Structural Logic In An Older Form

From an 1880s building in Tribeca: the double beam supporting the front facade (sidewalk vault on the left, cellar on the right) where it meets the side wall at a pier. That’s a granite block directly below the beams, which can be a little hard to tell because of the paint on all the masonry. It’s there because it was required by code – section 492 of the 1882 New York Building Code stated that where a beam was supported on masonry “a stone or cast-iron template must be built into the walls, said templates to be eight inches wide, in twelve inch walls, and in all walls of greater thickness to be twelve inches wide, and such templates, if of stone, shall not be in any case less than two and one-half inches in thickness, and no template shall be less than twelve inches long.”

Pretty straightforward, but that’s not the only reason it’s there. The code did not provide allowable stresses for materials, but referred to the Civil Engineer’s Pocket-Book by John Trautwine, a common engineering reference of the era. The 1883 edition of Trautwine gives the crushing compressive stress for granite as 300 tons per square foot (or 4167 pounds per square inch) minimum, with the maximum as high as four times that. The crushing stress for the middle range of brick (“Brickwork, good, in cement”) is minimum 30 tons per square foot or 416 psi. The safety factor used was four, so the allowable stress in brick was about 100 psi and in granite was about 1000 psi.

Using modern code, the allowable compressive stress in the brick pier is something like 200 pounds per square inch, and the allowable stress in the granite is maybe ten times that. Note that the old brick is fairly weak compared to modern brick.

The pier has to be able to take the load of the beams, so the pier has to be big enough in cross section for the actual compression to be below the allowable. But the granite serves an important purpose: it takes the loads that are concentrated below the webs of the beams and spreads them across the width of the pier. That takes a material considerably stronger and tougher than the brick.

So what we’ve got here is a system using three materials – wrought iron for the beams, granite for the bearing block, and brick for the pier – balancing each according to their relative material properties and cost efficiency. Sure sounds like engineering design to me.