November 21, 2002

Mixed-use and maximum value

  • There’s a lot to like about mixed-use, but unique design issues can be costly


    There’s a new vision of urban living sweeping the nation: dynamic, energetic places where the work is mixed with living. Neighborhoods with retail shops, cafes and restaurants are surrounded by attractive streetscapes, creating places for people to interact and socialize after work.

    This may be a throwback to the old European city, where the butcher and cobbler had shops at street level below living quarters, but it is also part of a hopeful trend toward sustainable development and revitalized modern cities.

    Spreading the risk

    The Seattle area has seen an increase in mixed-use developments during the last decade, particularly in the Belltown, Denny Regrade, Capitol Hill, and First Hill neighborhoods. Mixed-use projects include several functions. Typically, apartments are stacked over office space with retail shops, restaurants or a coffee bar at street level.

     Thea’s Landing
    Photo courtesy Mithun
    Thea’s Landing, Tacoma, designed by Mithun, is the first mixed-use project on Tacoma’s Thea Foss Waterway with public esplanade restaurants and shopping.

    There are good financial reasons for developers to invest in mixed-use buildings. True to the investor maxim — Don’t put all your eggs in one basket — a mixed-use project offers diversification. By having several different sources of revenue, the impact of a downturn in one market sector can have limited impact on the overall economy for the building. For instance, the current slump in the office rental market could have less of an impact on a mixed-use project than on a building with office space alone.

    And there are more advantages. Risk can be spread among different leaseholders in the building. Further, the cost of development can be spread over time. By phasing in different functions and parts of the project over time, initial development costs can be minimized.

    The larger the building created on a site, the lower the relative cost of the land. And with a larger project, development and construction costs can be lowered on a cost-per-square-foot basis.

    And then there are public incentives. As cities try to stimulate developers to revitalize neighborhoods, credits are being given for certain programming and design activities. A site’s floor area ratio, the ratio between the allowed floor area and the size of the site, can be increased by adding retail to an apartment-zoned site, or apartments to a commercial-zoned site. FAR can also be increased for streetscape-improving efforts, such as attractive marquis or street light fixtures.

    From a rental and marketing standpoint, the mixed-use project represents the trend of today. Developers market their apartments as life-style experiences. Downtown, where everything is happening, is the place to be. Typical offerings have attractive lobbies with concierge service and amenities such as gyms and libraries.

    Stacking it right

    Along with the many benefits of mixed-use development come some particular structural challenges. Different structural, mechanical, plumbing and electrical solutions are required by the varying program areas.

    For example, entrances and exits can become costly features in multi-use projects. Is there a desire to separate the entry to the office from the apartment complex? A separation may be needed to maintain a professional appearance of the office entry. Further, egress requirements for residential and office spaces differ both in terms of capacity and proximity. Located correctly, the same stair can serve both the residential and the office tenants. Entries, exits and means of egress are complex issues sometimes requiring costly separate entries, elevator systems and stairways.

    The lowest cost buildings have structural columns that are aligned. The greatest structural challenge is that the different uses of space require different column spacings. In other words, there is a “stacking” problem. And that leads to cost pressure.

    Although apartments and condominiums can hide columns within partitions and exterior walls, office and retail areas have high requirements for open space. A typical office bay has 30-foot by 40-foot spacing between columns. A typical retail bay has 30-foot by 30-foot spacing between columns. Problems arise as apartment units are stacked on top of office or retail space, because the column layout does not match.

    So why not use wider column spacing for the apartment floors as well? With wider column spacing, the depth of the structural members spanning between the columns would need to be deeper, and therefore more expensive. In general, for mid- and high-rise concrete structures, it is more cost efficient to have a tighter grid of columns than paying the price for thickened horizontal structural systems.

    Form follows parking

    In residential design, the saying goes that “form follows parking.” To maintain a cost-efficient design for residential units stacked over parking, the unit layout is somewhat controlled by the layout of the parking below.

    The layout of the structural columns must fit between the parking spaces and the drive isle. Code regulations for parking are very strict and define the minimum width for parking spaces and drive isles. With a couple of variations, these regulations determine parking garage layout.

    In order to avoid costly transfers of the structural columns, the challenge is to marry the layout of the parking structure below with the layout of the apartment units above. Add to the mix a few levels of retail and offices between the parking and residential units, and the columns don’t stack anymore. In the worst case, you might end up with two costly transfer levels — one between office and parking, and another between residential and office.

    Other stacking issues relate to mechanical and plumbing systems. An efficient apartment building is designed with bathrooms and kitchens of adjoining units located on either side of a common “plumbing wall.” This approach works well within a “stacked” apartment building, but what happens when there are office levels below the apartment levels? Plumbing risers and waste lines coming down into office space need to transfer horizontally to new locations that fit vertically for this type of floor layout. Without careful thought, the cost adds up quickly.

    A similar situation applies to heating and ventilation shafts. Shared and combined shafts and units will generally reduce construction cost, but acoustical considerations commonly lead to separating the venting and heating system for an office space from the residential system.

    Mixed-use projects also have unique security considerations. A mixed-use development results in a higher flow of people than the more controlled and restricted areas of single-use projects. For shared entries, access control and policies must be adapted for the different people coming and going to these different spaces.

    Different uses require different fire-rated separations and assemblies, escape routes and sprinkler systems. Eliminating noise transmission from public to residential levels can be challenging, but can also have inherent advantages. Locating apartments above street level can also minimize traffic noise.

    Successful design integrates the various needs of different functions into one building, with the different systems coming together and working as a unit. Combining functions and letting systems serve double-duty is one of the keys to reducing costs, such as office stairs being combined with residential stairs to provide egress. Whether structural, mechanical, plumbing or electrical systems, the most cost-efficient system is the one that does the job in a “path-of-least-resistance” fashion.

    The most important key to success in a mixed-use project is for each design discipline within the team to look for global solutions to the many conflicting-use requirements.

    Ola Johansson is an associate at Skilling Ward Magnusson Barkshire, a Seattle structural engineering firm.

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