Industrial mixer support structure 3

[elpoblano]

An old agitator (mixer) was replaced from a side motor unit to an in-line vertical motor. The support structure (2-W10x33, 4.25' between them, 16' span) was deemed acceptable. According to my observations, the beams are rotating 8~10 degrees and this is a concern.

I reviewed the beams cpacity for static loading (floor, agitator components, etc) and they are fine. I calculated the dynamic torque and the bending moment at the shaft.

Here are my questions: Are the dynamic torque and bending moment applied to the support beams? What is the limit for the rotation of the beams? How do I determine the distance between the compression flange lateral supports?

Please help,

 

[LeoSC]

The first thing that should be done is to provide intermediate supports to reduce the beam spans. From your description, you have a "failed" support structure, which may be still working due to redistribution of the load.

It is reasonable to assume that the dynamic torque will be transmitted to the motor base and to the support, and this should be taken into account in the design of the beams.

Steel W beams can develop their full section capacity only if they are laterally supported, to prevent buckling of the compression flange. In this case, the span is longer than the required distance between lateral supports of the compression flange, and as a result the beam has rotated.

The limit of rotation of the beam is an angle such that the angle is equal to the tangent of the angle, i.e. a very small displacement.

The distance between lateral supports is a function of the bending moment, beam dimensions, span, and the radius of gyration of the beam section.

 

[Lutfi]

One minor note, most motors when they start, they require tremendous amount of current and torque. Electrical engineers are aware of this; but most structural engineers are not. The inrush current can be as much as 5 to six times that required when the motor is normalized. Evaluate this condition.

I direct you to read the AISC design guide number 11 on open section torsional analysis. If you are AUSC member, you can download it from their web site

http://www.aisc.org

or you can buy it on line or fro their book store.

Torsion is a monster. I avoid it as much as I can. The best way to get rid of torsion is eliminate it, if possible.

Also consider fatigue stress due to cyclic loading. Make sure that you have vibration isolators.

You can brace the beams by using horizontal members or box them by suing floor plates on top and bottom.

Good luck