Skip links

Structural Investigation: Physical Continuity

There’s an aspect of losing at cracks that bears some discussion, and that’s the continuity of physical objects. In soil mechanics and foundation design, that continuity is usually referred to as “strain compatibility” which is the term I prefer but which is nearly opaque to non-engineers. (“Strain” is engineering-speak for deformation under load; “compatibility” is being used in the sense of two or more things fitting together.) The shortest version I can think of, which I’ll explain below, is that a change in the geometry of an object (such as a masonry wall) has to be accounted for across the entire object, because there’s no other way for real-life material to accommodate the change. I wrote about this before (here), but I want to give some specific examples.

The crack in the photo above has its bottom end a bit above the floor; its top (not visible in the shot) ends below the ceiling. In other words, the crack is less than one story tall. Cracks form because there is tension perpendicular to the line of the cracks, and masonry or a similar material (weak in tension and brittle) fails from the tensile stress. That does not necessarily mean there is tension overall (in this case, there may or may not be tension horizontally, perpendicular to that crack) because shear causes tension at an angle to its direction. But that’s not actually today’s topic. For today, what do we know because of the length of the crack and continuity?

We expect cracks to spread, but that’s limited here. if the sides of the crack were to pull apart, something has to happen at the top and the bottom: the crack can extend up and down, the cracks can turn and run horizontally or diagonally at the top and bottom, or the material at the top and bottom has to bend. All three possibilities allow the sides to pull apart. There’s no sign of any of the three taking place, which strongly suggests that the sides are not moving apart. That, in turn, limits the possible causes of the crack.

Here’s a more complicated example:

The green arrow mark discontinuous cracks, but they are connected by the voids of the window openings. The line of cracks ends abruptly at the base of the parapet, but the parapet brick is new. So this could have moved freely once, before the new (uncracked) parapet was put in place, and now cannot. However old the new parapet is gives us the most recent possible date for the cracks forming.

This very simple, if conceptually annoying, technique has a lot of uses. If a floor is sagging but the partitions above and below show no sign of cracking or gaps, then the sag is old enough that the partitions have been repaired or the sag was so gradual that they moved with it. If a window frame is out of square but the window moves freely, then the movement is older than the current window sash. If a horizontal seam is split in a gypsum-board partition but there’s no sign of movement at either end of the split or out-of-plane movement of the partition, then it’s likely not structural damage but rather local impact or poor workmanship.

Thanks to Tim Michiels and Ellen Key for the photos.