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Civil / Structural PE
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Discussion Starter · #1 ·
(I wrote this a few years ago for Nation's Building News OnLine. It goes very well with the current thread on TJIs)

Not long ago I got the call every engineer dreads. It was from a past client regarding something I’d engineered. In the world of structural engineering, this can be very serious for a couple reasons. First, if one of my structures fails, someone could be seriously injured or killed. Second, even if no one is hurt, the failure would likely be very expensive to repair – an expense I’d have to bear if it was my fault.

The call came from a good, repeat client, Wayne.

“Tim,” he said. “Remember the 20,000 square foot SR20 project you did last year? Well, the floors….”

My mind raced with visions of catastrophic failure, people pinned under tons of rubble. “Yes,” I replied cautiously.

“… the floors,” he continued. “They’re a little bouncy.”

Relief surged through me as the sweat spigot that had suddenly sprung from my forehead closed. “Really? I said, masking my relief. “How so?”

“Well, when a heavy person walks down the isle between the 2nd floor cubicles, people in the cubicles can really feel it. They’re complaining.”

“Sounds like a deflection issue,” I said. “I’ll recheck my calcs and get back to you.”

In doing this, it brought to mind one of my seminar quiz questions:

Bouncy Floors…

a) Are always an extreme hazard, and must be replaced.

b) Happen because the framer forgot to put the glue under the subfloor.

c) Are much more likely with a floor system that has not been designed by a qualified person.

d) May be perfectly fine from a strength point of view, but sure are annoying to the people who live there.

e) Occur because the deflection criteria of the floor system was not adhered to.

f) Should be reserved for trampolines, not buildings.

Answer a) is not true. Bounciness is due to deflection, not lack of strength. Deflection and strength do not depend on one another, though a lot of deflection (or bounciness if we’re considering a floor) may well indicate a strength problem.

Answer b) is not true. Glue helps strength and deflection a little, but generally less than 5%.

Answer c) is generally true.

Answer d) is true, and is the applicable answer to my client’s complaint.

Answer e) is generally true, but not always. The above scenario is an excellent case in point.

Answer f), is true enough, and could have also included big-time wrestling rings. But I digress.

When sizing beams and joists, there are three criteria that must be examined: bending strength, shear strength, and deflection. Deflection is not a strength issue, it is more comfort and cosmetic related. With floor joists, too much deflection can result in bounciness, cracked drywall ceilings, or sagging. It is possible that a poorly designed floor system could have both deflection and strength problems, but they don’t always go together.

Back to my SR20 project, I rechecked my calcs and found that I had designed everything correctly, and in fact used a more stringent criteria for deflection than code required. Still, the floor was a little bouncy. The primary reason was that the joist span was in excess of 20-feet; quite long.

Lesson learned: long floor joists, say 14-feet or more in span, are prone to bounciness, even if they are designed correctly. The best remedy is to use shorter spans. If that isn’t possible, be sure the designer knows what he’s doing, and doesn’t skimp on the deflection criteria.
 

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Well if anyone is ever around Pearl Ms. (Jackson suburb) pay a visit to the Bass Pro Shop ,second floor. It's a suspended slab and stand around the safes about midspan and wait for someone to walk by. It's very unnerving
to say the least.
 

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I had a job in Redmond where I had to repair bouncy floors in about 15 houses. The floor joists were spanned to code, but it was borderline. The minimum code requirements isn't enough to build a sturdy floor apparently, go a little beyond code.
 

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Pro
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rule of thumb is when you delve into larger spans (18+ feet) you have to go overkill on the floor joists to eliminate bounce and perma-sagging.

x-bridging is also not a bad idea for wood i-beams.
 

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Both bounce and deflection criteria can result in floors stronger than needed from a fail point of view. On the other hand, tiling needs a surface that doesn't bounce much. I say get the fat guy roller skates.
 

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I selected D.

Joists rarely fail in the middle, they fail at connection points. Wood gives you a lot of advance notice before it splits. Bouncy is bouncy, but not necessarily dangerous.
An interesting assertion. I've seen plenty of fails, but rarely at the connection, which would take a compression failure.
 

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Jeff
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I notice this alot with trusses and I joists. Everyone wants longer spans but for a strong feeling floor splitting the span in half is usually alot better. 24' span at 360 is just over 3/4" of an inch allowable deflection something people are gonna feel. Cut that in half with an appropriate sized beam and that 7/16" allowed seems considerably less. Both are "fine" though.
 

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Mercury Engineering
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From an engineering perspective a bouncy floor is the result of the floor’s “natural frequency” matching or close to the frequency of someone walking. The solution is to change the natural frequency of the floor which is roughly a function of the stiffness and mass.

Calculating a natural frequency is very difficult by hand but with a computer it isn’t that hard.

Next time you are on a bouncy floor try running across the floor instead of walking and see if an observer detects any difference. If it doesn’t work try running faster.
 

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Jeff
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From an engineering perspective a bouncy floor is the result of the floor’s “natural frequency” matching or close to the frequency of someone walking. The solution is to change the natural frequency of the floor which is roughly a function of the stiffness and mass.

Calculating a natural frequency is very difficult by hand but with a computer it isn’t that hard.

Next time you are on a bouncy floor try running across the floor instead of walking and see if an observer detects any difference. If it doesn’t work try running faster.
Whats funny about that is my house is 2x10 spf #2 13' span 16" centers so im roughly L/600. I can feel my 4 year old weighing 35 lbs run across the floor when i sit in the comp chair but 140lb wife walks and i feel nothing. just something about her running that creates the slight vib. On the flip side i had so much going on when the beam was sized the LVL company sized it. Gave the spans, room loads and stuff but hind sight im pretty sure they sized it for 360 and the deflection im feeling is the beam.

Kinda irritates me to be honest but thats what i get for no being totally descriptive in what i wanted.
 

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Carolina Surface Prep
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The feel of a floor can also be a matter of perception by the owner of the floor. People who have always lived on "slab on grade" floor can be uncomfortable when they move into a house with a wood framed floor designed to L/600 at 16ft.

Someone who has always lived in a house with a wood floor may be comfortable with L/360 at 16ft.

The complaint I usually see is the rattling china cabinet, against the inside of an exterior wall, with the front legs resting in the midspan of that joist 16" from the wall. Doesn't take much movement in that joist to make the top of the cabinet move when the cabinet pivots around those rear legs which are resting on the solid floor next to the wall.
 

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Mercury Engineering
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Deflection under a constant load is not the same as floor vibration (bouncy floor). Bouncy floors may meet L/360 maybe even L/600 but if the load is vibrating (like a person walking, dancing, etc.) the floors natural frequency is the important factor. Not to go to geeky but when the vibrating load frequency matches the floor natural frequency you get resonance which is a positive feedback loop. Think of pushing a child on a swing. You push a little bit each time and they can swing pretty high.

Note jkfox624 comment that his floor bounces when a 35 lb 4 year old runs but not when his wife walks across the floor (BTW does your wife know you are telling everyone in a public forum how much she weights :)). Note the bounciness is not a function of the weight of the person.

There are some guidelines about maximum deflection and L/XX that are supposed to result in floors that do not bounce. They are derived from more complex equations so if they are used then you have to be sure the particular situation meets the assumptions.
 

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I'm looking at a new set of plans today, and this structural engineer has a lot more to worry about then bouncy floors.

Sim
I'm missing something but I don't know what :eek:

We may get to frame a 4300 sq ft custom later in the fall. I'm really hoping we do. And next month we are going to frame a 4400 sq ft custom that is all 14" I-Joists for the roof.

And we'll form and pour the foundation. This year has been really slow, so I'm hoping we get to finish well.
 

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Civil / Structural PE
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Discussion Starter · #19 ·
I'm looking at a new set of plans today, and this structural engineer has a lot more to worry about then bouncy floors.

Sim
This is a good example that all engineers are not created equally. I bet this plan has a truckload of unnecessary holdowns too.

Did you know that shear walls can be 2x top and bottom, no blocking, studs at 24", with OSB or drywall? I spec them all the time and this is earthquake country (west coast.) Of course, sometimes I spec more heavy duty walls, too. But those are the exception rather than the rule.

I've written about over-engineering many times. Here is one such article from my blog: http://www.constructioncalc.com/blog/structural-design/over-engineer-kills-project/
 
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