ways to increase strength capacity of beam 2

[PowerRanger]

Hi all fellow

I am currently working on a project about a hotel in rural area. The owner want to change the upper floor from living to dancing area. So the live load changed. The case is that steel truss support the timber joist, and is being supported by a primary I beam. In my previous analysis, the truss and I beam just meet the design required for living case. But my client want this project go, so add more truss to reduce the spacing is ok. But he don't want to switch the primary beam, also, he want open area under, so added one more column or support is impossible. So do anyone have any idea.

Thank you for you time

 

[dbuzz]

A couple of ideas:
- Can you introduce additional lateral bracing to reduce the effective length and increase the beam's capacity
- Can you weld a beam-tee to the underside of the I-beam to increase the beam's stength and stiffness

 

[SlideRuleEra]

You may have other concerns in addition to increased live loading. Dancing can cause rhythmic floor vibration that were not a problem for a living area. Here is a link to an article on this subject

http://irc.nrc-cnrc.gc.ca/ctus/ctu22e.pdf

Other articles have appeared in Modern Steel Construction Magazine (I know of ones in the March 2001 & Septemeber 2001 issues. They are available as free .pdf downloads at the

http://www.aisc.org)

 

[pba]

The dynamic response issue is a very real problem and especially so with your construction of timber and lightweight steel work. This is because mass (dead load) helps to counteract the vibration effect.

There is also a possible problem with noise in the adjacent areas which needs to be considered - but is beyond my experience.

To deal with the vibration/resonance issue it will be necessary to check the natural frequency of the floor. At its simplest, an approximation of the natural frequency can be obtained by use of an equation which relates the frequency to the inverse of the root of the dead load deflection. I work in metric where the equation is Frequency in Hertz (cycles per second) is equal to 18 devided by the square root of the dead load deflection (in mm).

This is usually adequate for simply supported beams. It is not true of systems where there are multiple elements which all have their own natural frequeny and all interact to create a more complex modal shape. You may well fall in to the latter category.

If you have access to appropriate software, you can model the whole floor and determine resonant frequencies. I think all the better 3D analysis programmes can probably do this.

If you don't have access to such software seek out a book to help with determining the dynamic response of the floor.
At a guess, I'd assume that the timber joists would need to be checked and probably strengthened and that the truss will be your main problem area. It seem less likely that the primary beam will be a problem dynamically.

Having determined your natural frequency of the existing system you need to add stiffness to improve its dynamic response to sufficient extent that the applied vibration load can be considered as purely static. For normal floors, values for 4 or 5Hz are often used as the minimum permmitted natural frequency. In your case the higher the better. Maybe a value of 6 or 7 Hz would be a sensible target.

Now, given the above, it should be clear that there is a lot of work to do to fix this system. Simplistically, dynamic response improves as the deflection reduces. Deflecton is proportional to the cube of the span. Unless you are really close to making the system work, you should probably consider adding new props to the mid span of the trusses. If the owner does not like this then you are probably talking very significant money in terms of strengthening.