Adaptive reuse of historic structures is an important and growing sector of restoration engineering. Historic preservation, as well as aesthetic, environmental, and financial considerations, drive this niche in engineering and architecture. Adaptive reuse, as a distinct specialty, demands that the
restoration engineeringspecialist have knowledge of a fascinating catalog of outmoded, antiquated and forgotten structural systems which he or she will likely encounter retrofitting, renovating and remediating historic structures. In this article we will look at some of the most prevalent of these systems, some of their specifics and the manner in which a restoration engineering specialist must account for them structurally.
The rapid rise in adaptive reuse of historic buildings presents significant challenges to the restoration engineering specialist. First and foremost is the fact that drawings for many of these structures are either incomplete or missing altogether. This forces the engineer to draw on a variety of resources to determine the nature and capacity of the existing structural system. If industry resources are able to substantiate the undocumented system, than work can proceed to determine if the capacities and values meet current code needed for the intended new configuration, or the strengthening or remediation necessary to accomplish it. If such resources cannot be found, than the restoration engineering specialist must employ non-destructive testing or exploratory demolition to determine the character of the buildings structural support.
Prior to the 19th century, masonry and timber were the predominant materials for large structures. Primitive cement was used as far back as the Greek and Roman era as well as in ancient Egypt and China, though with varying success. Iron and steel production matured in the mid-1800s, and over the course of the next century they became the predominant structural materials. At the end of the 19th century, the introduction and refinement of Portland cement and reinforced concrete ushered in a new era of building material. Each of these building materials enabled or required vastly different structural systems, as well as different individual components and connection schemes.
The nineteenth century saw the evolution of construction from traditional methods to industrialized ones. The first changes came in the most fundamental arena, columns and beams. With the introduction of cast iron in the 1830s, followed by wrought iron in the 1850s, wood and stone were rapidly replaced as framing elements. Iron and steel allowed for much faster, stronger, taller buildings, with the skeleton system of structural support, using curtain wall facades, developing, replacing the bearing walls of traditional systems. However, the differences in the quality and type of steel used are often widely varying in 19th century buildings, something of distinct concern to the restoration engineering specialist. The first of the 19th century, wrought iron beams were predominantly used, but in many irregular shapes, such as bulb tees. Standardization to the modern I beam was decades away. Thus any changes in loading in an adaptive reuse project may necessitate the chemical analysis of the beam material. High sulphur/phosphorous content wrought iron beams were used in mid-century buildings and are brittle and unweldable. Similarly, the physical forms of antiquated structural members can cause unexpected results when new loads are applied in renovation. Combination girders do not necessarily distribute the load equally across all individual members, as might be supposed by the restoration engineering specialist. Phoenix columns, built-up box columns and hollow-section cast-iron columns were not designed with modern understanding of the gravity moments and axial forces at beam connections, significantly lowering their load capacity. Connections between structural framing members can also be tricky. Before welding matured, splices and beam connections to cast-iron columns used bolts that were inadequate for the shear. This was also seen in bolts for floor beams. Early hung lintels were often inadequately braced against movement affecting spandrel beams and columns.
This wholesale change in framing was not replicated in floor systems. There was a much slower transition, which saw hybrid forms of traditional floor systems begin to appear to adapt to the new steel framing systems, and begin incorporating the modern metal and concrete materials that were being developed. One of the earliest masonry floor/ceiling systems, prevalent in the late 19th century, were the masonry and tile arch systems which predated slab systems. Brick arches were holdovers from traditional architecture: Extremely strong and durable, but also heavy and difficult to construct. The replacement of brick with tile made for a much better floor system: lighter and much quicker to construct. But they retained the inherent drawbacks of needing skilled labor, constrained geometries and strength out-of-proportion to what was needed. Thus, new methods evolved in the late 19th and early 20th century. For the restoration engineering specialist, the main interest here is in the innumerable variations.
The multiplicity of forms and materials, some no longer considered to have structural capacity, and some used on the basis of experience, with little ability to analytically validate the load carrying ability, make the restoration engineers ability to modify loads a fraught process. There are only three basic ways to carry a load across a horizontal span: arches, catenaries and beams. Traditional construction used unit masonry arches and wooden beams. In the industrial ferment of the 19th century, curved arches were replaced by flat arches, catenary beams and eventually, concrete slabs (beams).
The Roebling floor system, developed in 1892, was one of the transitional phases between arched floors and beam flooring systems. It was a concrete arch, built without wooden forms, on a wire mesh arched between two steel beams, with concrete and rubble filling the space and producing a flat floor. Mesh was clipped to the bottom of the beams for fireproofing with a plaster ceiling. Eventually, the wire mesh that formed the arch was flattened into a reinforced slab, with tension replacing compression as the main structural loading vector.
One way and two way unit masonry and tile slab systems were used widely in the United States in the early part of the 20th century. In one way systems, tiles were placed on top of wooden forms, either connected in rows for one-way joists, or individually creating two-way joists. Clay, gypsum, concrete or slag block tiles were common. Reinforcing was situated in the joist channels, sometimes a full mesh, other times steel rods, and a thin slab was poured. The Natcoflor one way system dispensed with the top slab and only poured a concrete rib between the tile, whose corrugated sides were to transfer stress to the concrete. The tiles were usually 12 square, joists 4 6 and slabs anywhere for 1 -3 in thickness. The Schuster Floor system is an example of a two-way, early waffle slab system. It used up to 20 tiles without a top slab, or 30 rib spacings with a 1-2 top slab.
For the restoration engineering specialist, the dizzying array of floor systems from the late 19th and early 20th century can present tremendous challenges. Trying to substantiate the structural characteristics of the many tiles used is almost impossible. Most of these antiquated systems slabs are more fragile than modern materials, and determining exactly what floor system is present can be almost impossible task, as intrusive probes can sometimes cause significant weakening of load-carrying ability. Past performance is probably the best key in such cases. Examining the floor/slab from below can often provide conclusive evidence of sag or water penetration, particularly with period plaster ceilings.
Understanding antiquated structural systems and materials is essential for a restoration engineering specialist. Adaptive reuse entails many hybrid loading schemes, and interfaces between modern structural elements and antiquated systems, that present unique challenges in analysis, design, and execution.
