Steel staircase construction questions 2

[greznik91]

This is the 1st time Im designing construction like this. I d like to see some advices and confirmation of my design.

Columns are continuous from bottom to the top. Supports at the bottom are fixed.
Construction is approx. 8 m high. Columns are approx. 2,5 m apart.

BRACING OF CONSTRUCTION:
In Y direction bracing is done by steel diagonals between columns (outer frames). Connections between beams and columns can be pinned (connection C). Inner frames are designed as moment frames.
In X direction I cant design diagonals so the only option I see is to make moment frames. Connections between beams and columns have to be fixed (connection A and B)

Stair beam is also connected to the beam between columns by pinned connection.

What do you think about my connection/design?

 

[kingnero]

I see lots of bolted connections to a square hollow profile.
How will you tighten these bolts?

In X direction, can't you use handrails that serve as diagonal bracing? Beef up the connections of the stair beams, they'll help you achieve some overall stiffness there. You will see a lot of aid there, even with pinned conn's.

I also don't like gusset plates on square hollow profiles, as recently discussed here. Reason why, is because most people tend to place them in the middle of a side, where they don't have the necessary strength.

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[greznik91]

tnx for reply.

About bolted connections - I was thinking Hollo Bolts:

Well, there is always welding, but in my country contractors try to avoid that if possible (I think its cheaper, easier to make but most of all they dont like it because it destroy anti corrosion protection - galvanazing)

About gussets - Im aware of that, tnx.

 

[KootK]

1) Your lateral strategy here is to design each end tower with it's own lateral system and then just connect the two towers with the stuff in the middle, right? If not, I'd brace that middle roof bay.

2) As kingnero mentioned, you could potentially use the stair stringers themselves to brace the structure up to the level of the highest landing. Where the stringers are competing with flexible moment frames, the stringers are likely to draw most of the load anyhow.

3) Connection A seems to imply bolts that would be very difficult to access for installation.

4) If your roof and/or landings will be constructed such that they could serve as diaphragms, I'd use them as such to eliminate the horizontal bracing. Of course, if your landings are something like bar grating, that's another story.

5) It seems to me that connection C, in the location that you've called it out, would need to be a moment connection as well. That, since the bracing doesn't make it to ground in that end bay.

6) I'm not a fan of connection B for the simple reason that I wouldn't know how to design it or assess it's flexibility. A channel end plated into the size of an HSS with blind bolts is pretty exotic in my area. Perhaps the design standards in your region cover this kind of thing better.

7) I'd prefer to have all of the beams and columns -- other than the stair stringers -- switched to wide flange sections. All of the connections just get a lot nore conventional that way.

8) I'd be tempted to ditch the bracing altogether and just moment frame the whole thing. It sounds as though you're already half way there anyhow and eliminating the bracing would make for a nice clean structure. Depending on your location, weak axis column moment connections may not be allowed.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[greznik91]

KootK - tnx for your elaboration, I always learn a lot form your posts...

Well, I'd gladly use open sections (H), but architects allways insist on hollow/box profiles which Im not really fond of since there is always troubles when it comes to connections.

Looks like welding would be the most easy way for me, but most expensive for investor.

What about a connection in the picture - would you consider this pinned or fixed?

 

[KootK]

I would normally consider the connection pinned but, for small scale loads like you'll have here, I'd be on board with laying claim to some fixity. I like it a good deal better than the end plate connections.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[kingnero]

@ KootK, do you really?
I haven't got any experience with them, but they look flimsy and probably a sloppy fit (to allow for HDG, fabrication tolerances and erection purposes), I wouldn't count on much "fixity" here.
I agree the loads are minor but I'd still rather see endplated conn's than this. I'm no fabricator though, so I'm not taking into account the many additional manhours.

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[KootK]

@ KootK, do you really?

I really do. That said, I'd need for it to not be "sloppy". If over sized holes were used for whatever reason, then I'd need slip critical bolting etc. In terms of the expedient transfer of moment, I feel that the connection has two admirable qualities:

1) Good lever arm spread on the connection (out to out of the bolts).

2) Your delivering the moment as a shearing couple directly to the parallel web of the HSS. Short of somehow engaging both webs directly, I think that's the stiffest path available.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[kingnero]

I should have explained better: I meant I think the square "brackets" (English is not my native language, so sorry for my vocabulary!) are oversized with respect to the square hollow sections.
If only by 1mm (this time I'm not sorry, metric is simply superior ) over a height of, let's say 150 mm, that would mean the entire construction can "wobble" for 1mm * (h/150).
Let's estimate this platform at h = 2.5m, that means 17mm of possible lateral deflection, enough to make anyone seasick.

Even with a perfect fit, I'd say there is still lateral movement possible.
Plus, according to what I see, the prying action of the assembly (there's some place between the sides perpendicular to the web of the U section and the location of the bolts) would make even more movement possible. See attached sketch.
Again, I haven't got any real life experience with these things, so I might be totally wrong, but I'm not that keen to see these used over here.

http://files.engineering.com/getfil...6df4-4bc9-8740-5868d9514eca&file=brackets.pdf

http://www.fusionpoint.be

http://be.linkedin.com/in/fusionpoint

 

[n3jc]

So if I understand you correctly - handrail can be used as bracing between columns... do you mean this?

If yes, than handrail should be fixed as soon as possible, not at the end, right?

 

[greznik91]

Well Im inclined to weld everything together... that way i wont need any bracing at all as KootK said.
As i mentioned, contractors are not fond of that (welding on site), but if they want hollow sections i think thats the way to go. If Id deal with open sections id have no problem with moment connections but in this case Im just not confident enough.

What kind of anticorrosive protection is usually good in this cases?

 

[KootK]

Metalizing is good and zinc rich paint is okay. You can guess which is more costly. Could you have HSS columns but wide flange beams? If so, you might be able to do a convincing bolted moment frame that way. The do that kind of thing all the time in Japan I hear.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

You appear to have a lower storey that seems to be a bit light on bracing...

Dik

 

[greznik91]

Could you have HSS columns but wide flange beams? If so, you might be able to do a convincing bolted moment frame that way. The do that kind of thing all the time in Japan I hear.

KootK what kind of connections do you have in mind (I ll appreciate a photo or a sketch)?

 

[KootK]

This kind of thing.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TLHS]

I would never use the handrails as structure, unless it were incredibly obvious somehow. There's too much chance of someone coming and doing something to it later without realizing that it's the lateral system.

Plus, from an erection standpoint, it's a pain in the ass that your system isn't stable when they're trying to bolt up the handrails and things.