Floor vibration 4

[Gus14]

Not much experience in steel but I keep running into steel projects so these are my concerns. I have a 40 ft wide span floor which will support the full width of a residential house details are in the attached sketch (No columns inside the house are allowed). The floor will be a concrete cast in place slab. (Total dead load with partitions 125 psf and live load is 53 psf)

1) Which is better a composite steel joist or composite floor beam ? They were pretty similar economically.
Would a 32 inch deep steel joist have enough space for ducts to go through?

2) Do brick partitions help reduce vibrations or make them worse?

3) Are composite concrete steel joists designed similar to ordinary steel joists with steel deck from vulcraft manual or do they have a diffrent tables ?

4) Could fixing the floor beams or joists to girders by a moment connection ( to reduce deflections ) cause a significant problem in columns ?

5) How far is it possible to span floor beams when using a concrete slab to be economically sound?

6) Any red flags to check ?

 

[KootK]

1) Which is better a composite steel joist or composite floor beam ? They were pretty similar economically.
Would a 32 inch deep steel joist have enough space for ducts to go through?

- I'm surprised that the options are similar economically. Certainly, for larger projects, I'd expect the joists to come in cheaper. A house project may not hit the critical mass required for joist suppliers to be genuinely interested though.

- From a structural perspective only, and designed to the same criteria, I like the beams. They are more tolerant of strange loads and future changes having a more or less uniform shear capacity throughout.

- Running ducts through joists is handy and that's certainly easier with joists than it is with beams. 32" will buy you a fair bit of space. Grab a local joist supplier's catalog and they'll likely have a diagram indicating the max size of various shaped openings that you can expect to run through their webbing.

2) Do brick partitions help reduce vibrations or make them worse?

I'd say that they'd help. Firstly, the mass helps to dampen vibrations. Secondly, you'll want to design the joists supporting the brick to be wickedly stiff which will probably have the effect of stiffening much of the surrounding floor system.

3) Are composite concrete steel joists designed similar to ordinary steel joists with steel deck from vulcraft manual or do they have a diffrent tables ?

The design is quite different. If you can find composite table from your local suppliers, great. If not, it's time to run some numbers.

4) Could fixing the floor beams or joists to girders by a moment connection ( to reduce deflections ) cause a significant problem in columns ?

- For beams, it might help but it would probably be expensive.

- For joists, I wouldn't do this at all as it greatly complicates the joist design and detailing as well as the need for bottom chord lateral bracing. And it would probably be expensive.

- I don't understand your concern with respect to the columns though. Can you elaborate?

5) How far is it possible to span floor beams when using a concrete slab to be economically sound?

Target a span to beam depth ratio of 22 to 25.

 

[Gus14]

Thank you for your info.
With respect to columns.I was thinking that moment from floor beams will cause torsion in girders which will cause moment about columns weak axis Dont know if they will be large enough to cause problems.

 

[Gus14]

Update,
Floor beams will be (Ip 457×191×67)
which is similar to (W18×46) the girder requires less thickness but I'm forced to go with a deeper section to ease erection process. The connection is simple End plate connection with three bolts on each side ( has the least strength form AISC manual but will custom decrease bolt size to 16 mm instead of 18.75).

Is it possible to use a smaller girder with Floor beams resting on top (attched pic) ? I can't find a similar connection design example anywhere Would it cause problems like web buckling etc...

https://files.engineering.com/getfi...14-a5e0-b54fbd73346a&file=20200714_131517.jpg

 

[phamENG]

That is possible, but you're right about web checks. You also have to be concerned with connection stability. Stiffener plates between flanges are a must. What code are you using? AISC breaks it down pretty nicely for checking the web, bearing stresses between the steel interfaces (rarely controls for beams of that size loaded primarilly in bending), etc. Diaphragm shear transfer will be a bear to try to resolve here.

You're also going to increase your floor depth, unless you can conceal your girder completely in the wall and not interfere with any fenestration.

 

[Gus14]

Update
Floor vibration is (6 Hz) which is quite excessive. Floor beams span 40 ft long and are 457×191×67 ( which is similar to (W18×64)) spaced at 1.3 meters. Longest choridor/room span is 5 meters. Concrete deck is 10 cm depth. Does this seem reasonalable ?and is there any solutions.

 

[r13]

My first suggestion is to avoid the mixing of units - use either imperial or metric throughout for coherence.

what elevation is the duct with respect to the bottom of beam? Is the concrete floor made composite with the steel beam? Your beam spacing is too close, why not use joist? It is unlikely a 40' clear spacing is necessary for a residential building, Don't there interior walls you can utilize as support, or add interior columns. Does the beam support floor, or roof?

 

[KootK]

10 cm seems pretty thick. I'd expect something more in the 4"-6" range.

Are you making the beams composite with the deck as with a true stub girder system? The pic you posted previously was taken from stub girder literature but I'm not sure if that was really your intent.

 

[jayrod12]

10 cm seems pretty thick. I'd expect something more in the 4"-6" range.

10cm is 4"

 

[KootK]

Thanks. Had 10" stuck in my head for some reason. Other than it's inherent rationality, I hate metric.

 

[Gus14]

Sorry for the late response.
There is no joist factory in my country of residence (I thought there was). So I went with composite floor beams and I can't use columns inside the house (Owners request). I found that vibration are too much for a residental building (6 Hz). But I don't have real life experience so I want to check if the value of vibration (6Hz) is somewhat right which can be annoying for a residental building

Kootk, You are right I didn't mean using a true stub girder system. The floor beams are composite with the deck. It's just the only detail that I found were floor beams are on top of girders.

 

[r13]

The linked article my help you to improve your floor system for vibration response. Link. One other source pointed out that humans are most sensitive to vibration with frequencies between 4 Hz to 8 Hz, which should be avoided.

 

[Strong]

A rather unusual residential floor plan with two adjacent bathrooms!
Adding mass (brick wall) does not add much damping.
Increasing strength at wall-floor joint would probably not increase natural frequency very much.
Columns are your friend, if you are serious about raising natural frequency(s) and reducing deflection.

Walt

 

[canwesteng]

Is this a composite system in the end? If you have the space for it, try a lighter and deeper beam. To get away from 4-8 Hz I'd ballpark you need something ~2.5 times stiffer, like a W27 (ugly depth for a floor beam). Make sure you're taking advantage of the stiffness from the slab.

 

[haynewp]

I have a couple of questions. If your sketch is correctly drawn then you have beams at 82ft/6= 13'-8" on center? What steel deck depth are you using? I doubt a 4 inch total concrete deck thickness is adequate to get the cover over the studs for the deck depth required in that case. You also have 4 column spaces at 17 ft which only adds up to 68 ft. What damping ratio are you assuming?

I wouldn't fix the floor beams to the girder nor put the beams on the top of the girder. I expect the cost in beam stiffeners in that case to be more than standard shear connections.

 

[Agent666]

Could fixing the floor beams or joists to girders by a moment connection ( to reduce deflections ) cause a significant problem in columns ?

Remember as far as vibration goes, because the movements involved are so low, and very small changes in strain. Even pinned connections are typically assessed as fixed, the floor slab also creates an effective continuity over the girders. This is discussed in texts on vibration and can give a significant boost in the effective stiffness of your floor system.

 

[r13]

As the vibration of your slab is very much caused by the relatively long span, why not to provide intermediate supports along the interior wall in the N-S direction? 4" slab is a bad idea. I'll double the thickness for non-composite slab, and 10"-12" for composite slab.