Question regarding Design of Steel Beam with Unreinforced Web Openings

[oengineer]

I am working on several Design of Steel Beams with Unreinforced Web Openings and I am having an issue with the vaule of two-thirds times the Plastic shear capacity of unperforated beam, 2/3 Vp,being higher than the Maximum nominal shear capacity at the location of an opening, Vm.

Basically, I am getting [highlight #FCE94F]Vm = Vmb + Vmt > 2/3*Vp[/highlight] instead of [highlight #EDD400]Vm = Vmb + Vmt < 2/3*Vp[/highlight]. I am not sure why I am getting this?

I am using AISC Design Guide 2.

Suggestions and comments are appreciated.

 

[Agent666]

I'm not familiar with the inner workings of design guide 2, but I have designed quite a lot of openings in my day. Sounds like you are failing this particular check. So either your opening is too large or load is too high or web is too thin or opening is in the wrong location along a beam. With no other information provided on the exact scenario you are looking at, what exactly is the question? In some situations you are going to have to have stiffeners to get it to work.

Also check out the UK SCI P355 design guide, its probably more state of the art than design guide 2, but obviously follows Eurocodes, but still contains some good fundamentals. It's free from here.

 

[oengineer]

@Agent666

I was able to resolve the issue regarding the shear capacity. I used the lower value of Vm < Vp.

@Everyone

One of the beams I have is a W21x48. Since this beam is a non-compact beam for Fy = 50 ksi, how do I verify and/or determine the moment capacity of the beam for the perforated section to avoid local buckling of the compression flange? I do not see anything in AISC design guide 2 on how to deal with this issue.

 

[racookpe1978]

Again, How big and what shape and at what interval are/is the opening(s)?

21x48 is a very "light" member (3/8 web, 7/16 flange and 21 inches tall). Assumptions in a program (or in a designer) perfectly acceptable for a 4x30 or 8x100 rolled shape may become invalid for a very light piece of "near flexible sheetmetal".

 

[BridgeSmith]

As far as resistance to local buckling of the compression flange, it will depend on where the openings in the web are. If there are openings in the web at the flange, then flange is not 'stiffened' by the web and it becomes just a plate, subject to buckling as a plate.

 

[oengineer]

@racookpe1978

The penetration is in the center of the web of the beam and is 5" x 5". There is only one penetration in the beam web.

 

[racookpe1978]

5 x 5 square? That's not a smart decision! You'd be better with an angled square - one with the "points" of the opening up-down - rather than a horizontal top and bottom.

How long is the beam, how is the load supported, how is the beam supported? You have a "complete system" design problem here! Not some abstract "burned hole in the middle of a beam" problem.

 

[Agent666]

I'd like to review/correct a few misnomers that people are posting:-

Firstly, in terms of the overall way these things work, you have a hole, it leaves behind an unstiffened 'tee' top and bottom. You assess the local slenderness limits of the individual tees, the web still contributes and doesn't need to be ignored, in fact its integral to transferring the vierendeel moment across the opening that the web is there. Opening wouldn't work at all if the top and bottom flanges were the only thing resisting the secondary bending. If DG2 doesn't address this then, don't ignore it, because it is a effect you need to consider in the design.

The vierendeel bending across the opening is engaging these tees. So usually if you have different tee heights top and bottom you need to work out 4 sets of effective section properties (i.e. including the effects of local slenderness of flanges and web depending on the direction of bending and which side of the opening you are some may have flanges in tension so flange would be fully effective.). Often for a tee the plastic neutral axis lies in the flange anyway, so only the web in compression factors into the slenderness calculation. Review the publication I posted from SCI, as it covers in detail the effects of slenderness on the tees (stiffened or unstiffened), just relate what it is saying back to the AISC limits. DG2 is getting pretty long in the tooth now days, and the SCI guide is pretty much where its at in terms of the latest research as I understand it. In NZ we have adopted the SCI guidance into our latest composite design standard for example.

The way the SCI guidance works is downrating the web thickness based on the effects of slenderness compared to a compact web outstand, then calculations proceed as per normal. I think if you applied the same logic you are heading down the right track.

Secondly, You can have the opening arranged anyway you like as long as the numbers work out by following some recognised design guidance, rectangle, square, circle (either stand alone or as part of a cellar beam fabrication), hexagon (i.e. your typical castellated beam arrangement), or any other shape that you can apply the recognised guidance to. Circles tend to be more efficient as they equate to an equivalent square that is like 0.9 times the diameter (I forget the exact ratio, but its in the guidance). A rotated square is pretty inefficient for passing anything through it, if you have round ducts/pipes have a round hole, if you have rectangular or square duct or cable try, have a horizontally orientated square or rectangular opening. You are also unnecessarily reducing the shear capacity of the top and bottom tees if the top and bottom regions of the holes are not being used. If you need stiffeners, add them.

I'm not saying you can't have a 45 degree rotated square, but its actually reducing the effective depth of the tee sections by a huge margin and effectively the top and bottom of the rotated square are pretty useless if you are trying to pass a horizontally orientated rectangular duct or something through it.

5" by 5" is a very small hole for a 21" deep beam.

Its pretty scary how big you can go and how much depth of the web you can take out if you follow through the numbers. I've taken out 450mm(~18") deep by 1200mm (~48") long openings in 700mm(~28") deep welded beams before for stiffened openings and composite decked slabs, I would have thought a 3/8" (10mm) web would easily eat a 5" hole, its not what I would classify as a thin slender web by any means (slenderness ratio of 53.6 for the web, shear buckling due to slenderness starts to kick in at greater than 82 in my standard, but not sure how AISC deals with this.

When you see 5" of web depth top and bottom for 48" long opening it doesn't inherently feel/look right, but it works if you can put it in the right location along the beam and trust in the numbers. Generally about 1/4 to 1/3 of the span is the sweet spot for maximising the size/length.

For example here's a picture of one I did a few years back with only 5" depth of web top and bottom in ~28" beam , 10mm (~3/8") thick web, 20mm (~3/4") thick flanges, 6.3m (21') span. Easy to tweak it any way you like with a welded beam until it works, need more web thickness, no problem, bump it up a thickness.