Strongest Section Question

There is a problem I have encountered in the past which has several different cross sections of comparable size, such as a W-Beam, C-Channel, L-Channel, Hollow Tube, etc...

The question asks something like "which of the following sections is stronger." I am drawing a blank as to what they mean and how to solve it. I assume that they may be referring to which section would have the higher moment of inertia, thus can carry the most bending stress.

Maybe I need to be a bit more specific with my inquiry...

attached is one sample problem for the morning session. The solution is as follows:

"Tube sections are better able to resist lateral torsional buckling than channel, wide-flange, and double-angle sections and are therefore generally more efficient for use as beams with long unbraced lengths." The correct answer is A.

You do not see too many tube sections as beams, so intuitively, I would not choose a tube as my first choice. Therefore, how would you go about ensuring that you are not caught using bad intuition?

wstahlm80 said:

Maybe I need to be a bit more specific with my inquiry...
attached is one sample problem for the morning session. The solution is as follows:

"Tube sections are better able to resist lateral torsional buckling than channel, wide-flange, and double-angle sections and are therefore generally more efficient for use as beams with long unbraced lengths." The correct answer is A.

You do not see too many tube sections as beams, so intuitively, I would not choose a tube as my first choice. Therefore, how would you go about ensuring that you are not caught using bad intuition?

Click to expand...

try the attachment again...

While I know the attachment helps to make the question understandable, we need to ensure we don't violate copywrite laws. I've removed the attachment. Please reference people to their version of the practice exam instead of posting a scan of the document. Thanks, SapperPE (nick named Highway to Hell for the time being)

I wouldn't think of any question like this in terms of things you see...this is strictly a theoretical question. Answer it using what you know of statics and structures.

the other key to that question is the "unbraced length of the compression flange"

In most typical applications the top of the beam (compression) flange is embedded into a diaphram of some sort effectively bracing it. The tube section will provide the most resistance to bracing on its own so it's the correct answer.

wstahlm80 said:

wstahlm80 said:

Maybe I need to be a bit more specific with my inquiry...
attached is one sample problem for the morning session. The solution is as follows:

"Tube sections are better able to resist lateral torsional buckling than channel, wide-flange, and double-angle sections and are therefore generally more efficient for use as beams with long unbraced lengths." The correct answer is A.

You do not see too many tube sections as beams, so intuitively, I would not choose a tube as my first choice. Therefore, how would you go about ensuring that you are not caught using bad intuition?

Click to expand...

try the attachment again...

While I know the attachment helps to make the question understandable, we need to ensure we don't violate copywrite laws. I've removed the attachment. Please reference people to their version of the practice exam instead of posting a scan of the document. Thanks, SapperPE (nick named Highway to Hell for the time being)

Click to expand...

My bad on the improper attachment. For those still viewing this problem, please refer to:

Page 33 of the NCEES Sample Questions & Solutions book for the Civil PE

^ that's the way to do it lusone:

It's the closed section, the Tube, because it resists lateral torsional buckling.

wstahlm80 said:

Maybe I need to be a bit more specific with my inquiry...
attached is one sample problem for the morning session. The solution is as follows:

"Tube sections are better able to resist lateral torsional buckling than channel, wide-flange, and double-angle sections and are therefore generally more efficient for use as beams with long unbraced lengths." The correct answer is A.

You do not see too many tube sections as beams, so intuitively, I would not choose a tube as my first choice. Therefore, how would you go about ensuring that you are not caught using bad intuition?

Click to expand...

jillhill said:

It's the closed section, the Tube, because it resists lateral torsional buckling.

Click to expand...

Hey that's just what the solution says. Very helpful

You know what would be really helpful - from anyone with enough knowledge to provide it - is a simple description of when/why you would use each of the most common steel shapes. For instance, you'd likely use tube steel when..., you'd likely use a channel section when..., you'd likely use a t-section when... etc.

This would be for us non-structure people who just don't 'get' the whole Ix Ad^2 thing as applied to practical situations. If there is a similar problem on the test I'd be thrilled to have a snowball's chance of answering it correctly.

That's a difficult request because there is no definitive answer. The key to the example problem was in the problem statement itself (which was removed because it was a scan of a copyrighted document) where it specifcally said there was no lateral bracing. That is what made the tube section the best choice.

Sorry, I couldn't be of much help.

I understand my questions is difficult - but perhaps I could start with my very limited knowledge to give an idea of what I'm talking about. From Wikipedia I found the following, but was unable to find articles for angle, channel, bar, plate. Perhaps someone else would know.

Pictures of the various structural shapes can be found on page 58-2 of the CERM.

I Beam

Advantage:

The web resists shear forces while the flanges resist most of the bending moment experienced by the beam. Very efficient form for carrying both bending and shear loads in the plane of the web.

Disadvantage:

Cross-section has a reduced capacity in the transverse direction, and is also inefficient in carrying torsion, for which hollow structural sections are often preferred.

T-beam

Advantage:

Top of the t-shaped cross section serves as a flange or compression member in resisting compressive stresses.

Disadvantage:

Compared to an I-beam because it has no bottom flange with which to deal with tensile forces.

Tubing (Hollow Structural Section - HSS)

Advantage:

HSS, especially rectangular sections, are commonly used in welded steel frames where members experience loading in multiple directions. Square and circular HSS have very efficient shapes for this multiple-axis loading as they have uniform geometric and thus uniform strength characteristics along two or more cross-sectional axes; this makes them good choices for columns. They also have excellent resistance to torsion.

HSS can also be used as beams, although wide flange or I-beam shapes are in many cases a more efficient structural shape for this application. However, the HSS has superior resistance to lateral torsional buckling.

Angle (L-shaped cross-section)

Channel ( [-shaped cross-section)

Bar

Plate

Happy said:

I understand my questions is difficult - but perhaps I could start with my very limited knowledge to give an idea of what I'm talking about. From Wikipedia I found the following, but was unable to find articles for angle, channel, bar, plate. Perhaps someone else would know. Pictures of the various structural shapes can be found on page 58-2 of the CERM.

I Beam

Advantage:

The web resists shear forces while the flanges resist most of the bending moment experienced by the beam. Very efficient form for carrying both bending and shear loads in the plane of the web.

Disadvantage:

Cross-section has a reduced capacity in the transverse direction, and is also inefficient in carrying torsion, for which hollow structural sections are often preferred.

T-beam

Advantage:

Top of the t-shaped cross section serves as a flange or compression member in resisting compressive stresses.

Disadvantage:

Compared to an I-beam because it has no bottom flange with which to deal with tensile forces.

Tubing (Hollow Structural Section - HSS)

Advantage:

HSS, especially rectangular sections, are commonly used in welded steel frames where members experience loading in multiple directions. Square and circular HSS have very efficient shapes for this multiple-axis loading as they have uniform geometric and thus uniform strength characteristics along two or more cross-sectional axes; this makes them good choices for columns. They also have excellent resistance to torsion.

HSS can also be used as beams, although wide flange or I-beam shapes are in many cases a more efficient structural shape for this application. However, the HSS has superior resistance to lateral torsional buckling.

Angle (L-shaped cross-section)

Channel ( [-shaped cross-section)

Bar

Plate

Click to expand...

Thanks Happy....This direction is a start of how I was hoping my request would be answered!

Further googling has helped me find this site that explains strutural steel shape applications:

http://faculty.delhi.edu/hultendc/A220-Wee...ecture-Web.html

Really it comes down to what works, fits, and the cost for the forces required..

But here is a couple:

• Angles and Double Angles are good for roof trusses because you can stick a gusset plate down the center to attach the diagonals to. So a lot of roof trusses in warehouses will be a double angle top and bottom chord with a 1/2" gusset plate in between which the single/double angle diagonals can bolt to.
• Sq. or Rectangular tubes are good when you need a strong member in a small area.
• Round tubes are used a lot for temporary structures to clamp lights to (the 1 1/2" sch. 40 pipe is standard). Or Scaffold Structures.
• I-beams/W-Beams are the bread and butter of steel structures. They give you lots of welding space (unlike round tubes) and are relatively strong along both axis'.
• Solid rounds are used for tension rods
• Channels can be used in between I-Beams as perms or along the edge of a catwalk if you don't require much strength. Once again it's weaker than an I-beam and would mostly be used to save on cost.
• I don't find too many T-Sections being used for a standardized purpose (at least none comes to mind).

Also the above shows channels being used for decking but normally that's been substituted with corrugated decking.

This is just what i've seen in the field mostly.

jillhill said:

It's the closed section, the Tube, because it resists lateral torsional buckling.

Click to expand...

I personally disagree with this official answer given by NCEES. They seem to put too much stock in that word TORSIONAL. The section is not really in torsion, which would be a STRENGTH issue. It has a compression flange which is susceptible to BUCKLING, which is a STABILITY issue. The reason for that name (lateral-torsional buckling) is that unlike a column whose section is under UNIFORM COMPRESSION (and therefore the ENTIRE section tends to buckle at the critical load), for a flexural member, the compression flange 'tries to' buckle sideways (laterally) while the tension flange has no such tendency. This can be shown as if the bottom flange stays put while the top flange buckles sideways, thereby looking like the section is under torsion. The capability of a closed section to have greater STRENGTH against torsional loads, does not directly imply that it would have a greater buckling load. In my opinion, that question is poorly designed and cannot be answered as it is.

Just my humble opinion.

Thanks for the help/tips on this. Keep them coming if anyone has more to add.

The fact that a question asking one to choose a particular structural steel shape for a given application appears on both the 2008 and 2011 NCEES sample exams leads me to conclude that there is a high percentage chance another similar type question will be staring at me on Friday.

If a member is susceptible to lateral torsional buckling its strength will diminish. I've done up a little shindig on LTB over here:

http://www.wikiengineer.com/Structural/Ste...rsionalBuckling

As the unbraced length of the member increases it is more likely to be susceptible to LTB (see the first figure).

A box tube does not have a LTB issue. You can find the limit states of each type of beam on page 16.1-45 of the specs in the AISC 13th edition. You can see there that an I-Beam is suseptible to LTB (lateral torsional buckling) and a box tube is not.

That's what i'm guessing the problem is referring to. Although I don't have it to reference so I can't be 100% certain.

civilized_naah said:

jillhill said:

It's the closed section, the Tube, because it resists lateral torsional buckling.

Click to expand...

I personally disagree with this official answer given by NCEES. They seem to put too much stock in that word TORSIONAL. The section is not really in torsion, which would be a STRENGTH issue. It has a compression flange which is susceptible to BUCKLING, which is a STABILITY issue. The reason for that name (lateral-torsional buckling) is that unlike a column whose section is under UNIFORM COMPRESSION (and therefore the ENTIRE section tends to buckle at the critical load), for a flexural member, the compression flange 'tries to' buckle sideways (laterally) while the tension flange has no such tendency. This can be shown as if the bottom flange stays put while the top flange buckles sideways, thereby looking like the section is under torsion. The capability of a closed section to have greater STRENGTH against torsional loads, does not directly imply that it would have a greater buckling load. In my opinion, that question is poorly designed and cannot be answered as it is.

Just my humble opinion.

Click to expand...

Torsion and Laterial Torsional Buckling are two different things