At historic zoos, built at the beginning of this century or earlier, spectacular brick and natural stone buildings command notice. As when designing other public institutions, their architects selected masonry for its beauty and durability, often enlivening designs with exquisite ornamental detail. But since the mid-20th century, the philosophy of zoos has changed from exhibiting the widest possible range of animals in cages to breeding selected species in simulated natural habitats. As zoos have become animal-centered rather than people-centered, the role of zoo architecture has changed, as so has the role of masonry. Today, designers favor concrete block, often hiding it with foliage or camouflaging it with natural colors and textures or gunite. A zoo's priority is keeping the public safe from the animals. Concrete masonry units often are the first choice for zoos because of the material's strength and durability. Gorillas, orangutans, chimpanzees, elephants, giraffes, and other large animals each have their own design requirements based on their weight and temperament. The American Zoological Association makes its animal data available to designers, and some architectural firms have developed their own databases to calculate animal loadings. Conservative, empirical design prevails for masonry. For example, the elephant breeding facility at the Jacksonville zoo has 20-foot-high walls of 12-inch split-face block, with two #5 rebar in each cell. Reinforced horizontally at 8 inches on center, the walls are fully grouted with 4000-psi grout. To design this facility, the structural engineer pictured a pickup truck running into the wall, which approximated what a 3,000-pound elephant could do to it. To protect the animals from contamination and expedite cleaning, concrete block walls in contact with animals are always epoxy-coated. All wall coatings must meet USDA toxicity standards. In animal-holding areas, flush mortar joints are preferred. Because animals spend so much time in a limited space, walls need to be smooth and slick for cleaning and sensitive animal skin.