The Earthquake Code, Local Law #17/95, was promulgated for new construction; it does not affect an alteration where there is no enlargement of the existing structure. When proposed work includes an enlargement of the existing structure, compliance with Local Law #17/95 is required when new foundations are constructed or when existing foundations are reinforced to carry the enlarged portion of the building, pursuant to §27-119 of the Code, within any twelve-month period, unless a waiver is granted by the Commissioner.
Not withstanding the above, a separation shall be provided between any existing adjoining building and any new building or the horizontal or vertical enlargement of a existing building; pursuant to §2312(h)2.K of RS 9-6.
When compliance with RS 9-6 is required, consideration must be given to the interface between the existing and enlarged portion of the building. If a seismic joint (separation) is utilized, then only the new construction must comply with RS 9-6.
When no seismic joint is provided, the computed forces of the entire proposed structure (existing building plus the proposed enlargement), must be compared to the computed forces for the existing portion alone. The forces referred to herein are either base shear or overturning moment. If the increase in any seismic force resulting from the enlargement is less than 20%, compliance with RS 9-6 is required only for the new construction. If either of the seismic forces attributable to the enlargement increase by more than 20%, then the design force shall be the lesser of the following:
- Two thirds (2/3) of the combined force resulting from the combination of the new and existing construction but not less than the seismic force resulting from consideration of only the new construction;
- Twice the seismic force resulting from consideration of only the new construction.
This design force shall be resisted by either the enlargement alone or a combination of the existing building and the enlargement, depending on the design professional's choice of load resisting system and force distribution.
A flow chart (PDF) illustrating the above requirements is attached.