structural steel platforms: the fiddly bits

[charliealphabravo]

Hi guys. I have my grasshopper hat on again today.

I am designing a small extension to an existing maintenance platform. The loads are minimal and the primary members are ample so I am tempted to use minimal diagonal braces below the platform. My problem is that normally I would not attribute any bending strength to simple shear connections. My question is whether this can be done when the loads are negligible and whether bolted shear connections will snug up and be serviceable for small cantilever areas without considering typical moment connections or diagonal braces at each end of each remote member.

 

[charliealphabravo]

Here is the sanitized detail. I didn't mean for the fabricators title block to show up. Hopefully the moderator can delete the attachment in the OP.

 

[EngineeringEric]

I would think that with that application it should be fine. I agree that i am leery but under the right situation i would be able to still sleep at night.
As a second note, it may be worth field welding the angles to the plates, it is stiffer and will just button it all up nicely. I'd weld it.
As a third note, we typically use two (2) angles for our base of rail details as this is a more uniform loading situation, yes it is overkill but it looks cleaner than the single
And the fourth... If you do not have much force going to those kickers and 3BR3 then why have 8 bolted gusset-to-column connection?

 

[dhengr]

Charliealphabravo:
What’s a grasshopper hat? In my old age I’m missing half of the new age vernacular, these days.
Damn near every connection we design has much more cap’y, in every direction, than we give it credit for, all for simplicity of analysis, design, detailing. Doesn’t the connection of the grating offer another level of redundancy or another energy absorbing layer of structure, in the lateral direction? What are the chances of all of these load conditions happening at the same time? We have gotten so damn chicken shit about doing anything that we know is inherently adequate, except in the most extreme situations, if it is not explicitly covered by a code paragraph. We are literally relinquishing our authority as the experts on these matters to the tech writers or the democratically selected code writers, what the hell do they know? Look at thread507-363503, and put in a couple of diag. rods if you are really in doubt about the lateral cap’y of your design.

 

[charliealphabravo]

ahaha thanks on 4, i saw that too. that is just the fabricator's gloss unless they know something i don't.

oh no. grasshopper is old school. some days the master i am. some days the grasshopper. ahhh sooo.

 

[paddingtongreen]

I thought the original grasshopper was the seeker who sat at the Zen Master's feet, to learn the meaning of life.

When I was doing refinery design, I learned to try for an element of redundancy with this type of situation because the pipers and sparkies tended to cut holes in, and hang stuff from these platforms.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[wannabeSE]

I am assuming the new platform extension is framed with channels. I would be leery of the design because channels are not good in torsion. However, the member sizes, dimensions and loads are not given, so there is a chance that it will be OK.

 

[MikeHalloran]

A maintenance platform extension will make a nice attachment point for a snatch block when the crew wants to use the winch on the 6x6 to bend an adjacent tower into alignment.

Or maybe they'll just put the truck on the platform.

Don't bet that it can't be done.



Mike Halloran
Pembroke Pines, FL, USA

 

[TLHS]

It's definitely something you can run a reasonable check on. Do the math and if it works go for it. See if the new beam web can take the moment, check the bolts for tearout, the angle for bending and tearout in a few different directions and the web of the existing beam for some approximation of the bending and shear necessary to pass the loads to the flanges. If you get worried, double clip angles to spread out the tension perpindicular to the existing column webs would probably help.

Honestly, I'm more worried about torsion, regardless of how you connect. You could have strength/deflection issues on the existing members. You also have to look at how those existing members are connected to other beams and their supports. Throwing torsion into structures where it wasn't expected can get messy, even with loads that otherwise look really small.

It looks like a little bit of rearrangement could give you enough support that you don't need to rely on torsion. See attached.

 

[dik]

Can you extend the platform to the right to line up with the existing C Section?

Dik

 

[charliealphabravo]

Thanks guys. That's all good. Visions of snatch blocks and winches has me thinking about an additional brace to limit torsion and add some redundancy.

Torsion issues and section stability aside, my main question was a general one and it sounds like the consensus is that some calculated moment can be transferred through simple shear connections by doing a T/C calculation on the bolts through the web. Normally in moment connections the entire moment is designed to go through the flanges.

Thanks again all.

 

[rb1957]

"grasshopper" comes from the olde Kung Fu (David Caradine) TV, no? (ok, maybe that's when it went "main-stream")

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