Open parking structures revisited

When I first started writing this column back in September of 2002 my very first article was on open parking structures, and the fact that what would seem to be the simplest possible project for a plumbing designer can actually be quite tricky. Well, today that remains true — possibly more so than 15 years ago. 

My firm is bidding an open parking structure, which is the expansion of an existing parking structure as well as foray into a larger phase two medical office building. Since it is a design-build project, the bidding documents consist of a set of rough architectural plans. No drains are shown, and there is no plumbing criteria. We do have the drawings for the existing parking structure, but one cannot assume that what was done in the existing structure can be done today in the new structure. 

For example, the existing parking structure was not provided with a fire sprinkler system — just a dry fire standpipe. Surely this new addition will have to be sprinklered, since the So Cal fire marshals are on a mission to sprinkler the world. How odd will that be, to have half of the structure sprinklered and the other half not so? But that is just the tip of the iceberg. 

The project is in Los Angeles County, but it is in a city that is considered its own jurisdiction from a code standpoint. That makes it difficult to predict what the design expectations will be, and the small city officials are very hard to get in touch with. 

Some jurisdictions allow all parking structure storm drains to be combined on the various levels and piped to the site storm drain system. If this is the case, the storm water will require treatment of some sort to satisfy the SUSMP pollution control program. This may or may not be addressed by the civil engineer, meaning it may or may not be the responsibility of the plumbing designer. There just isn’t enough information in the bid package to tell.

Other jurisdictions require the top parking level to be piped to storm and the covered levels to be piped to sanitary since they do not receive direct rainfall. If piped to sanitary, some jurisdictions will allow the drains to be piped as an open system to a sand-oil interceptor that serves as a central trap. Yet other jurisdictions might require each covered level drain to be trapped and vented with a trap primer. This obviously makes the system a great deal more complicated and costly. Especially since the trap primers must be electronic since there are no flush fixtures to generate sufficient pressure drop to operate a conventional trap primer. If mechanical trap seal devices are allowed in the jurisdiction, this would be an appropriate alternative. 

In contrast to the examples above, L.A. City requires the covered level drains to be piped to storm, but through a riser separate from the top level riser and through a backwater valve at its base. The rationale for this has always eluded me, but it is yet another example of how varied the treatment of garage storm drain systems can be.

Another idiosyncrasy of L.A. City treatment of parking garages is that they do not allow hose bibs in parking structures, since parking structure floors are supposed to be cleaned and maintained using little vehicles similar to small street sweepers, rather than with water hoses for the sake of water conservation. The presence of a hose bib in the eyes of L.A. City implies that it will be used for car washing, in which case the drainage is required to go to sanitary through a sand-oil interceptor. 

The presence of a fire sprinkler system can have dramatic impact on the plumbing scope of a parking structure. Fire sprinkler systems of course have test and drain provisions. Once upon a time these drains were allowed to spill to the landscaping or a dry well. But the ever growing environmental awareness has put a stop to this in many jurisdictions, meaning that the sprinkler drain must instead drain to sanitary. As such, a project that would normally not require a sanitary sewer connection would now require one, as well as a domestic water connection for the trap primer — unless a mechanical trap seal device is allowed. But since fire sprinkler drains discharge through hub drains, the trap seal devices might not accommodate such an installation.

Another word of caution about parking structures is the possible need for overflow drainage. Depending on the slope of the parking structure slab, there is potential for saddled areas to be created that are as much as 12” or more in depth from low point to high point. As such, overflow drainage would be required in these deep bays unless it can be proven structurally that the slab was designed to support that depth of water. More often than not, overflow drainage would simply flow down the parking ramp or into the void between the ramp and the flat slab, but sometimes that is not the case.  

If required, overflow drainage can be handled in multiple ways. I have seen systems where the OFD is simply a drain body, open ended, to allow overflow drainage to be relieved to the level below should an emergency occur. In that case the drains on the level below the top level would have to be sized the same as the top level to receive the full overflow drainage. In other jurisdictions the OFDs are required to be piped to the ground floor. In other jurisdictions the OFDs have to be piped to the exterior (perimeter) of the building. 

Clearly when the bidding documents consist of a rough set of architectural drawings with no drains or grading shown, all of these variables are hard to predict and qualify in a bid, or as a designer involved in the schematic phase of the project. It is imperative to define with the architect as early as possible what the drainage scheme will be since it affects the structural design, the architectural design, and of course the plumbing design and associated pricing. 

As a final note, one other area of caution regarding parking structures is the possible need for condensate associated with elevator or electrical room AC units. If such units exist, the condensate maybe able to be pumped with a small condensate pump to the landscaping, but some inspectors might require it to go to an “approved receptacle,” meaning a floor drain or floor sink. Here again, this requirement might trigger the necessity for a sanitary system in a building that might not otherwise have one, as well as a water service for a trap primer in case insufficient condensate is produce to keep the trap primed.
So as you can see, a simple parking structure can have many variables that affect scope, design and price. It is important to iron out all of these issues as early as possible in the process lest they get discovered late in the program, making them more difficult and costly to resolve. 

Timothy Allinson is vice president of Engineering at Murray Co., Mechanical Contractors, in Long Beach, California. He holds a BSME from Tufts University and an MBA from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a LEED accredited professional. Allinson is a past-president of ASPE, both the New York and Orange County chapters. He can be reached at

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