Single Angle Bending Allowable BTH-1

[T_K]

I'm reviewing a design to the rules of ASME BTH-1-2014. There is a deadweight which is supported by two single angles (3x2x1/4 with some areas machined away). They are unconnected and support half the deadweight each. These angles are unbraced over 48 inches. I believe the angle cross sections meet the rules for non-compact for "legs of single angle struts..." in Table 3-2.2-1. I have no control over the design at this point (it already exists).

In section 3-2.3 for flexural members, the most appropriate sections is 3-2.3.2 I believe since this isn't explicitly major or minor axis bending because it's an angle, and this is a non-compact section since there is no compact section listed for angles in Table 3-2.2-1. However in reading through the allowable bending stress formulas in 3-2.3.2, there is nothing that appears to me to apply to single angles. 3-9, 3-14, 3-16 and 3-17 seem to apply to I-beams, box beams, and channels. The closest approximation is 3-18, but this is for Tees and double-angle members. This is the equation I would tentatively try to use, even though the words don't explicitly call out single angles. Is there something I'm missing? Anywhere else in BTH-1 that I would turn to for an allowable bending stress for a single angle in bending?

 

[racookpe1978]

Provide a dimensioned dwg or sketch. Your questions cannot be decyphered as written, at least not by me.

"Some areas cut away" ????? Those area removed specifically MUST be detailed, because those areas are the stress risers!

 

[T_K]

Sorry about that. Heart of the question is really what bending allowable to apply to a single angle member in bending, but fair enough to show a sketch. Having trouble uploading one, so I think you should be able to find a crude sketch at the following link:

http://files.engineering.com/getfil...d-6257-4c9f-8e49-60a0c1c0d719&file=sketch.png

The two angles are noted as part 9. You can see the 3" leg of the angle is machined down to 1.25" to support the drum at the centre. Not sure if this affects which allowable to use (obviously it affects the beam's local properties such as r, I, etc).

 

[JStephen]

T-K, I believe the allowable stresses in BTH are based on AISC steel design, with modifications to specifically include a factor of safety. With that in mind, I would refer to the latest AISC standard (should be available free online), work from that, and increase the factor of safety to match BTH requirements.
Angles in general are a mess, and it can be worth a few extra ounces to simplify the design by using flat bars or other symmetrical shapes.

 

[steele6707]

You may also want to look at the AISC 9th edition (allowable stress design). There is a specification and commentary (page 5-309 thru 5-323 in my copy). Like JStephen suggests I would also look at the latest edition to see if that is more conservative, but the 9th edition will have equations that look more like BTH 1-2014. You will need to modify the 9th edition allowable values to a more appropriate safety factor as JStephen noted.

 

[racookpe1978]

Your sketch indicates you have only 1-1/4 inch remaining below the large "arc" of the notch, but you did not indicate whether the 3x2-1/4x1/4 angle iron (piece 9) is aligned with the 3 vertical or 2-1/4 vertical. Either way, you have very little remaining strength - under the load - in the center of the notch - at the point of maximum stress.

Assume only a 1-1/4 x 3 x 1/4 angle iron, or a 1-1/4 x 2-1/4x1/4 angle iron all the way across. If you really don't like that assumption, use a single rib 1/4 x 1-1/4 high. But your structure is weak in the bending resistance of the angle iron's vertical web.

 

[T_K]

Agree with those saying AISC rules could be used as an analog, with the appropriate Nd safety factor applied for BTH-1. I do see single angle allowables in Section F of AISC A360. I'm assuming I'd work out the critical moment, Mcr, then express that as a critical bending stress by dividing by the section modulus, and apply the appropriate safety factor Nd from BTH-1 for the safety factor in place of Ω from section B3.2 of AISC A360 to get my allowable stress. It still seems unfortunate that BTH-1 has a slenderness ratio limit specified for a single angle, but no flexural allowable specified for that case.

racookpe1978, you are correct it wasn't labeled. I just took a screenshot of the sketch from the existing report. The vertical leg was the 3" leg and the remaining whole horizontal leg is the 2" leg. I agree that the basic assumption should be that this is a 1-1/4 x 2-1/4 x 1/4 angle iron all the way across to determine the bending stress at the critical centre. My issue was that I wasn't sure what allowable to compare that to. Pretty sure in this case the design is inadequate and that we'll have to ship it back and modify, but I wanted to be able to say with confidence what the exceedance of failure over the code specified limit was.