Technical Aspects

Everyone usually takes architecture for granted without giving it any thought whatsoever. Most people have not a clue of just how much engineering and technical considerations go into a design like this. Even most builders are oblivious to all the details involved and get caught short once they get into something like this.

Running through the centers of the concrete columns are pressure-treated 6x6 posts. The posts are extended 2 feet above the columns to tie the whole beam structure together. Yet, no portion of the posts are visible.

In order to accommodate the 4-way canopy pattern, the stub-beams had to be doubled, as opposed to the single stub-beam configuration in the earlier single-billow versions. However there was no need to double the corbels too, so they were left single and centered beneath the stub-beam pairs.

Look closely at the photo to the right to see how the bottoms of these corbels were used to lock in the tops of the two halves of the split concrete columns.

Because the intersection of the stub-beams around each post involves a large flat area on the top of the beams with seams, a pyramidal cap (visible in the second photo at the top of this page) was placed atop each intersection to direct water beyond the seams.

A similar thing was done directly above the columns atop their capitals, but there the pyramidal shape is segregated into four quadrants because of its intersection with the corbels. This not only deflects rain water but eliminates bird-nesting areas atop the otherwise flat capital corners.

Minor lateral bracing was gained from tying the underlying corbels on each post to the arches above them. However, the major lateral bracing came from the arches themselves: once the four arches were fastened between the pairs of stub-beams with three 16-inch bolts at each end, the entire arbor became absolutely rigid, eliminating any potential for induced vibration from wind or earthquake.

And you don't see any of those 24 tie-in bolts anywhere.

Every corner in the canopy involved a compound miter between incident 4x4s, where the angles were different for each course. Making this more complex was the fact that each pair had compound angles that were the opposite of one another.

Every 4x4 in the canopy was glued together with a polyurethane caulk before fastening it with multiple 6-inch long Dacronite screws. That's so water from overnight dew and rain would not seep in-between the one hundred 4x4 connections and cause dryrot in the future.

Every flat surface was covered with 1/8-inch thick PVC capping to prevent parching and cracking of the wooden elements in the harsh California summer sun and dry-rot damage from perpetual over-night dew and the winter rains. Since there was a lot of canopy (600 sq ft) this was no small endeavor nor minor cost.

Every 4x4 was covered with white PVC plastic from the skylight out to the end of the brim. All the components of the skylight, both grid and rim, were also capped with PVC. Every stub-beam and those flat diagonal 2x4 umbrella-canopies atop the columns were all capped with PVC. So was every 6x12 beam from the pyramidal cap right out to their carved ends.

Do you see all those little tubular spacers between the 4x4s running from top to bottom of the canopy? For the exterior portion of the canopy they are necessary for keeping the exposed ends of the 4x4s in the long brims (a whopping 6 foot overhang) from diverging as the 4x4s weather over the years.

And for the inner portion they were an engineering necessity for distributing the weight of the skylight (300 lbs) evenly among all the otherwise unbalanced 4x4s.

The canopy of this arbor is a first in arbor engineering -- it's what I call an "integrated open canopy". There are no rafters going up the corners in the interior nor are there any rafters at the outer end of the brims. There's also no cross-bracing whatsoever. The weight of every component of the canopy is channeled cumulatively down to the four supporting columns by strategically placed connectors. See this page where I am offering simplified arbors employing this specific engineering concept; these arbors require no major beams or arches and are relatively inexpensive because of it: ArborsAlready.

Arbors like this are one of a kind. That means nobody can be trained ahead of time to fabricate and erect this kind of structure only once and get everything right on the first try. So I had no alternative but to build this arbor solo without any help. Yet with six earlier brim & billow structures under my toolbelt, this arbor was still a challenge for me.  

At each stage in the construction there was no possibility to correct anything that was already done because of the tight integration of all the components. There were many intermediate precautionary measures that had to be taken just to make sure it turned out plumb and level in the end.

And you don't see any seams at all in those split concrete columns. Most contractors I know would have just left the seams exposed because after all, they are "split columns, what do you expect?"

Keep in mind this is a residential project, not a commercial one where money is otherwise budgeted for bringing in all the skilled trades as needed.  If this were a commercial project, it would have cost around $150,000.