Downspout Anchorage

[Trillers]

Greetings Everyone:

This might seem a bit trivial but it's driving me bonkers.

A client is constructing a pump station in a high wind, Seismic Zone E location. The pump station and wetwell are of PreCast Concrete Walls with CIP Foundation, Floor and Roof Slab.

The project QA is insisting that the 4" dia PVC downspout clamps and anchorage have to be designed to withstand seismic forces. I am of the opinion that this is a mechanical component, not a structural component and therefore qualifies under the ASCE exemptions for seismic requirements.

Nonetheless to satisfy my curiosity I started digging into the ASCE 7-16, the ACI 318 and the IBC and find nothing that shows me a downspout anchoring design should account for seismic loading. The closest would be as a mechanical "pipe" but I'm not sure if that is a correct assumption.

My reading and analysis are that the pipe weight is so insignificant that any seismic loading would be negligible even with the downspout flowing full. So I suggested that we just design for wind loading (190 MPH criteria) and shear from the weight of a full pipe. I would probably add an impact load of 15%...

Any thoughts on this?

Thanks.

 

[BridgeSmith]

Most likely, the reason you didn't find any seismic design criteria is that you're correct - it's not a structural item. Seismic design requirements exist to protect humans, not to prevent damage to drain pipes.

 

[JAE]

ASCE 7 has all sorts of mechanical component provisions for seismic. Perhaps not explicitly stated for downspouts but there are provisions for ductwork, equipment, etc.

I think you are correct that a downspout has very little mass and therefore negligible seismic load. Using wind is correct in my view.

And a failed downspout, tearing away from its attachment and hitting someone on the head is a life safety issue.

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[bones206]

I'm assuming you mean Seismic Design Category E. I think you could argue it is exempted from the requirements of ASCE 7-16 Chapter 13 using the highlighted clause below:

13.1.4 Exemptions. The following nonstructural components are exempt from the requirements of this chapter:

...

6. Discrete mechanical and electrical components in Seismic Design Categories D, E, or F that are positively attached to the structure, provided that either

a. The component weighs 400 lb (1,779 N) or less, the center of mass is located 4 ft (1.22 m) or less above the adjacent floor level, flexible connections are provided between the component and associated ductwork, piping,and conduit,and the component Importance Factor, Ip, is equal to 1.0; or​

b. The component weighs 20lb(89N)or less or, [highlight #FCE94F]in the case of a distributed system, 5 lb/ft (73 N/m) or less[/highlight];​

I wouldn't worry about seismic at all, but with that high wind, I think it deserves some attention so it doesn't rip off and become a missile in a storm. I would also check if the gutter system itself is rated for a certain wind pressure.

 

[BridgeSmith]

"And a failed downspout, tearing away from its attachment and hitting someone on the head is a life safety issue."

I suppose it could be, depending on the size and location.

 

[bones206]

Link

 

[SAIL3]

highly unlikely that the downspout would be flowing full during a seismic event....more likely during high wind(storm)...that leaves just the weight of the pvc pipe which would have a negligible seismic load...I still would design the attachment for the wind load which should govern anyway....

 

[wannabeSE]

Sail3
I agree. But OSHPD (California hospitals) requires seismic bracing of storm drains to be designed assuming they are full of water

 

[Trillers]

Thanks guys for for all your thoughts. I did consider the exemptions, but then taking into consideration what wannabeSE discussed about seismic bracing of downspouts assumed full - what could possibly be the forces? Seismic weight applied at the base of the downspout? Since the downspouts are not continuously braced but braced at intervals, where in the world would a seismic force be applied? would it be logical to apply the seismic forces at each point of bracing and assume the seismic weight is the weight of pipe plus water above that brace?

My thoughts now are to apply the exemption and just state that wind load governs. This is only a 2-story building, but what if the downspouts were on the face of a say 20-story building?

See this is the problem with we/us engineers - we just can't take "wind loads govern" for an answer.

 

[bones206]

I don't work in California very much, but I would assume the OSHPD requirement is to design the brace for the length of downspout tributary to that brace, with the entire column of the downspout assumed full of water. It seems overly conservative to consider a design rain event coinciding with a design earthquake event, but If it's a critical facility like a hospital I can see where the extra conservatism might be warranted.

If you consider the downspout to be full, then the component weight would be around 65 plf and the Chapter 13 exemption would no longer apply.

Personally, I would be totally comfortable assuming empty downspouts during the design earthquake event for most standard buildings.