Economical and safe support system?

[CSCPE24]

Hi All,

Just to preface, I'm a Mechanical Engineer by profession and not a structural engineer. I would like to get some feedback from all you structural experts on a particular concern as outlined below:

I have also attached an image for clarity.

What I have is a platform 3' wide running down the length of a 24' long W10X39 beam. The continuous load for the platform is 155 lbs/linear.ft. Right now, I have 4 of the shown supports evenly spaced within the 24' beam span. The support consists of a C6X6.2 channel attached to the beam as shown with (1) 3x3x3/16 angle. Is this method considered safe and economical from a structural engineers standpoint? Or is there another more efficient and cost effective method?

The 1400 lbs shown is the concentrated load calculated for such a condition for the (2) center supports of the 24' beam span.

I thank you all in advance for any comments and feedback.

Best Regards,
CSCPE24.

 

[JAE]

What holds up the right hand side of the C6?

 

[civilperson]

The loads are lower than most codes for platforms and walkways. Steel is cheap compared to lawyers.

 

[CSCPE24]

JAE, it's cantilevered.

Civilperson, Do you mean add more supports? Or totally change the design?

 

[JAE]

I guess I don't see how you can cantilever that C6 off the wide flange beam. There is no development of the end moment via the channel's flanges and the wide flange web will simply bend sideways.

Plus wide flanges have almost zero torsional rigidity.

Not a good detail.

 

[CSCPE24]

JAE,

Would the angle not pick up on the end moment? The angle will either be bolted or welded to both the channel and also the wide flange beam.

 

[civilperson]

Three foot wide with 155 plf gives approximately fifty psf for live loading, OK for walkways but too little live load for platforms. The picture looks like torsional load applied to the W10x39, J equals 0.98 in<4 for this shape. I would change the design to eliminate the twist force on the W10. Perhaps center the W10 under the center of the walk.

 

[CSCPE24]

Civilperson,

Yes, the 155 plf actually is derived from 40 psf for live load.

Assuming that I can only fashion some sort of support from the W10X39 while still adhering to the layout with the platform right beside the beam, would welding a tube to the underside of the beam flange to pick up on the loads be better then?

Thanks.

 

[hokie66]

The answer to your question is simply that the platform cannot be supported off the side of the beam because the beam is not strong enough to resist the torsion. You need another beam on the other side, hangers from above, or a totally different detail.

 

[JAE]

You've got the load path going through lots of relatively thin plates in bending. If the walkway edge angle is welded to the top of the wide flange, then the torque from the walkway would create a horizontal force path as follows:

1. Through the angle-to-walkway connection (rigid)
2. Then through plate bending in the vertical leg of the angle (very flexible)
3. Then through the weld from the angle to the WF flange tip,
4. Then through the vertical web of the WF as it torques against the 3x3 clip angle(flexible)

Everything would bend like crazy.

Instead of a W10x39, a 10x10 tube could be welded to the supporting beam below and horizontal channels (or tubes) could be welded to the side face of the 10x10 tube to form a cantilever system. The cantilevered members should be 8" deep or so to avoid bending in the side wall of the 10x10 and get closer to its top and bottom flanges.

 

[JAE]

Even with this idea, there would still be a lot of bounce in that walkway.

 

[steve1]

There appears to be another beam under the W10x39. IF this is true then dpending on how that beam is supported perhaps the cantilevered platform can be supported on it.

 

[gwynn]

An option may be to brace the W10x39 from another structure to the left hand side of the drawing. Placing a stiffener opposite the connection to the channel and running braces from the top and bottom flanges to a a more rigid structure further to the left could eliminate the torsion concerns.

In addition to previous comments, I also don't see the connection between the channel and w section providing much torsional restraint to the channel. This gives a large lateral torsional buckling length for the cantilevered channel. Can't say offhand if this causes capacity problems or not, but it is something I would try to avoid.

 

[CSCPE24]

Thank you all for the very insightful and helpful comments.

I was hoping to rely only on some sort of support members that can be attached to just the single W10x39 to pick up on the walkway but I guess it's not a good idea whatsoever.

There is actually another W10x39 to the left of the first one that is not shown. I'm thinking of just bridging the top of the two W10x39 with 5x2x1/4 rectangular structural tubes. These tubes would then act as the supporting members for the walkway. Although this would create an uplift force on the far side W10x39 but I think the loads will be well within the allowable design constraints.

 

[oneintheeye]

what about providing a knee brace support to the end of the cantilever?

 

[hokie66]

CSCPE24,
Now you are on the right track.

herewegothen,

A knee brace from what? A knee brace from the beam bottom flange doesn't change the fact that the beam is no good in torsion.

 

[oneintheeye]

agreed, I was thinking of a knee brace to the end of the cantilever not the bottom flange of the main beam. Obviously the brace would need to be attached to a column under the beam. Don't know if one is present. The platform could then span two ways onto beams supported by main beam on one side, knee bracing on other.

 

[csd72]

CSCPE24

Why not stick with the c6 channel, that way you can bolt it directly to the top of the two W10s with 2 bolts to each. Just an idea.

 

[CSCPE24]

csd72

That's a good idea. However, based on the overall support plan, welding will be easier because field drilling holes will be too cumbersome and preparing the steel with pre-drilled holes takes too much work since the support spacings won't have a consistent pattern.

 

[csd72]

In that case, welding makes sense.

Have you checked how the walkway material connects to your tubes?