Pedestrian barriers connected to long spans of I beam or PFC

[JPN_Kusch]

Hi Engtips,

On many occasions I have seen pedestrian barriers that are mounted to an open section member (like an I beam or PFC), in a 'fixed-base' fashion. When the barrier is loaded with a force resembling someone falling into it, the barrier would have to transfer an overturning demand at its base into the open section member. This overturning demand at the base of the barrier will induce a torsional demand in the open section member.

Given that open section members have such poor torsional performance, I was surprised to not find any commentary regarding imposing this torsional demand on an open section.

Is there a reason why this problem seems to be overlooked?

Cheers

 

[la belle vie]

May be ok as long as there is a suitable cross member framed in at the post location. Totally wrong if not. An inexperienced engineer can overlook this. I have seen this kind of mistake with microwave antenna attachments to buildings. You can’t just bolt to a flange. There needs to be some kind of outrigger to resist moments.

 

[human909]

Is there a reason why this problem seems to be overlooked?

Because most of the time it ISN'T a problem. And when it is a problem isn't really a strength/safety problem.

In my extensive experience with these you don't need a cross member frames at the post location as la belle vie mentioned. That is overkill. But if you do have a medium or long span without a cross framing member then you will get excessive twist.

An inexperienced engineer can overlook this.

I've overlooked this in one case of a channel member spanning 5m. There was excessive deflection, but it wasn't a serious safety risk but did need rectification. The simple solution was to provide a corner for the handrail rather than two end returns. It naturally stiffened things right up.

 

[bugbus]

There are ways to check the torsional capacity of the open section (e.g. Bending, Shear and Torsion Capacities of Steel Angle Sections (No. R810), Trahair)

But excessive twisting will probably become a problem before the strength does in my opinion, as human909 mentioned.