1800 Larimer St. is the first office tower built in Denver�s central business district in nearly 25 years and the first to achieve LEED Platinum. The 23-story building, which will be the new regional headquarters for Xcel Energy (occupying floors 3 through 16), offers 500,000 sq ft of environmentally friendly floor space.

Construction began in May 2008 and was virtually complete this spring. Xcel�s phased move-in from offices in three different locations will be complete by early August.

�Part of the project�s sustainable, transit-oriented appeal is the connection to the Union Station intermodal (now under construction),� says Dick Anderson, principal and project director for Denver�s RNL, the project architect. �By 2013, the building will be served by a shuttle�similar to the 16th Street Mall shuttle�that loops through the intermodal via 18th and 19th [streets].�

A Kinetic Skin

1800 Larimer St. has already garnered plenty of attention, not only because of its prominent location straddling LoDo and Denver�s central business district, but also for its perplexing patchwork exterior�an unusual collage of squares, rectangles, colors and textures.

�1800�s distinctive exterior presents an exciting new graphic and kinetic face to observers,� Anderson says. �Highly efficient, appropriately scaled office spaces ensure a comfortably attractive atmosphere.�

Built by the Denver office of Mortenson Construction, the building�s exterior is made of 60% energy-efficient glazing. Conventional strip windows alternating with custom-colored, textured precast panels by Rocky Mountain Prestress, Denver, seem to disappear beneath strategically placed squares and rectangles of raised blue-glass panels fabricated by Wausau Glass, Wausau, Wis. Daylight constantly changes the fa�ade�s reflected images and color.

1800 required 35,677 cu yd of concrete, 1,100 tons of rebar and 3,000 tons of structural steel for the poured-in-place foundation and structural steel frame. Aluminum framing helps anchor the reflective-glass skin. Poured-in-place concrete includes caissons and the building core of stairwells and 14 elevators.

Three levels of below-grade parking cover the entire site, while two levels above grade occupy the north half, opposite Sakura Square. Both are constructed of post-tensioned concrete slabs.

�The biggest challenge was lifting materials into place on the tightly confined site,� says Jason Miller, construction executive with Mortenson. �Materials off-loaded from a single closed lane on Larimer Street had to be immediately hoisted into place by tower cranes�the sheer volume of material required two tower cranes operating simultaneously.�

John McCorkle, Mortenson�s construction manager, adds, �Just-in-time delivery was carefully coordinated with construction activity. Space was further limited by dumpsters placed to sort waste for recycling.� He says that 89% of construction waste was recycled, along with nearly 80% of demolition waste.

Innovative HVAC System

The building is also the first office tower built in Denver in roughly 20 years to be equipped with a raised-floor air-distribution system. Buoyancy causes air delivered via the system to rise through adjustable floor-mounted diffusers and freshen workspaces by forcing stale air out through ceiling grills. The diffusers provide local control of air flow to workspace occupants. When conditions are right, air economizers reduce chiller use by pulling in outside air for �free� cooling. Haworth Systems, Holland, Mich., fabricated the system.

�The two 900-ton chillers consume an extremely economical .331 KW per ton of cooling as opposed to 1.5 KW per ton on standard AC systems,� says Greg Korstad, lead electrical engineer for Swanson Rink, Denver. �Meanwhile, five 2400 MBW boilers carry an 85% efficiency rating.� The mechanical installations, including three built-up air handlers, are installed atop the tower in the chevron-shaped 23rd floor.

�A demand-control ventilation system samples CO2 levels and adjusts airflow into the building accordingly,� says Tim Chiddix, Swanson Rink�s lead mechanical engineer. �Submetering allows tenants to monitor their own energy use, thus providing them with a strong incentive to conserve power.�

LEEDing Amenities

Miller says the extensive documentation required by the LEED precertification process to verify the absence of VOC materials, sources of materials, and so on, was difficult, but to accomplish it, �we made extensive use of BIM imagery.�

Heat-reflective, white TPL roofing covers the tower, dramatically reducing its heat island effect, while a landscaped 17,000-sq-ft terrace caps the above-ground parking. Additional LEED points were awarded for the terrace, which essentially counts as undeveloped ground.

�Dual-flush water closets, waterless urinals, low-flow shower heads and automatic water faucets are designed to consume an estimated 43.6% less water than traditional systems,� Chiddix says.

Sensors mounted in perimeter walls maintain uniform light levels by automatically dimming or brightening overhead reflected lighting in response to available daylight transmitted through floor-to-ceiling windows that also offer spectacular mountain views. Deck-to-deck height is 14 ft, while interior walls are 9.5 ft high.

At peak activity, 450 workers were onsite with 1,700 employed overall. The project cost for the core-and-shell was $102 million. Tenant finish, being performed by Mortenson for Xcel Energy, brings final costs close to $115 million.

Mortenson also finished the 18th floor for owner/developer Westfield Development and is close to completion on the Intermission Caf� in the 30-ft glazed lobby finished in Italian tile.

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div id="articleExtras" Westfield Development Mortenson Construction RNL Swanson Rink, Jirsa Hedrick & Associates, J.F. Sato & Associates Enermodal Engineering Wausau Glass, Haworth Systems, Encore Electric, RK Mechanical