PLYWOOD IS THE KEY to the seismic integrity of our sanctuary building, according to three structural engineers I have spoken with. Although no plywood seems to have been employed in its original construction, a layer was added overhead in 2004 during a re-roofing effort, as discussed elsewhere in this blog.
As of this past week, I can say with confidence that the roof’s plywood is adequate to ensure that our building provides at least a minimum life safety level of performance in the event of a serious earthquake.
 |
| Cross-section in a cut at the walkway alongside the roof's barrel, showing 3 layers: plywood, sheathing, and rafter. (I added a typical nail for comparison—but placed it too low.) |
Last month, in the process of making structural repairs to the roof, enough plywood was exposed that I was able both to see the size of the nails employed and to measure the nailing pattern used. Those two factors affect the seismic integrity of the roof.
Below the jump, I show and discuss my findings, while looking ahead to similarly strengthening the walls.…
Findings
I found that the nails are 8d in size, and they are placed at 7” on center (on average).
 |
| One of 5 samples used to assess the nailing pattern |
I measured to the nearest ½ inch the nailing pattern in 5 sections of the roof; each sample contained 3 or 4 intervals between nails. Then I tabulated the results and computed an average.
Discussion
When I reported these findings to two structural engineers, one told me that 8d nails at 7" spacing is not uncommon. (The roofing crew had likewise assured me that 8d nails are indeed the normal length for securing plywood.)
More to the point, both engineers said not to worry; what we have is adequate. True, it would have been even better if the plywood nails were 10d and at 6” apart. That is a more conservative standard. So the structure is not as sturdy as it could have been (with more and longer nails), but it’s maybe 85% of the way there—and anyway much better than how it was originally built.
I had wanted assurance that our roof is adequate to maintain a minimum life safety level of performance BEFORE we put a silicone overlay on the roof, as planned for this summer. Because otherwise, we should instead be stripping off the roof membrane in order to re-nail the plywood—a much more expensive task. Now that I have assurance for the roof structurally speaking, we can proceed with the re-roofing project.
Looking ahead to strengthening the walls
(Regarding the sanctuary building’s short walls, there is one partial measure that would be relatively easy: plywood could be added to the attic space above the bimah, because the studs there are already exposed. At the other end of the building, the impending renovation of the main-floor restrooms will probably involve seismic strengthening of the wall between the sanctuary and the lobby. Similarly, if we eventually convert the mezzanine back into offices, that will likewise involve adding some plywood to one of its upper-level walls.
As for adding plywood to the long walls, again some plywood could readily be added to the attic space above the bimah; it’s a relatively small area but could only help. And the impending renovation of the main-floor restrooms could include some seismic strengthening of the south and north walls in those two areas.
Otherwise, strengthening the long walls is currently slated for 2038, which is when the exterior stucco is expected to reach the end of its useful lifetime and will need replacement.)
No comments:
Post a Comment