STRUCTURAL ENGINEERS prepared one of the modules in the 2003 Historic Structure Report (HSR). Among other things, it assessed the sanctuary’s seismic integrity and offered a plan for improvement.
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| (As usual, click/tap on an image to enlarge it.) |
Below the jump, this post summarizes the seismic portion of the structural module, in light of more recent developments, while ending with a recommended plan for the future. That is, it compiles and updates my various prior posts about seismic issues.
2003 Seismic Strengthening Recommendations
The HSR recommended the following steps:
- add plywood to the roof;
- add plywood to the exterior walls;
- add bolts to attach the wood frame to the foundation;
- add hold-down hardware to prevent over-turning;
- inspect and repair tie-rod connections as appropriate.
I will now examine each of those elements, in turn.
Roof Construction
An unsettling opinion was expressed in the HSR, namely that our sanctuary building would be dangerous in an earthquake, because the roof lacked a stabilizing layer of plywood.
“The Temple structure … appears inadequate to maintain a minimum life safety level of performance. The wood sheathing diaphragm and connections to the walls are not likely to have adequate capacity to distribute seismic loads into the vertical shear walls.… Recommended mitigation:… It is important to sheath the entire roof with a thin layer of plywood over the wood sheathing before re-roofing.”
That sounds bad. However, in 2021, Rich Radenbaugh, a roofer, cited evidence that the desired plywood was already in place, simply by looking up from inside an attic:
“It looks like you already have plywood installed above [the sheathing]. … In between the cracks, you would see black felt if there were no plywood. [Yet] it is light tan (the color of plywood). Also, the long nails coming through the 1x4 wood are not roofing felt nails. These two things have me thinking there is already plywood up there. We won’t know 100% until the roof is off. We can also cut out a small patch of roofing to be sure.”
I can’t account for why the 2003 engineering report seemed so sure that there was no plywood. (It doesn’t appear that the roof has been replaced since then, such that plywood could have been added later.)
Rich’s inference was confirmed in August 2022, when we cut access holes in our roof — in the troughs along the long walls — as part of a more general exploratory effort. The workers encountered plywood.
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| Pulling up nails in the plywood, after having removed the bitumen membrane |
Furthermore, in order to confirm that this layer of plywood extended throughout the roof (not only along the side troughs), the roofer kindly cut out a 4"-square patch of membrane from the barrel vault, revealing the plywood there, as well.
Several weeks later, another structural engineer informed me that we should have observed the nail size and nailing density pattern, which determines whether the plywood is actually able to distribute seismic forces to the building walls. (Minimally, they should be 10d (i.e., 3" long) nails at 6" on center.) But the roofers could no longer recall what they encountered.
Whenever the roof membrane is next covered over (or replaced), a section of plywood should be exposed (which needs to be done anyway for termite treatment, as noted elsewhere on this blog), to observe the nails and confirm the seismic integrity. Alternatively, perhaps the nailing data can be ascertained from below, in the attic, by having someone familiar with roofing inspect it, more carefully than Rich Radenbaugh did in his initial note.
For what it’s worth, our roofing contractor, upon seeing the construction details that were revealed in the process of opening the access holes, volunteered his opinion that this roof was very well designed and built, and structurally speaking it appeared to be solid overall.
All told, it seems likely that our roof is adequate to maintain a minimum life safety level of performance. However, this assessment should be confirmed by further study.
Wall Deflection
The 2003 Historic Structure Report noted the walls are not properly braced for even a moderate earthquake. The only thing that keeps them from deflecting from side to side (laterally) is the stucco — which is brittle. Therefore a sizeable earthquake will result in cracked walls.
The report offered the following guidance:
The structure can be easily upgraded seismically. [The most] significant [step] would be to remove all the stucco on the exterior or plaster on the interior and install ½ inch plywood, repair all the deteriorated sills and add ½ inch diameter bolts at 24 inches on center.… During this work, the condition of wood studs and tie rod connections can be inspected and repaired as appropriate. (p. IE-106)
However, we have not had the financial or managerial wherewithal to undertake such a large project. Yet we will have a natural opportunity to do so eventually. For if stucco is well maintained, it is said to have a useful life of 75–100 years; after that, deterioration calls for replacement. Our stucco is 75 years old and has not been well maintained. In other words, we can expect that we will need to replace the stucco maybe a dozen years from now. (Presumably the need will first become apparent on the south wall, which is the most exposed to the sun. As it happens, some small patches of stucco did need to be replaced this summer, while most of the wall is holding up for the time being.)
Meanwhile, according to our waterproofing consultant, the silicone-based coating system now being installed on the south wall’s surface will have a 15-year lifetime if properly maintained.
By 12–15 years from now, the Mishkon leadership should be prepared to replace the exterior cement plaster (stucco), at least on the south wall. At that point, the long walls should be braced by installing plywood for seismic strengthening—after the stucco is removed and before it is replaced. Also the anchoring of the frame wall to the foundation wall should be checked and improved as needed.
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| Illustration of plywood that can be installed |
Wall Anchoring
In 2003, the structural engineers had no way of knowing whether or not the walls are anchored to the foundation. They assumed the worst (no anchoring), while making some “high-priority” recommendations that included: “Undertake limited cutting to [look inside the walls and] investigate the design and condition of structural framing.”
At the end of Nov. 2022, we made such test cuts, under the supervision of a structural engineer. We learned, among other things, that the walls are indeed anchored, at least to some extent. Both the load-bearing posts and the sill plate are affixed to the foundation with lag bolts.
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| Anchor bracket with lag bolts at base of post |
Given the prevailing practice when this building was constructed, we had not expected to see anchor bolts. It was a pleasant surprise. (However, we have not determined the size and spacing of those bolts, to ascertain whether they are considered adequate to hold our wall in place during an earthquake.)
Hold-down hardware
If I understand correctly, hold-down hardware is designed to resist twisting forces that could jolt the frame walls off of their foundation. That hardware looks something like this:
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| Illustration of hold-down hardware |
I do not know when it would be most efficient to install such hardware. At any rate, an engineering study is needed first, to decide what kind and where it should go (see below).
Tie-rod connections
One tie-rod connection on the north side shows evidence of having gotten damp from former rainwater intrusion. This was discussed in a prior blog post HERE.
A Key Step: Consultation
The next step seems to be to engage a structural engineer to specify the seismic measures needed (e.g., how the plywood should be attached to the wall), so that we can take those specs to qualified contractors to bid upon. Until then, we don’t know what the seismic strengthening costs would be. In 2022, I did get a formal proposal for an engineer simply to draw up those specs for comprehensive seismic strengthening of the sanctuary: $25K.
A Seismic Strengthening Plan
My recommended plan includes the following steps:
- Within the next two years, inspect the tie-rod connection that was the site of past water intrusion, repairing it as needed.
- Within the next two years, as a new roof membrane is being installed, peel away enough of the existing membrane to confirm that the nail type and nailing pattern of the roof plywood is adequate;
- In 12–14 years, engage an engineer to draw up specs for reinforcement.
- In 15 years, as worn-out stucco is being replaced, add plywood to the exterior walls and confirm that the walls’ sill plate is adequately anchored to the foundation, while perhaps adding hold-down hardware as needed, and checking the tie-rod connections.
The above steps have been to our online planning grid. This plan can guide our synagogue leadership to start setting aside funds for the tasks involved.
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This post is dedicated to Jim Farasatpour, in honor of his devotion to our synagogue’s physical facilities.
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