Seismic Design Requirements for Stormwater Detention Vault

[jputre]

Hi All,
I frequent this forum but this is my first post. I need help on a project I'm working on, it falls under the 2015 Seattle Building Code (amended from 2015 IBC).

The project site has liquefaction soils and our building foundation is supported by 60-90' deep auger cast piles. There is an independent, pre-fabricated below-grade stormwater detention vault that is outside of the building footprint, below a surface parking lot. Our code reviewer is stating the stormwater detention vault needs to be designed for potential geologic and seismic hazards (including seismic lateral earth pressures and differential settlement due to liquefaction). The Geotech report notes up to 15" of post earthquake liquefaction so that would require us to install the below-grade detention vault on a deep pile foundation similar to the building.

The reviewer has referred us to the 2015 Seattle Building Code (SBC) sections 1803.5.11 and 1803.5.12 for additional information and stated that they do not allow exemptions for underground water storage vaults or structures that are typically unoccupied.

However, this code section lists the requirements for the Geotechnical investigation, which were included, and doesn't note how the seismic requirements are applicable to below grade structures.

I believe the storm water detention vault should fall under the local stormwater management design manual which only states 'Detention vaults shall be placed on a stable, well-consolidated foundation and bedding material.'. I believe this rational is supported by the fact that none of the other site elements (retaining walls, electrical vault, electrical transformer pad) or the other stormwater infrastructure elements (catch basins, connections, up/down steam pipes) are designed for liquefaction settlement. Furthermore, the parking lot this is located under is not designed for liquefaction settlement.

Does anyone know of any code exemptions that exclude below-grade/unoccupied or stormwater structures from seismic requirements?

 

[Yikes]

2015 Seattle Building Code (SBC) sections 1803.5.11 and 1803.5.12 is subordinate to the scope of applicability SBC 1801, which states:


SBC 202 definition of "building" is:






I don't see how an underground stormwater storage tank qualifies as a "building" based on these terms, and I don't see how it is a foundation, unless the tank under the roadway is part of another life-safety system that is regulated by the IBC or IFC (for example, exit discharge path-of-travel, a fire apparatus access road, or maybe the soil is accepting a surcharged load from a nearby building foundation).

It's quite possible that civil engineering should still be required - - but I'm not sure that the 2015 SBC 1803 is the most appropriate mechanism for enforcement.

 

[Mark K]

If Seattle has not adopted a version of the IBC what have they adopted?

The tank is a structure. IBC 101.2 specifically states that it applies to structures not just buildings. A foundation is a part of a building or structure although I would not consider an isolated tank a foundation.

If the Seattle building code does not cover structures then it is not the concern of the building department.

The structural provisions in the IBC should be adequate to design such tanks. Likely a geotechnical engineer should be involved.

My understanding is that the building codes do not address the design of roads. If the road will become the responsibility of the city the City public works departments will establish the requirements for the road. If a road is on private property I do not believe that there is any formal regulation of the design of the road.

 

[jputre]

Thanks Yikes & Mark. Seattle has adopted the IBC with their own set of amendments (SBC) but I believe the reviewer is leaning on the broad definition of 'Structure' in their interpretation.

1601.1 Scope. The provisions of this chapter shall govern the structural design of buildings, structures and portions thereof regulated by this code.

1613.1 Scope. Every structure, and portion thereof, including nonstructural components that are permanently attached to structures and their supports and attachments, shall be designed and constructed to resist the effects of earthquake motions in accordance with ASCE 7, excluding Chapter 14 and Appendix 11A. The seismic design category for a structure is permitted to be determined in accordance with Section 1613 or ASCE 7.

Definition - STRUCTURE. That which is built or constructed.

However, I believe the applicability of SBC 1801 should address this since it's a below-grade structure which appears to be outside of the scope of Ch 18. We have a civil, geotech and structural engineer involved, who all agree that the detention vault should not be designed for seismic settlement.

Thanks again.