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Structural Implications of Vertical Expansion

By Keith M. Bouchard, P.E.

It was Mark Twain who first quipped, “Buy land, they’re not making it anymore!”  Nowhere in this country does that old maxim ring more true than in the crowded cities of the Northeast where every available parcel has been developed and re-developed.  With all the prime real estate taken, building owners and developers who are looking for more space are left to look upward instead of outward.  This often means exploring adding stories to an existing building where the zoning rules permit.  In some rare occurrences, the original developer will have the forethought to include provisions for vertical expansion in the original design, saving a lot of hassle.  This is typically not the case, however, and many times an Owner looks to add stories to a building that was constructed well before modern building codes were adopted. 

Structural Requirements

There are some obvious structural implications of adding stories to a building.  For example, if you are reusing the existing columns, are they strong enough to support the extra load?  Another obvious question is if the roof structure can support floor loading – which is heavier than the snow load the roof was likely designed for – or will it need to be re-built?  The existing footings and foundations will also need to be evaluated to determine if they can support the extra load.  If not, reinforcing them will be a significant cost, especially if the work is within the (occupied) footprint of the building.

Less obvious than the added weight of a vertical addition, but perhaps more of a concern, are the added lateral loads caused by expanding upward.  These include both wind loads and seismic loads.  Seismic loads are often the main concern since it can be very expensive to retrofit a building for earthquake loads if it was not originally designed using modern codes (seismic loads were first put into the Massachusetts Building Code in 1970).  Buildings with unreinforced masonry bearing walls can be especially troublesome. 

Chapter 10 of the International Existing Building Code, including Massachusetts specific Amendments, dictates that an existing building undergoing a vertical expansion be designed to withstand reduced-level seismic loads from the current code if the vertical expansion causes an increase in the “story shear” at any level by greater than 10%.  “Story shear” is the design lateral seismic force on one story of the building and is related to the weight of the story and its height above grade.  It is possible that a vertical addition over the partial building footprint with relatively lightweight framing built onto a heavy (i.e. concrete) building may not exceed the 10% load increase and therefore may not need a seismic retrofit.  However, it is likely that a single story or multiple-story addition over the entire building footprint will trigger the seismic requirements for most buildings.

Zoning & Fire Protection

This article will focus on the structural implications, however, the first item that an Owner should review when considering a vertical addition are the local zoning laws.  These laws will restrict the allowable building height in a certain district and if the vertical expansion exceeds this height a waiver will be required (which may be difficult or even impossible to obtain).  Additionally, the Owner will need to evaluate if the addition will make his building a “high-rise” structure (70’ mean roof elevation in Massachusetts) and therefore subject to stricter fire protection requirements, including full sprinklers.  The cost of these items alone may make a vertical expansion impossible.

In future blog articles, we will examine different methods of retrofitting existing buildings for seismic loads.