force, the buckling effect plays a vital role for safe carrying of the load or else it will fail.

DHAYANANDHAN.S