Wind Load on Guardrail w/ Glass Panel 1

[plausibly_civil]

Hey all,

I'm working on an aluminum guardrail design and running into some questions concerning the wind load. There are glass panels spanning between guardrail posts with a 4" gap at the bottom and top of the panel (see attached snip). The guardrails are located at an elevated low roof deck ~15' above grade. My gut is telling me to treat it like a parapet for calculating the wind loads, but I'm not sure what the standard practice would be for this. I can't really find any literature on how the 4" gaps will affect the wind load, and it feels unconservative to assume that the gap is enough to completely relieve the negative pressures on the panel. Thoughts?

 

[Tomfh]

The gap will not relieve the pressure on the remainder. I would just design it as a parapet of equal height to the balustrade. The codes have drag factors for various shapes, and for walls and elevated "hoardings" with and without gaps below them, if you want to assess it more accurately. It will come out much the same, though. A drag factor of 1.3 or thereabouts on the whole area.

 

[milkshakelake]

The guardrail loading per OSHA (I forgot the exact number; something like 200 lbs concentrated or 50 lb/ft uniform) might control over the wind loading.

 

[SE2607]

MSL - To make things worse, since ASCE-7 now defines the 200 lbf guardrail load as a live load, one must also consider D+0.75(0.6W)+0.75L. In the last guardrail I designed, this load combo governed even with the paltry 95MPH (ASD) wind load, so I can imagine it would govern in higher wind load areas.

 

[hawkaz]

Is the 200 lb. point load (or 50 plf) ASD or ULT? Most everything else in the code is being set up as ULT loads now- but this number has not changed in many code cycles. I suspect it is intended to be an ASD load.

 

[milkshakelake]

@SE2607 Didn't know that. But that's pretty wild. It does make a bit of logical sense that a 75% hurricane would happen and someone would cling to the handrail to not fall, but the 200 lb number should be reduced to account for this possibility. I don't think it was developed to be used in conjunction with a hurricane.

@hawkaz OSHA code was developed before ultimate load combinations came into wide use, so it's ASD. This is just theorizing, but 200 lb is a massive amount of force to put laterally by an average human. I think it was made to be a bit high so that things are "overdesigned" a bit, and to account for rare two-sigma cases.

 

[Aesur]

Is the 200 lb. point load (or 50 plf) ASD or ULT? Most everything else in the code is being set up as ULT loads now- but this number has not changed in many code cycles. I suspect it is intended to be an ASD load.

I think is a common misunderstanding, the load is neither ASD nor LRFD, it's nominal, putting it into the load combinations makes it ASD or LRFD. This applies to all loads in ASCE. You could argue that Wind is now LRFD, however it's actually Nominal and just happens to have a 1.0 factor for LRFD combinations and 0.6 for ASD combinations. It was brought to ultimate level for nominal loading only.

I'm not aware of any provisions for equalizing pressures with a gap under a wall, in fact if you look at the requirements for sloped solar there are strict limitations that a vertical wall element would not meet to consider pressure equalizations, so I would argue you don't have any and design this as parapet wind loading and the 50 plf or 200 lbs whichever controls.

 

[milkshakelake]

I think is a common misunderstanding, the load is neither ASD nor LRFD, it's nominal

Good point! To answer hawkaz's question more precisely than what I said, the load is not already factored. So if a load combination needs 1.6 on it, it will be 1.6 x 200.

 

[plausibly_civil]

Just getting back to this project, thanks for all of the replies! Reassuring to know I was going the right direction.

MSL - To make things worse, since ASCE-7 now defines the 200 lbf guardrail load as a live load, one must also consider D+0.75(0.6W)+0.75L. In the last guardrail I designed, this load combo governed even with the paltry 95MPH (ASD) wind load, so I can imagine it would govern in higher wind load areas.

Turns out this load case governs for my design as well, thanks for the tip.