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I have a 10-year-old cedar pergola (see picture) and I just discovered some moisture and carpenter ant damage in the base of posts. The damage had been hidden beneath decorative base caps. I scraped out the damaged wood and I'm wondering if the support provided by the remaining wood can still be considered structurally sound.

The posts are 5 3/4" square. For 3 of the posts, the lost wood is probably about 1/4" around, which would leave about 5 1/4" square of good wood. That seems like it would leave much of the posts' structural integrity intact.

One post, however is quite a bit worse (see picture), with a loss of about 3/4" to 1" in spots. Looking at the standoff post base it is mounted on, though, it still fills a lot of the area of that base. It looks like some of the post overhung the base and so I'm not sure that all of the lost wood was carrying the load. So, despite the damage, I think perhaps enough wood remains to carry the load, especially in combination with the other less damaged posts.

For brevity, I'm just showing a picture of this worst post. I know it is difficult to evaluate from just a picture, but with the understanding that the 3 other posts are much less damaged, does it look like the damage to this post has compromised the pergola such that it is unsafe now and needs to come down (or have the post replaced)? Or, does it look like there is enough wood left and that the post is still sound?

The pergola

Worst post view 1 Worst post view 2

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The post may not fail in gravity load but could fail in the event (wind storm, earthquake, kid shaking) that causes uplift and bending. Note that the post may have/soon lost its base connection.

We can estimate the reduction in bending strength using this relationship, R = (b'/b)^4. In which b = width of the post, b' = reduced width, and R = reduced strength of the post compared to the original for bending due to lateral load.

Post with minor damage, b = 5.75", b' = 5.25", R = (5.25/5.75)^4 = 0.7 (70% of the original strength)

Post with severe damage, b' = 4", R = (4/5.75)^4 = 0.23 (23% of the original strength)

I would not bid my luck but to replace the damaged posts ASAP.

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    Looks to me like the resistance of those rusty screws against being torn out of that rotten wood would be less than 23% of the original strength. Hopefully you can saw off and replace just the bottoms of the posts with appropriate fasteners. Maybe build solid pedestals out of composite material designed to look like the fake ones there now.
    – jay613
    Commented Jun 18, 2021 at 17:50

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