Replacing Giant Columns

Working the Corners

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The author's firm had been hired to replace the bases of six 100… Read More The author's firm had been hired to replace the bases of six 100+-year-old wood columns; engineer's inspections had indicated that the upper portions of the columns were sound, and there were other good reasons not to disconnect them from the roof structure entirely. As it turned out, much more was needed.

To temporarily support the porch roof while the columns were rep… Read More To temporarily support the porch roof while the columns were replaced, the carpenters built posts from 10-foot-long 2x4s and 2x6s.

To start, the assemblies were sheathed with 1/2-inch plywood. Fu… Read More To start, the assemblies were sheathed with 1/2-inch plywood. Full-length 2×4 strongbacks were added to the outside for rigidity.

Double-post assemblies were joined by top and bottom plates stac… Read More Double-post assemblies were joined by top and bottom plates stacked two high in order to support the roof while work was being performed on the columns.

The original plan was to replace only the column bases. Accordin… Read More The original plan was to replace only the column bases. Accordingly, the crew installed structural steel clamp rings around the shafts and shifted the weight from the pedestals to the concrete porch using 20-ton hydraulic jacks.

Rings of bending plywood served to guide the cutting of the wood… Read More Rings of bending plywood served to guide the cutting of the wood columns, which was done with reciprocating saws and circular saws.

When the bases were removed, it was discovered that the interio… Read More When the bases were removed, it was discovered that the interiors were delaminating and that the columns would have to be replaced in their entirety.

After cutting away the bases, the crew placed cribbing under the… Read More After cutting away the bases, the crew placed cribbing under the shafts while awaiting the delivery of new columns.

Sheets of 3/4-inch plywood protected the lawn from the lift that… Read More Sheets of 3/4-inch plywood protected the lawn from the lift that was used to transport and place the new columns..

The lift was fitted with a site-made cradle for moving the colum… Read More The lift was fitted with a site-made cradle for moving the columns.

Moving a column with the lift

Because of all the scribed trim and intersecting rooflines, the columns at each end took a lot longer to install than the middle four columns. We knew we would have to move these columns back and forth to achieve a tight fit, so we first built staging platforms in front of and in the same plane as the corner plinths. We covered the staging platforms and the plinths with plastic laminate, initially thinking this would protect the limestone plinth from the wood and steel dowels we placed under the column to enable it to roll back and forth.

After a couple of moves, however, we realized that there was very little friction between the polycarbonate ring on the bottom of the column and the plastic laminate, and that two men could actually slide the column back and forth without using the dowel rollers. Suffice it to say, it took many moves back and forth to properly align and scribe the new columns to the existing moldings and rooflines, including the large built-up entablatures that intersected the columns at right angles, several porch railings, and some radiused copper flashings protecting the rubber roof membranes over the flanking porches.

We also had to carefully cut beam pockets to receive the ends of structural girders from the abutting porches. Plywood jigs helped us align the flutes with the previously scribed moldings. We actually practiced the cuts on an old column first, which allowed us to rehearse the entire process before trying it on one of the new columns.

Structural plug. As mentioned above, one of the corner columns housed a structural post that supported the end of a porch girder. The girder had been added during a previous remodel, to support the extra load created when the rooftop deck was covered with heavy masonry pavers. The builder had inserted the post by simply cutting a wedge out of the column, then patching it back in.

We wanted to do something a little cleaner, so instead we built a structural “plug” that would be inserted into the column before we set it in position. We framed the plug like a small round room, with radiused top and bottom plates and studs in between. We used 3/4-inch pressure-treated plywood for the plates, and framed in a 5-1/2-inch by 5-1/2-inch Parallam post to support the girder. We also had to allow for the taper of the column, meaning the top of the plug was slightly smaller than the bottom.

It was more than a little tricky to position the post in its exact orientation inside the column. First we had to lift the column into place “empty” to test fit it — to the end of the beam, the entablature moldings, and the porch railing. Once we were satisfied, we had to move the column back onto a set of horses so we could install the plug, which we did by pushing the snug-fitting assembly inside and securing it with LedgerLok screws driven from the outside.

Then the lift operator had to once again stand up the column with straps, attach it to the cradle, and move it back into place. When the column was close, we manually slid it into place. We had to twist it as it went so that the projecting beam landed inside the beam pocket (which was only 1/2 inch wider than the beam) and so that the flutes aligned correctly with the entablature and the porch rails. It was a challenge we don’t care to repeat.

Once the column was home, we secured it with the Simpson brackets top and bottom, then used long Spax screws to secure the porch beam to the post inside, tying everything together solidly.