A Sprawling Green Roof in Mexico City

To view the figures and tables associated with this article, please refer to the flipbook above.

Strolling through the sprawling 70-acre public Parque La Mexicana in the Santa Fe neighborhood of Mexico City, locals and visitors enjoy paved trails, vegetation, sculptures, fountains, restaurants and lakes with the city skyline serving as a beautiful backdrop.
Amongst all the greenery, one would be hard pressed to spot a ½-million-acre Costco warehouse, parking garage and loading dock. Why? Because it’s largely concealed under what is arguably the largest green roof ever to cover a retail building.

Not only is the green roof a sight to behold, but it provides a healthy home for thousands of native plants, primarily succulents spanning 10 species, and a total of 15 insect hotels that were created in partnership with Mexico City’s environmental agency. A welcome addition to the city, the unique green roof was designed in the spirit of an agreement with the local parks commission which allocated 30% of Parque La Mexicana for commercial development in exchange for companies covering the park’s operating costs.

“Designs that provide green space and the use of cool and green roof technologies in cities can reduce heat-island effects, producing multiple benefits and cost reductions by helping to reduce emissions and air pollution, human health risks, and economic losses due to reduced labor productivity,” cited researchers in the Fourth National Climate Assessment by the National Oceanic and Atmospheric Administration.

While the green roof is not open to the public, action abounds next to it on a public soccer field, two basketball/volleyball courts, a padel court and a children’s skate park—all connected with walking, running and biking paths.

In building the massive Costco structure, the design team analyzed seismic codes and evaluated the options for the most efficient ways to use the space. Their research led to the design of three separate buildings: a metal building for the retail warehouse, a post-tensioned concrete parking garage and a loading dock of conventional concrete and metal decking. The warehouse is 144,522 square feet, and the entire project is 524,549 square feet.

The metal building’s advantages include sustainability, flexibility, durability, longevity, aesthetics and affordability, among others.

“The metal structure is made from 80% recycled steel,” said MG2 Architects/ Maribel Barba, AIA, in an Arcat Detailed podcast interview. “This minimizes the amount of new material utilized. In addition, the decreased thermal mass of the metal reduces building heat absorption, as compared to a typical masonry block wall.”

Working with the building manufacturer, Butler Manufacturing, and the structural engineer, Engineers Northwest, Inc., Span Construction and Engineering erected the metal building system, which comprised columns, beams, roof joists, roofing panels, an integral gutter system, wall girts and metal wall panels. These elements form the building shell.

Because the metal building can support large spans, it significantly decreased the number of joists and columns required for the building when compared to other forms of construction. As a result, Costco is able to create a more seamless shopping experience due to a reduced need for structural columns breaking up the floor plate.

Each roof for the warehouse, parking garage, and receiving area support distinctive loads and were consequently constructed with different materials. While there was initially a concern about the metal roof’s ability to support the load of a green roof, it turned out the loads were not as high as expected. As a result, MG2 could design the warehouse with the same amount of steel as the firm typically uses for most of the Costco projects in which they are involved. The warehouse, with the green roof above it, is made of metal that is composed of 80% recycled steel. While 20 psf is a base roof live load, the super imposed roof load was 30 psf overall and 49 psf for a 2-foot perimeter around the roof.

Durability also tops the list of key metal roofing benefits. According to a study conducted by the Metal Construction Association and the Zinc Aluminum Coaters Association, the projected service life of a standing seam metal roofs (SSMR) is a minimum of 60 years. This is generally considered the longest service life of any commonly used roof system currently on the market. Placing a green roof over the roof should not have an impact on the lifespan of the roof due to the introduction of a complex drainage system and the inclusion of a Sarnifil membrane, a multi-layer, synthetic roof waterproofing product.

At the same time, the three-level parking garage roof houses the sports courts and had to support the adults, athletes, and children using the courts and occupying the space as spectators. To support these loads and meet city restrictions on how high the structure could be, MG2 went with post-tensioned concrete.

“This is a very efficient design that utilizes less concrete than other common concrete structures while still holding heavy loads from the sports park,” Barba said. “The thickness of the floors is a little over 5 inches, which is quite thin compared to other concrete structures.”

As for the loading dock underneath the skate park, the team was challenged to create a column-free span large enough to enable a semi-truck to enter, unload its merchandise, make a U-turn and exit the structure. It was constructed by combining metal steel columns and conventional concrete metal decking.

The solution involved custom fabricating a 90-foot-long, 12-foot-tall beam, providing adequate clearance and maneuverability inside the dock area. The three-level parking structure is post-tension concrete. “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,” Barba said.

With the facility built on the site of a former sand quarry that later became a landfill, the civil and structural teams also had to address the site’s poor soil conditions. In addition, Mexico City and its surrounding suburbs are in an earthquake-prone region considered to be one of the highest seismic zones in the world. In 2017, a 7.1 magnitude earthquake devastated Mexico City, killing 369 people and damaging hundreds of buildings. And in 1985, an even larger 8.0 magnitude event took the lives of at least 5,000.

For the Costco project, the team worked with three different building types, each of which react differently to earthquakes. “Per code, we needed to add a seismic gap between each building type,” Barba said. “However, people still needed to be able to move between the buildings, so different types of seismic joints were used throughout the project.”

The use of a metal roof provided an extra value from a variety of perspectives.

“Today’s metal roofing is a strong choice for even the most vulnerable structure,” wrote Robert Zabcik, PE, in a recent white paper “Metal Roofing Performance and Wind Events.” He added, “It is well documented that buildings with secure, storm-resistant roofing materials like metal are less prone to damage from hurricanes and high-wind events. Metal roofing’s strength and durability provide safety and security for building owners and occupants alike.”

In the case of the metal building, it is important to consider that modern seismic design focuses on providing sufficient ductility in a structure to absorb and dissipate the massive energy produced by an earthquake. The International Building Code (IBC) serves as the basis for legislative building codes in Mexico and three steel moment frame systems are currently defined and permitted in the building code for resisting seismic lateral loads. Each has a different design rule that recognizes the amount of ductility that is anticipated, primarily based on the inelastic rotation that can be expected at the beam-column connections.

Mexico City’s local code doesn’t use the term Seismic Design Category (SDC) as in the United States’ ASCE 7, but it does use it as a reference to provide the required level of detailing. The current version of the Mexico City building code includes two design procedures with different levels of complexity, with the most complex rules aimed at large, important structures.

Note also that Seismic Code Evaluation–Mexico, created through an evaluation conducted by Jorge Gutiérrez, confirms that “the evaluated Norms may be considered state of the art but they are specifically meant for Mexico City, with very specific seismicity and site conditions. National authorities should be encouraged to issue a National Code with due consideration to the country’s seismicity and site conditions.”

Also note that the steel moment frames used in metal building systems differ from the prototype steel frames evaluated in the post-Northridge research program. Metal building system frames are optimized to match the strength required at any location on the frame, so the frame members are built up from welded plates that are commonly web-tapered, with the web thickness and flange size selected to optimize material along the length. The members are slender, with thinner flanges and webs than hot-rolled steel shapes that are typically used in multi-tiered conventional steel construction.

All structural systems defined in the building code for carrying seismic lateral loads are assigned design rules. These rules, including the maximum building height permitted, depend on the seismic design category which includes the seismic hazard at that location and inherent ductility that each system embodies.

Research by the Metal Building Manufacturers Association (MBMA) and industry partner American Iron and Steel Institute (AISI) evaluated the seismic behavior of full-scale metal building frames at the University of California, San Diego (UCSD) under the leadership of Dr. Chia-Ming Uang. Many observations were made in this study, including that metal building moment frames behave quite differently regarding ductile design philosophy. The metal building frames demonstrated little conventional ductility. A hot-rolled shape in a multi-tiered moment frame exhibits ductility by forming a plastic hinge at the location of highest stress, typically in a beam near the connection to a column. However in a metal building, the performance of slender, built-up tapered frame members is governed by buckling of the flange or web, or a combination of both, before a conventional plastic hinge can be achieved. Also, the location of the buckle in a metal building gable frame is closer to a web depth transition that can be several feet away from the column.

More recent research sponsored by MBMA and conducted by a team led by Dr. Benjamin Schafer of Johns Hopkins University used high fidelity finite element models of metal building frames that were able to reproduce the inelastic response noted here. This study confirmed that the current R-values used for seismic design of ordinary moment frames is appropriate for the metal building type frames.

In line with the community building aspect of the project to develop the rooftop for a variety of public recreational uses, the design team was also challenged to blend the buildings into the natural landscape.

In addition to siting some of the building underground, small berms, called Montañitas, were planted around the walls to conceal the building even more. These green screens offer benefits of natural ventilation and daylighting inside the facility. As the vegetation grows, the look of the building will change over time, resulting in a dynamic and ever-evolving feature.

Costco is also making an additional contribution to Parque La Mexicana in the form of water. The three different types of roofs and surfaces were custom engineered, using a combination of siphonic drainage and gravity drainage systems, to collect and transport stormwater runoff to the park to help sustain its fountains and lakes.

In addition, a wastewater treatment system creates greywater for reuse, thereby reducing the building’s water consumption in line with building regulations governing this drought-sensitive locale.

In gaining a primary location in the heart of Santa Fe—and in exchange for providing greenery and sports courts to the community, and funding for park maintenance—Costco’s arrangement with Parque La Mexicana is an ideal private-public partnership.

“Every hurdle we overcame and every milestone we reached added to a growing sense of achievement that culminated when we witnessed the impact of our work firsthand—the creation of a space that enriches lives and fosters a strong community spirit,” Barba said.

About the Author

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