Stiffening a Bar Joist Floor for Vibrations 1

[cedent]

I have been reading AISC's design guide 11 on floor vibrations. I need to stiffen an existing bar joist floor for a future loading with vibration. (The existing floor is okay for stress and deflection under the new load.)

From my reading, it seems as if a 'queen post' configuration on the bottom of the existing steel joists is the most effective means. For those of you without this design guide, it states:

"A technique which has been shown to be effective if there is enough ceiling space is to weld or clamp a queen post hanger to the bottom flange of a beam or joist as shown in Figure 7.1c. This arrangement substantially increases the member stiffness. The hanger can be placed around existing ducts and pipes in the ceiling space. Repairs can be carried out at nights or on week-ends by temporarily removing ceiling tiles below each member to be stiffened. The hanger should be prestressed by jacking up the floor before welding or clamping) the last connection."

The figure they reference is a small diagram of a bar joist. The 'queen post' configuration essentially adds a new bottom chord below the existing bottom chord (and any ducts or piping).

My questions are: Has anyone implemented this method? Was it successful? Or, do you have more of a cautionary tale to tell? If a series of joists are stiffened... shouldn't there be some nominal lateral bracing between the queen post bracing system of each joist?

 

[271828]

What you're asking about is A LOT harder than it sounds. I've been involved in several such projects, usually after a non-vibe specialist tries to implement something like this. A queen post, in particular, will do absolutely nothing if done wrong. Extending the bottom chord extensions will also do nothing if done wrong and little if done right.

I'd recommend that you hire a vibration specialist to save you and the owner a big headache and a lot of money.

 

[271828]

If you want to do anything without a specialist, try to extend partitions from below and screw them into their tracks. If you jack up the floor, screw them in, and then unjack the floors, that might work. I can't think of a time that I haven't seen that work.

 

[bkgd]

Cedent, not sure what the problem is in terms of stiffening the joists like anything else if done wrong it will not work.....right?

The queen post is one way to change the moment of intertia and thus the stiffness which in terms changes the vibrational characteristics of the joists, basic vibration theory applies. The only difficulty arises in what level of acceleration are you trying to limit the floor system to, does your client understand the limits and what they requrie. Obviously you will need to include the supporting girders if you have them since they obviously affect the acceleration of the "system".

The only time we have ever hired a consultant to help out is if you cannot get enough stiffeness or mass increase to reduce the vibrations to the criteria needed and we needed to add tuned mass dampers or similar devices.

Regards,

 

[271828]

bkgd, have you ever tested one of these floors before and after to see if the fix did anything?

 

[bkgd]

271828,

we designed a grand stair into a theater space, there was one stringer (20x8x5/8 HSS) that was located on one side only. The HSS stringer was curved to follow the opening above it. The treads and risers were then cantilevered 5 feet from the HSS so not only did the curve induce tourque but the cantilevered risers and tread did as well. The HSS span was about 35 feet, we were very concerned about any vibration.

We followed the AISC guidelines for vibration and modeled it with various input functions of single to multiple people walking up and down the stair. We are going to have someone come in an measure the vibration characteristic of the stair to see how closely it matched our design for future reference. The stair has performed very well, I think the handrail provides a certain amount of unaccunted for stiffeness.

The only other time we have had measurements taken is for a floor in a gym that was having huge problems, TMD's were used to retrofit that floor. There was one other time for a MRI to check the existing structure.

regards,

 

[271828]

bkgd, that stair problem sounds like loads of fun!

I hope you post the results of the stair measurements. I'm thinking about writing a paper on the subject and would be interested to know how accurately your model predicted natural frequencies and accelerations due to walking.

I do lots of FE modeling for vibe and can sometimes do well, sometimes not--mainly depends on damping assumed, which is a shot in the dark.

What kind of software did you use to apply footsteps that walk on the stair? I know that SAP allows this and I'd be interested to know what other packages do.

Have a Happy Easter!

 

[bkgd]

We used Sap 2000 for the modeling, definetly will share the results.

blake

 

[271828]

Thanks Blake. I wonder if this would be something worthy of a journal or conference paper.

 

[CROWN06]

cedent
When I look at FIG 7.1c, Isn't the new bottom chord, below queen posts, sufficiently long to be unbraced in both horz. & vertical planes.Also now the joist is deeper than before.

 

[cedent]

crown06 & others:

Thank you for you comments.. I will attempt to answer a few questions. The building is not currently in service for rythmic activities that would require consideration of vibration. This particular floor area will be repurposed as part of a renovation. Yesterday, I went through the f_n & f_req calculations shown in Design Guide 11 and found that one floor area is borderline ('okay') and another area is borderline ('n.g.'). Due to the varibility of the computations, everyone involved would prefer to take preventive measures as apposed to corrective ones. The first area is over an open lobby, and the second area is over a series of existing mechanical and storage rooms. Both areas have drop cielings.

Crown06, When I referenced bracing, I was thinking of lateral bracing for the queen post itself.

 

[NS4U]

Why not just weld some bars to the top and bottom flanges of the joists to increase the moment of inertia. Thus, reducing the amplitudes of vibrations and pushing the natural frequency up.

One of the reasons for the L/360 deflection requirement is it provides adequate stiffness for vibration control. However in recent years this doesn't really work, thanks to increased technology. To get an idea of the type of Ix you need, try L/480 or L/560.

 

[271828]

Mike, if he does that, it probably won't do anything. We've tried things like that in the lab, testing before & after, etc.

For example, I had a 30' square bay and tested it to get the natural frequencies, acceleration response, etc. Then jammed screwjacks under the girder at midspan. I put them in there as hard as I could. Retested and the dynamic properties didn't change enough to talk about.

The problem is that the existing members already have plenty of strain in them. If I would've jacked it up really far, enough to relieve most if not all of the strain, installed them, then let the jacks go, then that might've done something.

We even jacked up bar joist floors and welded in bottom chord extensions, then let go of the jacks. This barely did anything. Putting the BCX in there without jacking resulted in practically no change. There's just too much slop in a real structural system for this kind of thing to work.

This stuff falls into the "seems like it would work, but won't" category, LOL. Not saying this stuff never works--sometimes one gets lucky.

 

[NS4U]

hmmm... I'd be interested to know more about that test. You're saying that vibs control would essentially need to be planned for prior to erection and joists should be fabricated accordingly then? Because once they start carrying load and you try to build-up the members, dynamic properties won't change? And this is becuase the built-up section is carrying the "live-load" and the previous section was carrying all of the dead loads, which are substantially more than the live? Very interesting.

I was just basing my assumptions off the equation of motion for a SDOFS. Which is essentially what design guide 11 assumes.

 

[271828]

Mike, I agree it's an interesting a kinda baffling thing and it emphasizes the need to do it right the first time if possible.

I think it's just because the initial member is already supporting the mass (think mass, not load--I think that helps).

There's also the matter of slop in the structural system. Even if the system is jacked up, BCX welded in place, then released, various elements "squirm" around enough to cause the bottom chord to not accept much or any load.

It reminds me of PT stressing if the tendons aren't stretched far enough, then let go. All the elongation just gets taken up in anchor seating, etc.

I don't understand your first question. It would be much better to select a joist that needs no retrofit, but that's too easy to be what you're talking about.

 

[271828]

Oh yeah, one more thing. I have seen partitions below work well when the floor is jacked up, studs screwed hard to their tracks, then the jacks released.

I think this works because there might be 1/4" or more of displacement when the jacks are released, so the studs actually do get load. Displacements of that size result in significant axial force because PL/AE=1/4" results in a lot of P. Probably get some deformation at the new screws so there's little danger of buckling anything, IMO. There's also the matter of increased damping which one doesn't get from a BCX or something else welded to the member as a flexural element.

For a BCX or a plate welded on, the 1/4" (or whatever) displacement when jacks are released results in a microscopic elongation in the thing that was welded on. It's just hard to make this work. Again, think PT tendon that hasn't been stretched far enough before release.

Anyway, we're typing about my favorite subject, so I have to keep myself from going on and on and boring everybody to death!