Rhythmic Excitation in Balcony 1

[SteveGregory]

If the balcony is framed with regular rectangular bays, then Design Guide 11 would apply directly and you should use the walking excitation and the rhythmic analysis.

However, if the balcony follows some type of a curve producing some very irregular bays, then vibration may not be a problem.

In either case, you may want to quiz the architect to find out if they intend to hang or fix some type of video projection equipment to the balcony. This could be a problem and you may need to use a vibration isolation suspension system for the projector.

 

[271828]

graybeach, such devices would be heinously expensive to use in a new structure. Also, keep in mind that the Millennium Footbridge had low frequency, high amplitude (relatively speaking) vibrations. The vertical displacement in a structure such as the OP's is going to be a fraction of a sixteenth, most likely. I'm not sure if such devices even engage at very small amplitudes.

ajk1, such remedies are anecdotal. I've also heard of people doing that sort of thing, and much more, and the problem either remained the same or got worse. These are definitely into the emergency fix category, not a fix one would plan on ahead of time.

csd72, there's no doubt that increasing damping will decrease vibrations. However, I'd take exception to the 1% increased damping statement because damping estimation is voodoo (no offense to actual voodoo practitioners on the board LOL). I don't think anybody can look at an arrangement of strl and nonstrl elements and say that the damping is 2% or 3%, 4% or 5%, etc. I think there's something else going on in most cases. For example, we use 5% damping with full height partitions. However, what really seems to be happening in a lot of these cases is that the partitions are like shear walls on the slab, adding stiffness and driving the frequency way up. That's too much of a hijack to go further, so I'll stop with that.

I thought you were talking about devices and materials specifically designed to increase damping. I'm very interested in this subject, but have yet to see a solution that looks promising for routine design problems such as this one. The first link is a primer. Second link provides some tips for estimating damping, and there's no doubt that it should be estimated as accurately as possible. Third is a general article on vibrations. Very useful, but not what I'm personally looking for.

To my knowledge, the current exotic options are tuned mass dampers, damping concrete, active control. There are currently problems with each of these approaches.

TMDs get out of tune if the mass or stiffness changes, so they're problematic for applications like this one. I think they have been used in somewhat similar applications, but they shouldn't be considered a primary choice. Also, keep in mind that your client can buy tons and tons of steel for the cost of the analysis alone, plus site visits to tune, re-tune, etc. Again, these fall into the emergency category IMO.

I have heard of a prelim study using damping concrete that was supposed to do a lot, but it didn't do much of anything. Maybe the full study will show that this is an option.

Active control is extremely effective, but would be prohibitively expensive for an application like this one. I forgot the exact number, but I think it's in the large 5-digit cost per bay. I imagine that it would take a multiple shaker system to control a floor with a group of people bouncing. I'm guessing this is a $200k range option. One can buy A LOT of steel for that!

The bottom line is that 99% of the time, one should change member sizes and try to satisfy the Design Guide 11 criteria.

Just my opinions and they're worth what you paid for them!!