To any contractors or structural engineers:
In lowrise commercial construction, I almost always see HSS columns with I-beams. And I wonder why this is the standard practice.
As I understand it, HSS (tube steel) is commonly used for columns, because:
-it is stronger than an H-section pound-for-pound, and
-it has less surface area (cheaper to paint)
By the same logic, shouldn’t rectangular HSS be preferred for beams? Why I-beams?
IANASE,nor contractor,yet have experience with aspects of your question,and will make a SWAG.
The paint issue IS a big concern-lots of inaccessible area inside a tube.
Connections might be an issue (flanging).
The critical factor in a beam is depth of web and I or H sections can be had with a variety of flanges,whereas mechanical tube,though having some range of sizes, is always the same gauge (section thickness) throughout,and will always have a seam.
Last I knew (about a decade now) odd sizes of tube,say 14 x 2,though listed by vendor were actually impossible to obtain and pricey.Meaning one had to order sufficient tonnage before being made.Thus long lead times even if price wasn’t a factor.
It’s only stronger in small sections.
Consider a corrugated cardboard box. A box of, say, 6 inches square can be quite rigid and strong. A small and short enough box can withstand being stood upon by an average person.
Now consider a 2 ft by 2 ft box. Do you think it can hold up a person’s weight?
I-beams are necessary for the load they must carry. Structural shapes are chosen by the task to be performed.
I’m sure someone more qualified than I will be along shortly to explain further.
*paging BobLibDem … pick up the red courtesy phone … *
It possibly has something to do with inertia, or second moment of area, I can’t remember exactly which one is the correct term (If either are). The I beam has the largest inertia value for a given cross section (cutting across so you get the I shape). Deflection under load is inversely proportional to the inertia value.
I don’t know why they don’t use I beams for the columns, but its possibly just an aesthetic thing.
In summary, a whole heap of guessing, and a question…aren’t H and I beams the same thing more or less?
I-beams taper from web radius transition to end of flange.
H-beams have parallel surfaced flanges on the inner face,post radius.
Cooky,I and H beams are often used for columns.
How do square-tube or round-tube colums compare to I- or H- beams in end-to-end compression when they’re vertical? Likewise, how do they compare in lateral deflection when they’re horizontal?
And, for a given strength, how do they compare in price?
I’m not a structural engineer or ironworker, but I’d wager that the tubes are cheaper than beams in the same load-bearing capacity when used as vertical columns, and the tubes are more expensive and/or structurally unsuitable for use as horizontal beams.
FWIW, around here, I’ve only noticed tubular columns being used in one-story buildings where the only horizontal members attached to them are the roof trusses. They may be tall buildings (eg: “Big Box” stores like Costco or Bed Bath and Beyond) but one story nonetheless. Multilevel buildings invariably seem to be made of I- or H-beams, and a goodly amound of diagonal bracing. (We do get a lot of earthquakes here.)
Tubes resist deflection equally in all directions. I beams resist defelection verically more than they do horizontally. Pound for pound I beams are stronger per-weight than tubes when used as beams.
Take a simple beam supported at both ends and with a down load in the center. The beam will bend so as to be concave upward. The top of the beam is in compression and the bottom in tension. In the center between top and bottom there is the neutral line where there is no load whatever. Thus, the I beam is designed to place the greatest amount of material at the top and bottom where the maximum stress is. A box does the same thing but uses more material.
I agree with Dan Blather. Tubular shapes are good for columns because they have equal section moduli about either axis. I beams are better for beams because they can provide much more strength per pound about one axis than it does another. Looking at my steel book, for example a 16x16x1/2 tube has a section modulus of 150 in^3 and weighs 103.3 lb/ft. You can get a W 16x89 to give you a section modulus of 155 in^3 about the strong axis at a weight of only 89 lb/ft. Of course you pay for this by having a section modulus of only 31.4 in^3 about the weak axis, but since it is a beam this won’t be a concern unless you have loads in the other direction to consider.
You’re close. It’s the second moment of inertia, or the moment of area.
If the fancy math is too complicated, think of it this way: The columns are being squished, but the beams are being bent. Those are two completely different forces, and it’s not surprising that two different kinds of structure are used. After all, a stack of bricks makes a pretty good column, but a really, really, lousy beam.
Just to be more complete about the columns- when you compute the buckling capacity of a column you use the minimum moment of inertia of the section. Having both axes of the section the same will result in the maximum buckling capacity for the given weight.
Thanks - that’s very helpful.
Is a W16x89 also 16" wide? If not, how heavy is the comparable rectangular tube?