Civil and Structural Engineering Key to Creating a Massive Green Roof for Costco

By W. Lee Shoemaker, Ph.D., PE, F. SEI, Director of Research and Engineering, Metal Building Manufacturers Association

In Mexico City, on top of Costco’s metal building warehouse, parking garage and loading dock rests a ½-million-square-foot roof of sports parks and recreation. 

On the way to pick up some macaroni and cheese, fish sticks, and paper towels at Costco Warehouse in Santa Fe, Mexico City, one might initially have a hard time finding the store. That’s because the 144,522 square feet warehouse is camouflaged underneath what is arguably the largest green roof to ever cover a retail building. 

A welcome addition to the city’s 70-acre public Parque La Mexicana, the green roof is adjoined by a soccer field, two basketball/volleyball courts, a padel court, and a children’s skate park connected with walking, running, and biking paths. In addition, the green roof hosts 21 varieties of native plants and grasses as well as 15 insect hotels.

Through a public-private partnership, Costco constructed sprawling roofs on top of its warehouse, parking garage and loading dock for public recreational use. Three different types of roofs and surfaces were engineered, allowing a siphonic drainage and gravity drainage system that collects and transports rooftop stormwater runoff to the park’s water features.

THREE BUILDING STRUCTURES

The 524,549 square feet Costco was designed as three interconnected buildings. The retail warehouse is a metal building, the parking garage was created from post-tensioned concrete and the loading dock combines conventional concrete and metal decking.

For the metal building portion, supplied by Butler Manufacturing, Span Construction and Engineering erected the building shell, consisting of columns, beams, roof joists, roofing panels, integral gutter system, wall girts, and metal wall panels.

The structural engineer, Engineers Northwest, Inc., worked with the metal building manufacturer to reduce the number of joists and columns required, thereby creating a more seamless shopping experience with fewer structural columns breaking up the floor plate.

The selection of steel, with its decreased thermal mass, as compared to a typical masonry block wall, helps reduce the building’s heat absorption, which is significant in Mexico’s hot climate. Additionally, the structure is made from 80 percent recycled steel.

STRUCTURAL IMPACTS

Because the green roof is not open to the public, the roof designated for the metal building warehouse didn’t have to account for additional load. 

The base roof live load is 20 psf, the super imposed roof load is 30 psf overall and the 2-foot perimeter around the roof is 49 psf.

For the parking garage, restricted to three stories to blend in with the park and meet local height requirements, post-tensioned concrete was deemed to be the most suitable material.

 “This is a very efficient design that utilizes less concrete than other common concrete structures while still holding heavy loads from the sports park,” reports Maribel Barba, AIA, project architect and manager, MG2 Design, Seattle. “The thickness of the floors is a little over 5 inches, which is quite thin compared to other concrete structures.”

“The rest of the receiving area was designed with steel columns and conventional concrete metal decking to hold up the weight of the children’s skate park,” she adds.

For the loading dock, the project team designed a column-free span large enough to allow a semi truck to enter the space, unload its contents, make a U-turn, and exit. To enable this, MG2 custom fabricated a 90-foot-long, 12-foot-tall beam to provide the required clearance and maneuverability inside the dock area. 

SEISMIC REQUIREMENTS

Considered to be one of the highest seismic zones in the world, the codes and standards in Mexico City and its surrounding suburbs are naturally stringent.

With metal buildings, engineers must take into account providing sufficient ductility in the structure to absorb and dissipate the massive energy produced by an earthquake. Research by the Metal Building Manufacturers Association (MBMA) and industry partner American Iron and Steel Institute (AISI) evaluated the behavior of full-scale metal building frames at the University of California, San Diego (UCSD) on their shake table to study the behavior and ductility of a metal building system under various earthquake conditions. Their work revealed how metal building moment frames behave quite differently regarding ductile design philosophy and that metal building frames demonstrate little conventional ductility.

Mexico City has adopted the International Building Code (IBC), which permits the use of three steel moment frame systems for resisting seismic lateral loads. Each has a different design procedure that recognizes the anticipated amount of ductility, primarily based on the inelastic rotation that can be expected at the beam-column connections.

As opposed to the hot-rolled steel shapes typically used in multi-tiered conventional steel construction, metal building systems utilize slender members with thinner flanges and webs. The web thickness and flange size are optimized to match the strength required at any location on the frame. Consequently, metal building system designs can save on material costs while still meeting seismic codes.

The metal roof structure can also perform well to protect facilities facing high-wind events. In a recent white paper “Metal Roofing Performance and Wind Events,” Robert Zabcik, P.E., president, Z-tech Consulting, Houston, wrote, “Today’s metal roofing is a strong choice for even the most vulnerable structure. Metal roofing’s strength and durability provide safety and security for building owners and occupants alike.”

For the Costco Santa Fe project, the use of three different building types required different seismic designs. At the same time, the buildings are all connected.

“Per code, we needed to add a seismic gap between each building type,” Barba explains. “However, people still needed to be able to move between the buildings, so different types of seismic joints were used throughout the project.”

LANDSCAPING THROUGHOUT

As noted, a big effort was made to blend the warehouse into Parque La Mexicana’s natural landscape.

To hide the building, tall berms called Montañitas were placed strategically and green screens were designed for the walls.

“The green façade screens, with native crawling vegetation at its base, cascade down from the roof above and obscures the warehouse and parking structure’s appearance,” wrote Barba in the Mexican landscape architecture publication Landuum.

In addition to concealing the structure, the native Montañitas require less irrigation than typical vegetation.

IN RETROSPECT

Working closely with the community, city, park association and environmental groups, the design engineers and architects went above and beyond original expectations. 

 “Working on Costco Santa Fe was both incredibly challenging and deeply rewarding,” says Barba. “Integrating innovative solutions with community service meant that we were not only addressing the current needs of the city, but also engaging with Parque La Mexicana to tailor solutions that were truly beneficial and sustainable. Seeing the positive impact of our work on the community—how it improved daily lives or solved a specific problem—was profoundly fulfilling. Personally, it has enriched my understanding of how innovative design and community can work hand-in-hand to create meaningful change.”