COMMENT OF THE DAY: A STRUCTURAL ENGINEER EXPLAINS PARKING GARAGE COLLAPSES “. . . Structural design is generally done by computing an anticipated load (how much will the stuff in the building weigh?), multiplying that by a safety factor, and then designing a structure with enough strength to support that “factored†load. Modern building codes also estimate the anticipated strength as less than it actually is too (for errors in materials/construction). Structures generally end up with at least 3 times as much strength as they need. That said, parking structures typically have the lowest factor of safety built into their design. They fail much more often than other building types because a) they’re so cheaply constructed, b) the loading is so much lighter than other types of structures that factoring doesn’t increase the loading by as many tons, and c) failure rarely results in loss of life.
@TL: You mentioned that there was a loud creek and then it failed? Any guess how long that creak lasted? 5 seconds, 3 minutes, an hour? I ask because concrete structures like this (ESPECIALLY pre-stressed concrete structures) are designed so that IF they fail, the rebar in them is the last thing to go, which will stretch and stretch and stretch gradually so that people have a chance to GTFO. The alternative is what’s called a brittle failure, where there’s just one loud pop and then bam; no warning at all.
. . . Engineers always talk about this stuff in terms of ‘strain’ and ‘yield.’ Strain is how much a material can deform (stretch or compress). When the strain gets too much, the structure ‘yields’ or permanently deforms. For a concrete structure, deform === collapse. The last thing to go before a concrete structure collapses is the reinforcing steel, which has a maximum strain of about 0.02 (2%) before yielding. That means if the clear span (beam-to-beam distance) is 30 feet, you can have a sag of 30 ft / 2 * 0.02 = 3.6 inches before it actually damages the structure. Parking garage widths are typically 64 feet, which can have ~7.5 inches of sag in the middle.” [Ornlu, commenting on A Top-Down View of Last Night’s Parking Garage Collapse at One Riverway] Illustration: Lulu