steel section, IPE beams, I beams, etc why do dimensions differ? 2

[lolobau]

Hi

I was always wondering why do the real dimensions from the standard steel section always differ a bit from the Designation?

1. For example I-Section 533x210x82 can vary from 528,3mm height up to 544,6

2. H-Section 305x305x97 is 307,8mm high there is non of 305mm


One other question. Why are I beams called in the end with the kg/m (sample H-section 254x254x73 (kg/m)
and angle section (sample 25x25x3(mm)) on the end with the thickes?

lolobau

 

[dcarr82775]

On your 1st question, the fabrication is based on a certain depth of roller between the flanges (T distance in AISC terminology). The flanges are made thicker to make a heavier section, but the T distance remains the same.

The rest is I believe is rounding from imperial measurements and whole numbers.

 

[lolobau]

Mr dcarr, oh my goodness you are right. Only now I see it in the table. The height of the web is always fixed only the flange thickness differs and therefor also the total height. So question 1 is answered

 

[JStephen]

On question 2, note that 305mm = 12 inches, so it was a nominal 12" section converted to metric. Would have made more sense to just call it a 300mm. With imperial dimensions, the height will be shown in decimal inches, but won't be exactly 12" either.

Historically, with the tabulated properties, I believe they were a summary of what various mills produced, but not necessarily a directive that mills ought to produce those specific dimensions, so there was some variation. So in the tabulations, you might have the inside corner radius shown as the largest regularly produced by any mill (detailed for clearance purposes), while the cross sectional area or torsional properties were based on the smallest radius regularly produced, etc.

 

[racookpe1978]

Structural steel definitions are just about as complicated by tradition and practice and 1850's technology as nominal pipe sizes.

Do NOT ever assume the "conventions" are exactly logical nor "exact" but use them as they are. Fighting the system won't work. Understanding it will let you adapt, adjust, and cope.

First thing to do is remember ALWAYS that the definition of the steel member IS the shape itself (how it was rolled and formed) and the "label" of the steel member IS what you must "call it" to identify the member to other engineers and purchasing groups and the actual dimensions of the member ARE NOT necessarily related to either of the above terms. Just be content to ALWAYS properly label the member, regardless of metric or ANSI.

Angle iron (L and AN) is defined by depth and width and thickness of each side. Those are easy. Both legs are always the same thickness, unless you special order something.

Hollow square shapes are rectangles (HSS - Which should be anything else!) and are defined also by depth and width and wall thickness.

"Hollow Round Steel things" are further refined into Structural Steel Tubes, Structural Steel Pipe, and Mechanical Tubing, and "Pipe". Order what you really want to get: Structural round hollow things are NOT rolled and toleranced much more finely than liquid-carrying "pipe". The round hollow structural steel shapes use different specifications for thickness and straightness and mill wall thicknesses and strengths as liquid-carrying "pipe" - EVEN THOUGH THEY MAY HAVE THE SAME DIMENSIONS as "pipe". Tube and Pipe shapes are named differently and dimensioned differently.


Solid rectangles are plate or flat bar, with some overlap depending on the company. The difference depends really on how the rectangles were originally just hot rolled and trimmed, hot and then cold rolled to more precise dimensions, or re-formed from billets as thin sheet metal rolls. The larger but thinner flat bat (6 or 8 or 12 inches) overlap with cut plate at some thickness - 1/8, 1/4, 3/16, 1/2 for example. Typically the thicker plates are bought as "square foot" (sq meter) by thickness in standard widths, specified often by thickness or weight/sq foot. the thinner plates are sheet steel, most often by standard "gage" numbers - which have to be translated into dimensions. Some overlap again.

"Flat Bar" steel is always depth x thickness, cut or ordered to length required. Make sure you specify cold-rolled or hot rolled - The corners and tolerances are very different. If there is ANY chance somebody ANYWHERE will confuse things, specify "round bar" "hex bar" or "Square bar" or "flat bar". Usually just specifying "diameter" is enough to make sure you get a round shape, not any other.

C and MC and CH all are terms for "C" shaped members. They are defined by nominal depth - NOT actual depth! - and then by weight/length. Lbs/ft in ANSI units; kg/m in metric units. Note: The "legs" on a C shape are sloped, and require special washers or nuts to bolt up square and perpendicular if you are bolting through the legs, and not the web. Thickness of the leg is NOT defined (measured) at the tip nor the root.

If you specify a rolled or bent member formed from plate into a "C" shape, the legs will be same the same thickness of the web, and the legs will be perpendicular and the same thickness across their whole length. The "outside corner" on a rolled "C" is rounded, that same outside corner on a hot-rolled C shape will be exactly a right angle. This often makes a difference in bolting up C-shape members, fitting them up exactly, coping the flanges and webs, and welding them.

WF, W, I are also defined by nominal height of the web and then by weight/length. Again lbs/ft or kg/m. The difference between the three are the actual shapes of the upper and lower flanges: Flat all the way across or sloped (like a C channel). The "family or nominal height of each W/WF member is set so a single column in a high-rise building can "stack" a single family (W16 or W24 or W30 for example) all the way up a single column. Each joint between vertical shapes of each different weight/ft designation will exactly match the inside dimension of the next higher weight member. The result is a good, strong, consistent load-bearing member from the bottom (highest stress, highest weight/ft, thickest web and flange sizes, strongest shape) to the next lower weight member, to the next, to the next and so on up to the top. Every joint can be defined and connected with standard sizes and offsets by rivets, bolting, or welding. Since each member of a family has the same inside dimensions to allow this stacking of columns, each successively larger member is thicker and wider and higher than its lighter weight brother or sister.
Most metric shapes I have checked have been designated by nominal sizes (NOT exactly translated by 25.4!) and by weight/meter. Other countries use a "EXACT" shape designation: "304.5" mm NOT just the "300 mm" family.

So, is it clear as mud now? 8<)

 

[racookpe1978]

Order what you really want to get: Structural round hollow things are NOT rolled and toleranced much more finely than liquid-carrying "pipe"

My error above!

This should be:
Order what you really want to get: Structural round hollow things are MUCH MORE CLOSELY rolled and toleranced wunder many more controls than liquid-carrying "pipe" shapes.
This is because structural steel and structural tubing is required to resist external bending and twisting forces; and (as a column) must be able to resist the fatigue and collapse and buckling end forces that an internally-pressurized liquid or gas-carrying system will never see. Those same internal pressure forces that straighten a bent pipe, that round it out to a better and stronger shape, and that act in the strongest direction of the pipe's wall are not only absent in structural analysis. The sideways twists at joints and intersections from loads and buckling forces common in structural analysis of most concern are absent in piping.

 

[JedClampett]

racooke1978, one important thing to point out is that the fact that they can roll hollow shapes to such tight tolerances has been used to actually reduce the thickness of the shapes.
i. e. when you order a Round HSS 4.5 x .375, the design wall thickness is actually .349 inches. The properties in AISC are based on the minimum thickness that can pass the ASTM.