15 Bytes | 2012 | Architecture & Design

Stone Timber and Steel: The Logic of Structure

by John Griswold


Standing on the tee box of a local golf course I saw a peculiar sight. A nearby house sported a second-storey deck, clearly a cantilever as there were no support posts, and rising from its corners to carry the safety rail were two stone columns.|1| The carpenter side of my brain kicked in: “Those must be steel or timber posts clad in faux stone, they’re far too skinny to pass the Drunken Allen Test.” My partner Jeff and I have jokingly applied this rule for decades while repairing yet another failed deck rail: got to make the connections strong enough that Allen could lurch his 230-pound frame into our product without going through it like a Hollywood stuntman.

It was only later that I was struck by the inherent silliness of placing a stone structure, any stone structure, floating by itself on the second- story. Since frame construction has come to dominate housing and light commercial building, materials once used as structural components have been reduced to decorations, applied as siding or embellishments with less and less thought given to their original structural roles. As I started looking for these architectural non sequiturs they popped out like spring mushrooms: second-storey stone dormers,|2| massive timber columns supporting tiny entry porch roofs; constructs no self-respecting mason or master carpenter would have considered sixty years ago.

Builders in the Craftsman era reveled in their use of materials, cobblestone foundations and broad tapered porch newels, timber trusses and deep wooden beams;|3| they wanted to display their structural thought process. As aesthetically pleasing as this style can be, it’s also pretty spendy. And as the nail gun replaced the well-swung hammer, master carpenters gradually were replaced by nail-gun operators. The structural rules of thumb once passed down from master to apprentice faded as the masters retired, and we are left now with timber arches spanning openings in stone walls.

Why shouldn’t you span an arch with timber? Why doesn’t a 6-inch column look right in the middle of a 2-foot stone base? The answer lies in the physical properties of the materials.

An overview: Stone is heavy, resistant to rot and oxidation, weak in tension and strong in compression. Timber is light, vulnerable to rot, and possesses considerable tensile strength. Steel is heavy, vulnerable to oxidation, and hugely strong both in tension and compression.

Obviously, stone is an ideal material for foundations. The more you load it, the stronger it gets, it won’t rot, and the bugs won’t eat it. As the Plains Indians said, “Only the stones last forever.” Its low tensile strength makes it less useful as a beam to span openings, but masons long ago devised the masonry arch as a practical and visually pleasing workaround. This assembly redirects loads down and sideways along the compressive paths around the opening. This loading places all the involved stones under compression and can actually increase their individual strength.

Timber’s greater tensile strength allows it to perform well as a true beam, transferring loads from the center of a span to point loads at the sides of the opening. Picture a timber as the proverbial bundle of sticks, gaining strength through the multiplication of many relatively weak members. The “members” here start out as thousands of cellulose tubes in the cambium layer, the living outer sheath that conveys water and nutrients to the tree’s crown. Each year the innermost layer dies and solidifies while the tree adds a new layer on the outside. We count these layers as rings on the stump of a felled tree.

This cross-section of the tree, which shows the “grain” as rings, is the hardest and is fairly resistant to crushing; the sides of the trunk, which become the sides of the timber, are far softer. When you lay a timber horizontally to form a beam you concentrate the load on small sections of these softer sides, which will crush if not sufficiently large in area. While timber is well suited to carry the vertical load of a living crown, it is also designed to flex under lateral loads. Trees that are too rigid break when the wind gets high. Timber columns must either be thick enough to resist this natural tendency to bend or need proper bracing. A single 2X4 can carry thousands of pounds, so long as it is prevented from bending under the load.

Steel can be used for almost any structural need, so long as it is kept out of the weather or coated to prevent rust. It is often used in modern structures for columns, beams, roofing and siding, but not commonly used in the sort of traditional style buildings that so often violate proper use of stone and timber. We see it most often in the gusset plates connecting larger timber members.

So the basic rules of thumb? Stone belongs on the ground or on top of other stone. Masonry chimney “stacks” can obviously raise two or three storeys so long as the footprint at the bottom is large enough to carry the apparent weight. Stone or masonry columns must start with large enough footprints to carry their apparent weight without “post holing” down into the earth. While they can taper, they need to maintain enough cross section to resist lateral loads and thrusts, to which their low tensile strength and rigid mortar joints are vulnerable.

Wood columns that rest on masonry columns should have the largest practical footprint in order to place all the base stones or bricks under compression thus increasing their strength, both individually and as an assembly. Timber columns should be thick enough to resist bending under their loads but not so thick that they suggest a wasteful carpenter. Wooden beams should appear to be deep and broad enough to resist deflection (bending) under their loads and rest on large enough beam pockets or columns that their bearing surfaces don’t crush over time.

Steel gusset plates are so much harder and stronger than the timber members that they connect that their main structural consideration lies in the placement of the bolts that tie them to the timbers. Any bolt hole drilled through wood severs the fibers, or “grain.” Loads placed on these cut fibers can tear them out, particularly close to the end of the timber. The old rule of thumb was to place them at least seven nail or bolt diameters from the end of the wooden member.

The sum of these rules is nicely demonstrated in the typical two-storey masonry house of the turn of the nineteenth century, the kind commonly found in Salt Lake’s older neighborhoods, like the Avenues. Rising out of the ground on a large block sandstone foundation, the brick walls extend to the eave lines of its roof slopes.|4| The roof and gable end wood structures place the brick walls below under compression, lending strength to the upper courses which otherwise would be unloaded and fragile. The bottoms of window openings are dressed with massy stone sills (to place the brick courses beneath under compression); the masons show off their artistry in the brick arches that span the window openings at the top.|5| The front porch roof is supported by wide wood columns, which terminate either on broader masonry columns or transfer their load through the floor framing to the sandstone foundation below. The beams spanning between columns appear to be at least 12 inches tall and 6 inches deep.

Other examples of the authentic use of traditional materials are on display in the Westminster Heights neighborhood above Westminster College. There, several Craftsman-style homes built in the 1920’s and ‘30s recapitulate the bungalow style, with overhanging roofs that cover the front porch, and deep eaves on the gable end sides of the building to add shading and weather protection for the shingle siding.|6| These houses most commonly are frame construction resting on cobblestone foundations, with robust timber brackets to support the barge rafters of the deep eaves and broad tapering cobblestone or wood columns that rise to the beams supporting the porch overhangs.|7| Often a tapered cobblestone chimney stack will embellish one gable end; the final impression is a composition in functional structures and materials.

The contrast between these authentic buildings |8| and the new, faux traditional, business that I pass every day on my way to work strikes me every time. This building is stone clad, but the stone siding covers the gable end to its peak. Were the stones actually structural, those running up the rake and at the apex would be relatively unloaded, not locked by weight into the structure and easily loosened. Two-piece timber arches span the windows, joined at the peak by small steel gusset plates. To cut these curved shapes out of straight timber would have cut every fiber of grain, and placing weight on them would have split the cut fibers apart, assuming that the bolts attaching the gusset plates at the top of the arches didn’t pull through the end grain first.

And just to confirm that the bold elements applied to the building were false, the 10-foot-wide car passage through one stone wall is spanned by what looks like a deep timber arch. The curve cut in this arch is deep enough so that in the center, where bending forces would be greatest, the timber has tapered to a mere 5 or 6 inches. The ends of the “beam” are, in contrast, so massive that they could support a timber spanning twice the opening, were the beam of even depth.

This all may seem like quibbling, the stones and timbers used are attractive enough, if you ignore the lies they are telling about their functions. But the structures are built to last, and for the next fifty years or so no competent builder or architect will glance at them without at least a snicker.

 

1 reply »

  1. I could add to this list of sins against visual logic, but I’ll limit myself to one: the arch over the two- or three-car garage, bricks or stone spanning thirty or forty feet, with a rise in the center of six inches. Good luck getting that to stand up IF IT WERE REAL. The lower a dome or arch, the less likely it is to work. The closer the sides are to vertical, the better. Compromise is the difference between Hagia Sophia in Constantinople collapsing a handful of years after construction, then its being rebuilt a few feet taller, and standing for centuries.
    Thanks for calling attention to the sad fact: most of today’s construction is an implausible veneer glued to the outside of a dull frame or pole construction. That’s not, in itself, an impossible situation, but for it to work, the illusionist must know and observe the rules of how real materials work.

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