1-Center Beam Floor Support. (tile deflection) 5

[EngStuff]

This is one of those projects that I need major opinions on.

I have a Mezz. that will be considered a "stage floor" that will only be supported by 1 Beam. This beam will be an HSS member which will be supported by girders that are also HSS and HSS columns. the cantilever beams might be tapered plates or tapered W-shape beams with steel cladding or combination of both. Moment connections are everywhere including a fixed base. The floor is tile. see image below.

When it comes to deflection. Since we have both cantilever beams deflecting at their ends. At the center beam, I am worried about a tension strain on the thin set and tile(cracking). I think I will end up calling for a uncoupling membrane but that might not be enough. I am also worried about a "bump" at the center beam due to deflection on both sides.

I analyzed with full dead and live load. I also analyzed with full dead with skip live loading one each side.

My question is, what do you guys think I should hold the deflection to in this case? Also, is there anything I should take into account when designing this that I might not have thought of. Any pointers? I have admittedly not dealt with a floor like this.

Floor dead load will consist of
1. Steel beams
2. 1.5B metal deck
3. 3/4 ply
4. Thin set
5. 3/8" tile
6. Misc.
(uncoupling membrane?)

 

[dik]

I would be looking at something like this... maybe bring the beams in a couple of feet...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[MIStructE_IRE]

To be clear, if going for a flat slab option i’m talking something like this, ie supported on 4 legs so its just a big table - not a flat slab on a central beam which still has the torsion problem.

Dik, i like your solution also but I believe OP said the architect won’t accept that earlier on.

 

[EngStuff]

Architect slapping seems like a great choice. haha.

I wouldn't be able to make it a flat slab due to other information I didn't think was necessary to bring up. There will be floor glass at some locations to peek through. Plus More issues I would have to deal with when it comes to a flat slab. I will put this option in my back pocket. I have pushed to make the whole floor system glass, but at the moment they are against that idea because they are worried about peoples comfort looking down.

So I ran it and got a preliminary design with a worst case of .3" at each end. I did use larger member sizes. while I am typing this up, I realized I forgot to add the railing load which is a glass rail. I'll have to rerun it with the additional load to keep the .3" :/

Back to the steel option. I think I can make it stiff enough to not worry much about vibration.... and... how can I determine vibration in this case?

BTW, It's about 8 foot long on each side with a total of 16'-0 wide floor.

Also, I am thinking in this case, I should base my deflection limit on a differential deflection at every 1'-0" feet. I can perhaps look into making them create a "joint" in the thin set every so often. This will cause the tension strain release. they might see cracks through the grouted joints, but as long as the tile is good.

What you guys think about that?

Dik,

That was an option to allow beams to shoot through like that. I haven't heard back from them to determine if it effects the visuals. I should hear from them when they are back from vacation.

 

[r13]

Watch out, the edges likely will suffer from two way deflection. Damn, I am scared already just from thinking, would never step on it

 

[MIStructE_IRE]

Where’s your lateral stability in the long direction? Are you cantilevering the columns from foundation? I hope you’re tied into something stiff and not relying on that centre beam as part of a sort of offset sway frame..?

Incidentally, if you do glass floor it, how are you achieving diaphragm action between the two end bay portals? While 8 foot cantilevers arent massive, theres a lot of first and second order movements happening here and a lot of deflection/vibration issues to be considered. A slight degree of rotation on the centre beam will result in significant deflection at the cantilever tip. The relative deflections between short distances will likely govern particularly under pattern load.

Vibration is key also. You have vertical vibrations and lesser checked lateral vibrations to consider here (Ive only ever had this once and farmed it out to a specialist who did a full FE vibration analysis - I wouldnt know where to start!!)

The architect clearly has zero appreciation of what they’re asking for here.

 

[r13]

Try space truss. If necessary, clad with sheet metal for looking and feel.

 

[dik]

Take my word for it... that's easy to design... and gets rid of much of the torsion, to boot...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

You nailed it... design, including second order issues are nothing... vibration, if used as a stage, would be my only concern...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

...and that's the reason I like it. We were the first class at U of M to take limit states for both concrete and steel, and that was back in 1965... it was just coming out.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[mtu1972]

Dik - we were way behind you. Structural classes in 1971 & 72. Steel was ASD, and that’s Stress not Strength, and concrete was both, ASD and LRFD, as they were not sure what is now USD would be totally accepted.

gjc

 

[HTURKAK]

Although Stage floor gives a rough idea regarding the dimensions , will you post the dimensions and section view ( architectural ) to see the concept ?.

You mentioned one beam will support the stage together with cantilever beams and the cantilever beams will be tapered plates or tapered W-shape beams with steel cladding .What is the is the min. depth of cantilever at tip what is the max. allowed depth of main beam?.

IMO, the top deck and bottom deck plates should be designed as structural elements.

If i were in your shoes, i will consider two orthotropic options;

- Steel grid main frame composed of cold bended channels or box profiles top deck checkered plate thk. 3 mm and bottom deck 2 mm.

- The whole platform could be aluminum structure with extruded profiles and aluminum skin top and bottom deck.

Consider the analogy of aircraft wing concept..

Yes..vibration shall be major concern if used as stage..

 

[dik]

hturkak: never thought of it as an airplane wing... good analogy. but, the force is on the opposite side...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[HTURKAK]

I fully agree with you.. crazy structural engineering solution for the crazy architectural demand ...

 

[dik]

I hate to admit it, but I have a perverse enjoyment from this type of project... only done a handful of them... but, neat nonetheless...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Ron247]

I get a headache just thinking about all the loose ends to tie up for an "Architects Concept" such as this. Make sure you check fatigue with DL+LL+IL. IL is Idiot Load. As soon as the "artists" using this stage note it rocks when they walk out to the ends, they are going to laugh as they make it rock more and more and more. Tacoma Narrows will become a footnote.

Before you slap the Architect, tell them this will hurt you more than them. Might as well lie to them before you slap them.

 

[dik]

...occasionally engineers need a slap, too.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[EngStuff]

Hi all,

Hope all is well. Sorry just got back into town and jumped back on this project.

to respond to some comments:

in advance, mind the grammar errors.

The most important one,
I definitely need to slap everyone involved in this project and myself. haha.
--
So the stage is about 13’-6” high from first floor(slab on grade) 3 columns will be supporting the stage and second floor. one column will be only supporting the stage. all of them will be fixed at the base and top. The second floor is about 8 to 9 feet above first floor. the 2nd floor is enclosing the stage at 3 sides. The column at the lower left near Node N12 will not be attached to the second floor, just the other 3 columns will. See picture below, this shows a partial 2nd floor. The red lines are 2nd floor girders attached to the columns. I will be counting on the stage columns to be a lateral resistant system as well for the stage and 2nd floor. There will be other columns elsewhere that will be resisting the lateral. I think for the most part, all of my columns will be cantilevered and part of the lateral resistant system. looks like I won't be able to put any x braces anywhere useful.

the stage is 21.5' long with each outrigger being 8 feet long, thus it puts it around 21.5'x16' floor. it's not an exact rectangle, it's curved at the edges into the columns. the long frame end is about 13' long, the short frame is about 6.5'.

They want the stage to look as thin as possible, that is why we were required to taper the outriggers. But the center support is going to be a massive HSS member.

Based on your comments, full glass floor is out of the question. only a 1/4 of one side of the cantilever will have glass. located at the edges.

2nd order issues:

I have already taken Pdelta into account. no thought about vibrations though.

I hope you’re tied into something stiff and not relying on that centre beam as part of a sort of offset sway frame..?

I kinda was, What's the bad news lol


I also have another possible idea. Which kind of follows the idea from DIK, but keeping the center span. What if I used a rod or steel wire gauge at the column ends. This will reduce the cantilever span and also help with the deflection. It already sounds like a horrible idea due to the "pull" at the top of columns.

 

[Enable]

I can’t help with the structural aspects as this is beyond my competence, but there are a few things you should consider about the tiles:

1. Don’t do tiles

2. See number 1

3. Should you not see 1 or 2 please have a drink of scotch

4. Uncoupling membrane won’t hurt but it wont help. Uncoupling membranes are for differential shear usually due to dissimilar coefficients of thermal expansion in environments subject to temperature swings. They are not meant to accommodate differential vertical displacement.

5. Make sure you use SMALL tiles (DO NOT DO LARGE FORMAT TILES. YOU CRAZY?!). Much smaller than the architect will like. The additional grout joints allow for more movement so a) cracks in the grout become less obvious (read: a problem is not a problem if the client never sees it) and b) cracks happen in the grout not the tiles.

6. Use the most flexible grout you can get your hands on. Typical cement based grouts are out. Acrylic modified grouts are in. Mapei Flexcolor CQ might be a place to start.

7. Ideally you make the tiles someone else's problem.

Really cool project. Would love to see the final design. Please come back to post the solution (and hopefully successful service record)!

 

[EngStuff]

@Enable

I think I need a whole bottle.

Use the most flexible grout you can get your hands on. Typical cement based grouts are out. Acrylic modified grouts are in. Mapei Flexcolor CQ might be a place to start.

Speaking of tile, idk if you had a chance to read the posts, but what do you think if I looked into/called out something like this. see quotes below.

Thanks!

Also, another thing that is on my mind. Deflection for tiles have always been used for floors that cause a "compression strain" on the tiles. In this case, we will see a "tension strain". Perhaps on the thin set, but that thin set will transfer the force onto the tile as well. Which it seems like regardless if we use an uncoupling membrane, it doesn't really solve that issue. Thus will cause cracking.

Also, I am thinking in this case, I should base my deflection limit on a differential deflection at every 1'-0" feet. I can perhaps look into making them create a "joint" in the thin set every so often. This will cause the tension strain release. they might see cracks through the grouted joints, but as long as the tile is good.

 

[dik]

One of the long time members used to use the tagline, "If you're not living on the edge, you are taking up too much room." Pretty much sums up my life.

OP: I don't understand the framing you are proposing... keeping the centre beam is just inviting problems with deflection and vibration... my earlier comment about vinyl tile is still correct.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik