How to analize large web opening and missing flange?

Question

How to analize large web opening and missing flange?

johnsonSDSMT (Civil/Environmental)

(OP)

2 Jul 04 12:13

I am designing a strong back beam that will be holding a 100 kip truck bed up. The beam must be lifted by a crane and there are severe height limitations on the overall system so a oval shape must be put in the center of the beam in the web. It needs to be 20" in depth and ranges from 5" to 8" in diameter.

Also the above flange needs to be removed for 20" directly above the web opening.

My inital thoughts are to basically readjust my section properties (Ix, Zx, Sx) by taking into account the effective area of the flange and web with these missing parts and then just do my Shear, LTB, FLB, WLB and yielding checks.

However I am wary that the web opening may be to big to treat it like a normal "hole in a beam". My text book references a design guild specifically for large web openings however I am working for a company just for the summer since I am starting grad school in the fall for my masters in structural engineering so I am not really in a position to get this book and unfortunatly the engineering department here is more project engineer then design so there is no one with any experience in this more detailed analysis.

Any fast and dirty methods would be appreciated, I am already taking my factored load moment and multiplying it by a F.S of 1.5 to provide extra safety since the loading is dynamic to an extent and I am also planning on having plates welded around the area that will be extruded.

If I do it the way I mentioned above I am having some difficulty in calculating my new Zx, but I have my new Ix and Sx. Thanks

JedClampett (Structural) · Answer 1 · 2004-07-02 12:36:26

This sound dangerous. You're cutting an opening in the region of maximum stresses. Worse than that, you're reducing the moment capacity by cutting the flange away in the maximum moment area. Even if you can get the stresses low enough using your revised properties, there are stress paths and concentrations to be considered. Plus the fact that the strongback will be subject to impact and fatigue loading.
This is not a task for the intern to do. Have your bosses hire an experienced engineering firm.

johnsonSDSMT (Civil/Environmental) · Answer 2 · 2004-07-02 14:16:52

Ever heard of being thrown into the fire?

Hiring it out to some one else is not an option. I do have a structural emphasised civil engineering degree from one of the better engineering schools in the nation and am trying to take all neccassary procations to make this safe.

I am simply asking if my above described method is the correct approach. Not only did I multiply the live load by 1.4 for dynamic aspects before coming up with my factored load but I also put an additonal F.S of 1.5 on the final factored load moment and shear load. The plate stiffeners will be completely surrounding the opening and substantially thick. Sizing the beam is taking along time due to so many variables in finding the reduced section properties unfortantly so I haven't came up with the size yet but it will be sized to handle roughly a 600+ kip load safely according to AISC standards. I know there are things I might not be taking into consideration or naive on but with such a large over estimation of loading I feel confident it should be ok. Thanks for the concern though...

VoyageofDiscovery (Structural) · Answer 3 · 2004-07-02 15:20:29

Hi,

If this strongback is being lifted with two cables in the form of a triangle, you will also have compressive force and perhaps a slight additional end moment at the connections due to the eccentricity.

Regards

VOD

johnsonSDSMT (Civil/Environmental) · Answer 4 · 2004-07-06 10:08:39

A crane will be attaching to the strongback via a pin mechanism through the hole in the middle of the strongback.

johnsonSDSMT (Civil/Environmental) · Answer 5 · 2004-07-06 17:20:57

Alright I found a design guide for web openings of this type however it does not describe how to include the fact that I have a flange opening as well. Are there any design aids that may help me with that or do I just try and adjust my unperforated values accordling.

trainguy (Structural) · Answer 6 · 2004-07-07 17:49:38

Hi johnsonSDSMT.

Where, along the beam length, are the openings? How many are there?

Are the beam ends restrained against rotation?

I don't see any attention to stability in any of your posts, both local, and global.

Please be more specific, at this level of detail, it sounds very dangerous.

With all due respect, there's more to struc. eng. than Ix, Sx and Zx properties calculations. You must determine the failure modes, and design against them.

Please add some info., and maybe we can help...

tg

trainguy (Structural) · Answer 7 · 2004-07-07 17:56:03

I forgot to answer your question:

Your method is not correct. Beefing up the loads and safety factors without attention to actual failure modes is not the way to go.

It also sounds to me that having a web hole without a flange could lead to instability at the top (compression) part of the beam.

Also, Zx is only valid for doubly symmetric sections, which can be shown to attain plastic moment without local buckling of all webs and flanges.

tg

johnsonSDSMT (Civil/Environmental) · Answer 8 · 2004-07-07 18:07:20

I found away to design it without the web and flange openings as I did not feel confident in the design, thanks for the insight.

johnsonSDSMT (Civil/Environmental) · Answer 9 · 2004-07-07 18:16:32

However, what exactly should I be concerned with?

Lateral torsional buckling seems to be the determing factor between LTB, WLB, FLB and Yielding as the beam itself will be compact.

Also checked max shear and deflection and both were fine. Again the scenario is a beam connected to a crane hook at midspan (will have a pickpoint welded on to it). At each end of the beam it will be connecting to the side of the truck bed (100 kips total).

Thanks.

CSEllc (Structural) · Answer 10 · 2004-07-08 00:14:19

Think about this....Do you remwenber the commercial for crazy glue where a man stands beneath a heavy object (I think it was an elephant) "glued" to a hanger with crazy glue?

Now apply this to your problem, You have a 100K truck hanging over your head that has a oval hole thru the web and aabout 20" of flange missing.

I hope you bought some crazy glue....

Listen to your gut....it doesn't work

johnsonSDSMT (Civil/Environmental) · Answer 11 · 2004-07-08 09:52:11

The only reason I initially went that route is because there is an existing one that is made similar with the open flange/open web area which handles the 60 kip truck beds. However the beds I need to lift are wider and larger so I explored that option.

And my "hanger" is a 200 ton capacity crane, but you are correct I don't feel safe with the idea.

Ron (Structural) · Answer 12 · 2004-07-11 00:01:44

Assuming you have now gotten that beam cut-out issue out of your mind (yes, that was dangerous as several have said), I understand your problem to be one of a single crane pick point at the midpoint of a beam, with the 100k body suspended from it at either end, giving you a 50k load on each end.

This system has no lateral support, potential dynamic loading, and unknown failure parameters and consequences. At the least the bottom flange of the beam will be subject to buckling and torsion. Concentrate your efforts, assuming you have adequately sized the beam for the static and dynamic loads, on the flange compression and torsion. You will likely need web stiffeners.

johnsonSDSMT (Civil/Environmental) · Answer 13 · 2004-07-12 10:53:14

How does one check if web stiffners are required? I checked if they were needed due to shear which they weren't. The only other design work I was taught with them is if your factored load strengths are greater then your limit state's strengths then you can alter your Lb by placing web stiffners and then recalculate the limit states with the new Lb.

The beam itself is compact, at the moment its a W40 X 199 beam. The factored load moment is 2550 kip-ft which is based off the live load X 1.4 for dynamic, then 1.6(L)+1.2(D) and then at the end an additonal factor of safety of 1.5 to the resulting moment. So the beam is designed to take a much larger load then it will be. My concern is that I am not taking something into consideration that I should be.

Also, when you say the flange compression and torsion aren't these being looked at in the limit states that I sized the beam for? (LTB, WLB, FLB, yielding, shear, deflection)

My understanding of lateral torsional buckling is that due to the bending there is an axial force being subjected to the flange. This force can cause a twisting of the member and cause it to buckle and must be taken account for.

Also I understand that due to the bending the bottom flange is in compression but how does one analyze if the flange is adaquate?

I am already planning on placing 2 web stiffners below the pickpoint at the midspan of the beam.

Any clarifiaction would be appreciated on the above questions, thank you for your time and help in this matter.

trainguy (Structural) · Answer 14 · 2004-07-13 14:08:37

johnsonSDSMT:

Most beam design equations in most building/bridge standards are tailored to more or less standard applications, i.e. floor beams, roof beams, bridge girders, etc. In all these cases, the ends of the beam, or at supports close to the end, sufficient lateral and/or torsional restraint is provided, and then the engineer checks the intermediate spans for whatever unbraced conditions / lengths exist.

Your case does not compare with the relatively "stable" cases I am describing above. The only thing resembling stability in your system is, I believe, provided by the load itself. While this may work in certain limited circumstances, it is NOT directly comparable to design code applications.

You can put all the web stiffeners you want, I still can't see how this stabilizes your beam ends against twist.

This is a tough one, and in my humble opinion, not the kind of problem to be resolved on an internet message board, even this highly refined one.

That's my 2 cents...

tg

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How to analize large web opening and missing flange?