Single Angle Design Based on the Steel Construction Manual, 13th Ed.

[vcueto80]

I have two questions regarding single angle design. First question is in Part 16 Section F10.2. The yield moment is specified for section (i) and (ii) but not for (iii) and (iv), so I assume the yield moment is calculated using the principal axis for (iii) & (iv)?

Question 2.
Say I have a beam that is a single angle (similar to a lintel supporting brick above an opening).The brick load does not pass through the shear center of the single angle causing torsion. The single angle has no lateral-torsional restraint in between the supports. So I have flexural, shear, and torsional stresses. Part 16, Section F10 is for flexure of single angles, Section G4 is for shear of single angles, what about torsion? Would this scenario be Section H3.3? Say I can restrain the angles at the supports for all the reactions including torsion.

I don’t know if this is standard but I seen it many times where the single angle lintel carries the brick above a small opening and it just bears on brick 8 inches on each side without any lateral-torsional restraint. Many architects show this feature on their arch details. Most details I have seen have a structural wall, then about a 1-inch gap and then the masonry veneer. And at an opening it has a steel angle supporting the veneer with no attachment to the structural wall. Is the steel angle required to have some type of lateral-restraint because if an angle just bears on the brick how is the torsional restraint achieved at the ends?

Regardless if I have a single angle lintel or not, if I have flexure, shear and torsional stresses acting on a single angle what do I check for torsion, H3.3?

 

[Lion06]

1) I would say Yes, use the principal axis (since those sections are for bending about the principal axis)

2) You are still bending about a geometric axis, and F10 deals with this using Me. As long as you don't have some ridiculous eccentricity, I believe you are ok neglecting the torsion and using F10 as-is.

 

[vcueto80]

I guess I don't see it bending only about the geometric axis, unless somehow the brick is providing lateral restraint. I see the resultant force of the brick acting a certain distance away from the shear center of the angle. Is this only if there is arching action? I agree in some cases torsion is negligible. See attachment

 

[Lion06]

It is bending about the geometric axis. You can obviously break this into components about the principal axes, but the spec takes this into consideration by reducing the yield moment where there is no lateral-torsional restraint at all.

Analyzing a single angle in flexure is really not th easiest task in the world to do from a mechanics standpoint, but I believe AISC takes those pesky mechanics issues off the table with the design provisions.

 

[cntw1953]

The tendency of angle to rotate is somewhat prevented by the brick. However, I think you can resolve the torsion into shear and add it to the shear produced by weight, then check combined shear and bending stresses.

 

[Stillerz]

Read Commentary pag 280, second & third paragraphs.

 

[vcueto80]

Thanks, I guess I am looking into this too much. I was also reading the commentary on pages 16.1-279 thru 16.1-283. I thought the bending for the geometric axis only was when the load was through the shear center of the angle.

 

[Stillerz]

See also the Design Examples of AISC 13, Example F.11 on page F-39.

I think this is exactly the case you'd have.

 

[vcueto80]

Let me repeat that I thought bending for the geometric axes was only if the load was through the shear center or if the angle is restrained torsionally

 

[vcueto80]

or restrained in one direction (ignore is restrained torsionally)

 

[vcueto80]

Thanks Stillerz, I just read Example F.11 and the problem has to assume bending about the geometric axis. From there I gathered that the loading was through the shear center.

Thanks for the help guys and/or gals. I have enough information for what I have been looking for.

 

[Stillerz]

AISC might be well served to change the text to read "loading in the direction of the geometric axis" or something to that effect. It is confusing because loading in the direction of the geometric axes can still cause bending about the principle axes.

 

[Stillerz]

Also, be sure (like the example notes) to check leg local buckling since the toe of the angle is in compression.

 

[BAretired]

Is the steel angle required to have some type of lateral-restraint because if an angle just bears on the brick how is the torsional restraint achieved at the ends?

If the brick above the opening aligns with the supporting brick, torsional restraint is not required at the ends because the reaction is centered on the brick, i.e. the bearing pressure under gravity load is uniform.

The torsional moment in the angle at the support is R*e where R is the reaction and e is the eccentricity from center of brick to heel of the angle. Torsion is felt by the angle because its shear center does not align with the reaction. Torsion is not felt by the brick supporting the angle.

Torsional rotation is zero at each support and maximum at midspan. The rotation and torsional shear stresses can be easily calculated.

Wind loads on the angle, however, must be resisted by some form of lateral bracing to the structure.

BA

 

[Stillerz]

Don't turn it into a science project. Engineering is more or less an applied science. Scientists and research engineers did the science part. Take what they gave you in the book apply it.
I kid somewhat...i understand the concern and the wanting to know the subject thoroughly.

 

[ishvaaag]

Furthermore these things give problems. We had one 25 years ago when a junior architect checked an equal legs L and put weak supports too far apart. On the rotation, a parabolic section of supported thin brick veneer fell.

 

[southard2]

I have wrestled with how to interpret the single angle design spec for years. I've also noticed that no one does single angle design in any structural software package I know of. That said I just avoid it completely. See technical notes on brick construction 31B. This is what I use to design steel angle ledger angles for brick.

http://www.gobrick.com/html/frmset_thnt.htm

To provide torsional restraint I usually just redhead in an expansion bolt at 2 to 4 feet on center. Keep in mind too that brick ledger angles only have to carry the weight of a trangular portion of the brick weight above due to arching action.

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[HouseBoy]

Keep in mind too that brick ledger angles only have to carry the weight of a triangular portion of the brick weight above due to arching action.

The above (posted by Southard2) is true but I want to point out a "problem" that I have seen numerous times when there is not adequate brick above the lintel to create such "arching action. There are a few different scenarios that cause a lack of arching action such as insufficient height or unsymmetrical layout or the odd location of a window above the lintel.

Just want to highlight this issue...

 

[spats]

An interesting article related to this subject, although it's quite old, is from the 1st quarter 1984 AICS Engineering Journal entitled "Safe Load for Laterally Unsupported Angles". The article presents load tables for angles used as beams where the applied load is within half a leg length on either side of the shear center. Right or wrong, I've used these tables many times to design single angle beams.

For anyone that is an AISC member, this article (and many more) can be downloaded from aisc.org by clicking on "ePubs".

 

[JrStructuralEng]

I have seen buildings (on several occasions) with 3-4ft wide openings in the brick and no supports except for the door frame. Although I don't recommend this, it shows that the opening has very little loading on it and the door still works fine! (i.e. negligible deflections).

as houseguy mentioned, remember the science behind the arch. I would suggest the mortar even has some tensile capacity and thus, almost no angle is needed in many occasions.

And I agree, I wouldn't turn this into a science project either, oversize the angle enough so you can neglect torsion. Recognize that the brick will help to resist torsion, and don't spend too much time on the design because it is just an angle. If you upgrade the angle to and 4x4x3/8 instead of a 3x3x3/16" and spend 1 or 2 hours less design time your client will still be ahead.