There’s a lot of talk about structural integrity concerning houses and buildings during an earthquake–for good reason–but what about minor or superficial damage to a home’s interior? Without discounting the importance of a building’s structural earthquake resistance, let’s talk a little bit about earthquake resistance for the aesthetic portion of the home interior; in particular, wall and ceiling finishes.
We understand the why, and we want to know the how. But for this we must first understand the basic design and construction of your home interior. Simply put, a home’s interior is a combination of walls, floors, and ceilings. If your house was built to code, any moderate earthquake generally won’t affect your floors (in the case of wooden flooring or a comparable flooring finish).
But because walls and ceilings are generally constructed with drywall–which is a non-structural, butt-joint-construction pre-finish layer–they are especially weak at these joints. And as you can probably guess, weak or moving drywall joints cause breaks, cracks, and tears in wall and ceiling finishes such as wallpaper and paint.
The 2 primary causes of cracked drywall joints are:
1. Structures with high lateral displacement potential
2. Inadequate drywall framing
Point 1 applies to structures that absorb seismic energy and dissipate it through controlled elastic movement of the structure. Such structures are generally made of steel or wood, as opposed to concrete or masonry structures which absorb the same energy with minimal lateral movement. Without significant steel reinforcement, the latter structural types have the tendency to fail in tension (break apart) during intense seismic activity due to their brittle nature.
Point 2 applies to interior drywall framing that is subpar or inferior, in that it doesn’t adequately support the drywall — in particular, the drywall joints. Aside from the simple fact that it makes the drywall needlessly weak to impact and natural contact with occupants, insufficient framing equals insufficient drywall fasteners, which by this fact alone compromises the shear strength of the wall.
There are some structures that take drywall into account in terms of the overall strength of the structure. Such structures are generally 2 by 4 houses or similar diaphragm structures that rely solely on shear walls, as opposed to diagonal bracing, to support the structure in shear. Interior drywall therefore, is treated the same as the exterior structural plywood, in that fastener pitch is very small and stringent — 100 mm around the perimeter, and 200 mm down the center stud.
Drywall joints in these designs generally hold up better during seismic activity when compared with structures where interior drywall is purely cosmetic and therefore does not have the same level of framing and fastener support. In the latter case, it is advised to apply glue to all drywall joints regardless of the framing. Be sure to wipe any excess glue that oozes out as it will interfere with the joint-puttying and will take the drywall housing-paper with it when removal is attempted.
Other Important Points to Note:
– When drywalling, overlap natural areas or points of structural movement, such as the beams between the first and second floor or floor levels in split-level structures.
– Be generous with glue and fasteners around all windows and door frames, as these are points of high stress concentration.
– Joint the paper-enclosed sides of the drywall as much as possible, as the open gypsum ends will simply break apart under high stress even if glued due to its ‘crumbly’ nature.
– Stagger drywall end-joints as much as possible to limit the intersecting of joints and crack propagation.
Although drywall is generally a superficial layer that simply supports the interior surface finish, it’s important to ensure adequate framing and to glue all joints to prevent cracking over time or following seismic activity. Paying extra attention to ensure framing is strong and adequate–especially corners wrapping around window and door frames or similar points of stress concentration–goes a long way in preserving the integrity of your interior finish.