Allowable floor beam deflection

[nickky]

Hello,

I am designing a 34 feet simply supported floor steel beam with a uniform load and a number of point loads on it (including two 13 kips loads in the mid span areas). I know it will be the servicability criteria which would govern the design of this beam plus the fact that I have floor thickness limitation. So among the many available sections I should choose wisely!!

Question: what is your suggested deflection limiting criteria for such a beam to avoid floor vibration?

My suggestion: I want to design so that the deflection is limited to less than 0.5 inch regardless to the [span/deflection] and [span/beam height] ratios.

Thank you!

 

[csd72]

nickky,

deflection is not an accurate way of asessing vibration. I would suggest you get a copy of the AISC design guide and follow that. I think they also have a free demo program for this.

0.5 inch deflection limit for this span will lead to an excessively large beam, particularly if you are talking about total deflection.

When it comes to vibration, dead load is normally your friend as it is more mass that needs to be moved with each vibration.

 

[rscassar]

As a starting point for vibrations I have always limited the deflection to 1mm under 1kN point load. (1/25" under 225lbs).

34' is a long span and I think you should perform a more detailed analysis. A 225lbs point load applied centrally may not give accurate representation to control vibration and recommend you perform calculations as per the AISC design guidelines.

 

[steellion]

1. Use composite construction if you can. The concrete floor will increase your effective I and decrease deflection and vibration.

2. Is this beam spanning between two columns? If not, the movement of the girders also plays a role in the vibration design. Most of the time, you cannot look at one beam autonomously, you need to look at the whole floor system. See AISC Design Guide #11.

3. While the solution to reducing deflection and vibration can be similar (i.e. increase moment of inertia), there is no conversion from one to the other. Two very different concepts.

 

[dcarr82775]

What is your vibration from? People walking, machines, etc?

 

[nickky]

Thank you for valuable advises. To answer "dcarr"'s question, this is a high end residential building and this beam will be at the edge of a mezzanine where two large curved stair ramps (typical left and right curved stairs in huge foyers) will be hanging off of this beam. The load type though is just human walking live load.

 

[a2mfk]

Nickky,
Even if you are delegating those stairs to the specialty engineer, beware of long, single span stairs and vibration. I had a project involving installed stairs with a pronounced "bounce" from just one person walking up and down the stairs. The stair reactions and connections may have some dampening or amplifying effect on your beam.

My vibration research has shown it is very hard to analyze anything but pretty standard floor-beam layouts without some specialty knowledge and/or software. I will keep tabs on this discussion.

It also sounds like unlike open offices or hard-walled interior wall offices, you will not have much in the way of live load dampening..

 

[nickky]

No, As I said, this beam is at the edge of mezzanine accross a huge foyer (34 feet long) and the owner does not want any columns. As a matter of fact I had designed the system with two columns in mid span area where these stairs were joining the floor but then the owner said he does not want any column. He also limits me in floor thickness because he does not want to lower the height of those huge windows.

 

[omw7]

Be careful of the interaction between the stairs and the beam. The overall vibration of the system needs to be considered.
We have a staircase that is hung off the side of a walkway which has a close-to-human-activity natural frequency. Alot of factors such as real connection behaviour (articulation or encastrement) change the natural frequency quite spectactularly.
You might want to consider fixity at the ends of the beam which would reduce the natural frequency but can lead to problems of how to achieve this real fixity (what are the elements to which the beam is attached).
A last piece of advice, always use the simple equation 18/sqrt(dead load deflection in mm). This can be checked against your modal analysis and makes sure that you are looking at the right modes.

 

[NITTANYRAY]

Get a copy of FloorVibe by Tom Murray.
He wrote most of the vibration criteria for AISC Design Guide 11. "Floor Vibrations Due to human Activity"
The Web site to get this reasonably priced program is:
www.floorvibe.com

 

[gyung]

If you can get hold with these standards, it might help you.

AS 2670-1983 or ISO 2631
Vibration and shock - Guide to the evaluation of human exposure to whole body vibration