Balcony Railing Design Load combination 2

[rolkamatic]

thread507-250820

I'm an architect, the engineering submittals I see for balcony rails typically state that wind and railing live loads are not considered concurrent because the balcony is assumed not be occupied during a 50 year wind storm, 98mph wind speed in my area.

The IBC and ASCE 7 has a load combination for allowable stress design which include live and wind loads discounted 75%. Does the assumption of the balcony being un-occpied have any basis other than the engineers judgement? Is there any standard to or published method for approaching this?

 

[slickdeals]

There is very little surface area on a balcony guardrail for wind to have any appreciable effect on it. I don't think there is any location in the code specifically stating that wind and live load need not be considered concurrently, but I think it is common practice not to include them concurrently.

The picture is different if the balcony has some sort of fascia. Then the loading can become significant from wind loads.

I am sure others here will chime in and you will have a broader idea.

PS: I am super excited about architects flipping through structural codes and asking practical questions. That's a rarity

 

[JAE]

0.75L + 0.75W is a pretty specific code requirement that they DO act concurrently.

The 0.75 is a discount to account for the fact that during extreme events you won't have full LL applied.

Balcony guardrails do indeed sometimes have significant wind force if they are solid panels, perforated panels, or even a series of steel bars @ 4" o.c. Wind drag on individual pieces does occur.

 

[dcarr82775]

If the guardrail is open I would ignore wind, but if it is solid I would combine wind and live per the code.

 

[PMR06]

I would combine them. Think about it... debris can collect and cover any open pickets (there's your wind load), then some idiot film crew trying to get the perfect shot will slip and fall into the guardrail (there's your live load).

 

[msquared48]

NO...!

After he hits the broken pickets, it will be a dead load.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[csd72]

Wow, there is such a thing as a structural engineering joke!

Personally I would say that it depends on the circumstances. A domestic house has only small railing loads that can be achieved by only 1 or 2 people, a public balcony has large railing loads to allow for crowd surge. I would think that crowd surge and wind gust acting concurrently would be very very unlikely.

 

[cvg]

I can't believe after some of the highly publicized balcony failures in the past where many people were killed that any registered engineer would want to skimp on the design of a balcony. Seems like there is very little reward for assuming extra risk.

 

[rolkamatic]

Thanks for all the comments, this is on a big building, 45 stories, we have a wind tunnel study and there are substantial wind loads the infill panels guards are glass. So it is substantially solid.

Thanks Again. John Rolka

 

[paddingtongreen]

I don't know why we would consider that there would be a net pressure on a balcony handrail; whether windward, leeward or on the sides, the pressures front and back of the railings would equalize.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[rowingengineer]

Paddingtongreen,
Exploring your thought, what if the door behind had blown in? or the wind was blowing in a cross balcony direction?


An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[dhengr]

John:
Ask your wind tunnel guys for guidance on this, you paid dearly for what you got from them. You can have a positive pressure on the windward face of your glass, assume that’s pushing toward the building; you will have a negative pressure on the leeward side of the glass; and you can have three or four really dumb people holding onto the handrail to keep from being blown into the building face; these loads are all additive pushing the railing toward the building. And, you can have quite different wind pressure scenarios (+ & -) at building corners or significant wall plane changes, and at higher elevations. Mostly, what I worry about is the people leaning against the handrail and pushing it off, thus they fall off the balcony. I worry a little less about them pulling it inward, because they just end up on their butt, on the deck, not on their head ten floors down.

You said as much, of importance to the problem, in your last post as you did in your first post; so some of the earlier posts, while not wrong, were at a bit of a disadvantage without the latest info., although elevation only changes the wind pressures or velocity. However, many minds change about the strength of handrails when people are viewing the street from 45 stories high, instead of only 8' off the ground. Add to that the view through the glass baluster plane and half the people won’t want to get within arms length of the handrails.

But, Mike McC is dead on.

 

[racookpe1978]

Paddington:

No.

For example, you could not claim a roadside sign has no wind load because atmospheric pressure is the same on both sides. If anything, it is the balcony that will shield the lower part of the windows and doors in the wall from direct wind and water blasting: but the wall and balcony supports must absorb loads from both the balcony and the total wall area. (If you have solid or near-solid balconies.)

The gusts and turbulence hitting the balcony rail on one side of the building pushes that side; later, - when the storm is coming from the other direction - the balconies on the opposite side of the building's face their load.

 

[paddingtongreen]

I stay with my opinion. A balcony railing is not a sign in the open; a balcony has a building behind to trap the wind. The only way to get additive pressures is if the wind does blow over the railing. I concede the open door may change things locally, always provided that there is also a door open on the other side of the building and remembering that doors open outward and the pressure side would close the door.

In any case, 98mph would vacuum up anyone foolish enough to be on the balcony.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

Michael,
A decent argument, but I doubt if you are going to convince anyone else. I'm pretty sure that if I crouch behind the balustrade, I will feel less wind than if the balustrade doesn't exist.

 

[SteveGregory]

It seems to me that the solid infilled guardrail would be analogous to a parapet wall and ASCE 7 has speciofic wind loads for windward and leeward parapets.

 

[paddingtongreen]

That's true hokie66, but I think that even if you are standing you will not feel the full force. I think of a barn, with a large open door on one side, what ever pressure there is on the outside of that wall exists inside the barn, the inside pressure cancels out the outside pressure on that wall. You could stand in the doorway and not feel any great air flow because it only takes a small amount of air to enter the barn to equalize the pressures.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dhengr]

Michael:
I’m with rowing & hokie on this one, us old dogs usually gotta stick together or all these young dogs will run us over trying to do an elaborate FEA in a hurry to arrive at an incorrect, exact, answer. But, in this case it seems your slide rule is stuck in ‘won’t compute mode.’ Flip the darn slide end for end, you’re readin it wrong. We are not talking about ambient air pressure at sea level here. That is the same all round on the building, except as it changes with elevation. We are talking about a varying pressures around the building, as it stands as a broad surface (not unlike the road sign) or a funny shaped air foil in the larger wind flow.

To carry your argument to the extreme, if the pressures the same all around; why do we have any wind loading on a bldg.? Any bldg. is a bit like the road sign; except, much larger in mass, surface area, volume, stiffness, etc. and potentially shielded, or not, by other building or terrain. The bldgs. face does offer a boundary layer in the fluid flow, but it is not very thick in relative terms, the wind normally does blow over the hand rail. And, in fact, this solid handrail/baluster surface does shield the bldg. surface so things are not additive in that respect.

SteveG’s parapet analogy is a pretty good one; except in the case of the parapet there is a long reach of unobstructed roof on the leeward side of the parapet, thus its pressure profile will be somewhat different than the solid handrail/baluster (glass) surface. I think this is were your argument comes into play. We don’t know if the handrail is 1' away from the bldg. face or 8' away, or its exact orientation to the wind and this will affect the pressure profile. There could actually be wind pressure packing, a high pressure region btwn. the handrail and the bldg. face in some situations. Nonetheless, there will usually be a positive pressure on the solid baluster/handrail on the windward face, and a lesser pressure or maybe a negative pressure on the leeward side of that surface. If the wind is coming slightly from below, the suction behind the handrail will be even greater. I am not suggesting that we should always design for all this minutia. Let’s have the OP’ers. wind tunnel guys splain it to him, and he should report back to us, so he/we get our money worth out of that darn wind tunnel test.

If my argument above has failed on you, then let me put it this way: this is the age of all things structural and mechanical being determined by committee, if this were not so, why would we even have these forums where we ask a question, tally the yeas and nays, and that determines our design. We outnumber you so you better get on board, and I don’t mean drafting board either, because you and I are probably the only ones who have one of those in our office any more; or else we’ll leave you at the station.