On the up and up
In the high-rise building development market, time is money. The faster buildings can be designed and constructed, the quicker owners and developers can secure tenants and start to generate revenue.
One way to bring a building to market faster is to eliminate bottlenecks in the design and construction process. For high-rise, steel-framed buildings with below-grade parking structures, traditional construction methods require the garage to be fully complete before erection of the above-grade structure can proceed. Depending on the size and depth of the garage, construction can take from six to eight months, creating a delay in the erection of the tower.
Two novel steel construction methods - up/up and up/down - are being employed to cut several months from construction time by permitting simultaneous construction of the below-grade parking structure and the superstructure. Up/up construction
'To put it simply, it's called up/up because once the excavation and foundation system are completed, the building's structure starts at two locations - at the bottom of the garage and at the ground-floor level - and progresses simultaneously upward,' says Gino Baroni, executive vice president with Boston-based general contractor Beacon Skanska Construction Co. The firm is currently employing the up/up method to speed construction of One Lincoln Street, a 37-story, 1 million-sq.-ft. office tower under construction in downtown Boston. Developed by Gale & Wentworth of Florham, N.J., the $360 million project includes five levels of below-grade parking and will be completed in 2003.
'With up/up construction, the foundations are constructed using conventional methods, followed by erection of the steel columns to the ground level and of the ground-level framing,' explains Minhaj Kirmani, principal with New York City-based Weidlinger Associates, structural engineer for One Lincoln Street. 'Then, erection of the superstructure framing and below-grade structure take place simultaneously, essentially removing the basement levels from the 'critical path.''
'If we were to use the traditional 'bottom/up' method on One Lincoln Street, six to eight months of below-grade construction would have to take place before construction of the above-grade structure could begin,' says Kerim Evin, project executive with Beacon Skanska. 'With up/up, it'll take us only about four weeks to erect the free-standing steel columns and then get the ground floor framed, decked over and the concrete slab poured in time for the superstructure construction to begin.'
Essential to using up/up is accurate coordination among all building team members during construction. 'Because we're starting with five stories of relatively unbraced steel columns below grade, there's a limitation on how high the superstructure erection can go before the concrete slabs are poured in the garage,' says Norman Adams, project manager with Boston-based Jung|Brannen Associates, architect for One Lincoln Street. 'On this project, for instance, it was determined by Weidlinger that the superstructure can go as high as 10 stories before some of the parking slabs are laid. So there's a timing issue there.'
Adams adds that compared to traditional methods, up/up requires that the architectural and M/E/P design plans be finalized much earlier. 'Since the ground floor slab is poured first, the design of the first-floor lobby area - which is usually the most complicated space in high-rise buildings - must be pinned down early,' he says. 'On a standard project, construction begins from the bottom of the garage upward, so we're not as much under the gun to have the first floor all worked out.'
Kirmani says Weidlinger has been involved with several other Boston-area projects that employed up/up construction, including the 19-story, 530,000-sq.-ft. 10 St. James Place office tower, the $135 million CambridgeSide Galleria retail development, the Back Bay office tower and the East and West office buildings at the World Trade Center. 'It's estimated that between three and four months have been saved in the critical path schedule on each of these projects,' adds Kirmani.
The other uncommon approach, up/down, also skips construction of the below-grade structure to advance the erection of the steel superstructure. With this method, however, the foundation wall is installed, followed by installation of caissons, columns and the ground-level slab. Then the superstructure is constructed while at the same time the area for the garage is excavated and the garage is constructed from the ground level down.
'With up/down, you're literally using a tedious mining operation, where you're removing the soil through small openings in the [already placed] ground-floor slab, called 'glory holes,'' says Baroni. 'It sometimes requires specific areas of the ground-floor slab to be heavily reinforced to handle the weight of the cranes and tracker trailers that use the ground level slab as a working platform. This adds to the cost.'
Kirmani adds that the basement levels are usually constructed within a constricted space, 'generally only about 10 feet high.'
Although more costly than up/up construction, up/down does allow for the erection of the superstructure to begin sooner. Moreover, there's virtually no limitation on how fast the superstructure can be erected before the garage levels are completed, because the above-grade structure is not temporarily supported by unbraced steel columns, as with up/up.
Evin says the building team considered using up/down construction for One Lincoln Street, but it would have presented two problems: it was too costly and the design documents were not far enough along to start the above-grade structure any sooner. 'It's no good to spend the extra money to get to the superstructure construction faster if the design documents are not ready,' says Evin. 'I think one of the main drivers of the up/down method is building projects where the schedule is the primary factor, above and beyond cost, say the building is for a tenant that has to be operational at a particular time.'
Beacon Skanska last employed up/down for construction of the 75 State Street building in Boston, completed in 1989. The method was also used in 1987 to speed construction of Rowes Wharf, a 16-story office/retail building on the Boston Harbor waterfront.
'To make up/down worthwhile, the project must have at least five levels of below-grade structure. Other important factors to consider include the soil conditions and the site and garage layouts,' says Evin, adding that One Lincoln Street's tight site conditions would have made the up/down method difficult to employ.
'Of the three scenarios we considered for One Lincoln Street,' says Evin, 'the traditional method has the longest duration and, in most cases, the least cost. Up/down has the shortest duration and the highest cost. Up/up offers the best of both worlds, with a better schedule than the traditional method and a lower cost than the up/down approach.'
'[Up/up and up/down] are not cure-alls,' says Baroni. 'We would rather use traditional methods in many cases because it's less costly.'
But time is a major factor for owners today, and they're always searching for ways to deliver a building faster, which may involve putting more money into the construction process.
'Having tenants moved in and paying rent on a building six months early is a huge financial benefit to owners,' concludes Baroni.