"I have limited these to the determined and purposed suggestion of a mode of support other than the true one. The architect is not bound to exhibit structure; nor are we to complain of him for concealing it, any more than we should regre t that the outer surfaces of the human frame conceal much of its anatomy; nevertheless, that building will generally be its noblest, which to an intelligent eye discovers the great secrets of its structures, as an animal form does, although from a careles s observer they may be concealed."And in Section VIII Ruskin (2) continues:
"With deceptive concealment of structure are to be classed, though still more blamable, deceptive assumptions of it - the introduction of members which should have, or profess to have, a duty, and have none."These two statements also apply to reinforced concrete. Reinforced concrete is a material which is formed by the combination of two materials: steel and concrete. One cannot see inside a reinforced concrete section to observe how the steel is placed or how it is functioning: it is hidden. The means by which the cross-section can carry or transmit it's load is only obvious if the cross-section is working in compression. The clues as to how the structure and its components function are given in the shap e of the cross-section of the elements and in the form of the structural system. Thus, the form of a reinforced concrete structure is its singular expression of the magnitude of loading and thus of its function.
There are four major stages in the development and use of a new building material: imitation, inappropriate application, conceptual application and appropriate use. As a new building material is introduced, such as steel or reinforced concrete, the appli cations which would take the greatest advantage of it's unique physical properties are not usually immediately obvious. The initial uses of the new material often mirror known forms and building technologies. An understanding of its potential must be dev eloped because industry must adapt to the new conditions before the material can be effectively utilized. Understanding the physical characteristics takes time and usually involves a transition phase with many ludicrous applications. The accumulation of a greater body of knowledge and practical understanding results in new methods of design and construction. Thus, specific forms begin to appear which are appropriate for the new material.
The classic example of the imitation stage of a new building material is seen in the design and construction of the cast-iron bridge in Coalbrookdale, England in 1777. The builders had little to no experience with the new material before the erection of the bridge. Thus, they relied upon their experience in wood construction. Wedges were rammed between elements at the joints to place them under a higher compression. This is a perfectly valid application for wood, but creates local stress points which induces failure much more quickly in cast iron. Such applications are usually doomed to failure. It is only through the careful analysis of such failures that the body of knowledge of the new material can be expanded.
A similar process was seen as concrete was "rediscovered." The early period was for the most part dictated by Joseph Aspdin's rediscovery of hydraulic cement. He named his new cement "Portland Cement" due to its resemblance to the P ortland Stone which was in popular use at the time. He perceived that his discovery provided him with a means to inexpensively imitate stone and thus earn him great profit. This concept of concrete as an artificial stone permeated the earliest appl ications of the new material. Concrete was indeed cast in blocks to not only imitate stone or brick in appearance, but was laid in the same manner in the construction of houses. These first houses were known in the British professional literature of the time as "concrete-stone houses."(3)
The initial construction methods of ramming plain concrete into shuttering without the addition of reinforcement for walls resulted in structural elements with properties only a little better than rammed earth. Reinforcement was added to increase their b ending strength. The early reinforcement was cast-iron, but this did little to increase the strength of the walls. The material properties and production technology of cast-iron advanced so that successive new generations of improved Irons could be intro duced. The very brittle cast-iron reinforcement was soon replaced by the more ductile wrought-iron. The labor intensive wrought-iron was replaced by puddle steel. Technology advanced at an ever increasing pace so that within a short period of time vari ous types of higher grade steels with a variety of qualities became available in dependable quantities. The demand for structural steel members increased as the quality and dependability of steel increased. Reinforced concrete underwent a similar maturing process. Its initial unreliability was gradually improved as manufacturing processes were refined. The physical properties of the final product depended not only upon concrete, but also upon the steel. Thus, one could say in a very simplified manner, as the strength of steel increased, so did the strength of reinforced concrete: as the reliability of steel increased, so did the reliability of reinforced concrete. This process was instrumental in the expanded use of the new material.
The factors which spoke for the increased use of Steel in the construction of structures were varied and many. It's application quickly permeated almost all sectors of the building industry. It was found in almost all public structures as its popularity as a load-bearing material grew. As its use increased so did the building failures of structures built with the material. Within a short period of time there were catastrophic fires which resulted in the sudden collapse of load-bearing steel structures . It was quickly realized that the naked steel structure required a fire-proof cladding. This cladding was stone, terra-cotta, brick, plaster, concrete or combinations thereof. As the quality of concrete became more dependable, and its price more reason able, it gained increasing acceptance with the design professionals for use as a Fire-Proof construction system. The Fire in Baltimore was widely published around the world. It was a catastrophic, yet crucial, event for reinforced concrete constr uction.
By 1900 there were dozens of different systems for the erection of reinforced concrete fire-proof floor slabs. This brought a rapid expansion in application, and thus practical experience in and theoretical understanding of reinforced concrete; thus the material began to be seen as not just a fire-cladding for steel, but as a building material in its own right. The combination of the two was found to have unique advantages which could be utilized to build a wide range of structures: e.g. water tanks, ske letal structures of all sizes and shapes, harbors, stadiums, and halls of all descriptions.
As the advantages of the combination of concrete and steel were observed, the two were soon seen as a single inseparable monolithic unit. A good example is Hennebique's patent of 1892 for a method of reinforced concrete beam and slab construction. He proposed a structure based on the post and lintel as seen in traditional wooden or steel structures. The unique aspect of Hennebique's method was found in the conscious application of the "fluid" properties of concrete to create a str uctural whole of the beam and the slab. He perceived the two as one structural entity, and not two layers of separate construction. This is described as a Plattenbalken in German, which translated means Plate-Beam. This is known in English a s the T-Beam, which describes its geometry, but not its structural function. The T-Beam took advantage of the capability of integrating the floor slab as the expanded upper flange of the supporting beam. The T-Beam introduced a structural member with pro perties that were unique to reinforced concrete.
The T-Beam, as perceived by Hennibique, was a convenient way to increase the load-bearing capacity of the members of a reinforced concrete skeleton frame construction. His structural design repertoire did not really extend beyond the post and lintel; his success did not require him to explore any further capacities of the new material. He would allow his licensees to do this in his name. The T-Beam is a structural member that was uniquely qualified to be created of reinforced concrete. It was the firs t step in designing reinforced concrete according to its own properties. But, it was not until after the introduction of load-bearing surface structures (shells) in the early 1920Ős that the structural design of reinforced concrete would find a form bas ed solely on its own unique properties. Hilbersheimer(4) described this very eloquently in 1928:
"The elements of reinforced concrete construction were in the first place the same as those that are found in steel construction: columns, beams, and various combinations thereof. Later, the elements of reinforced concrete construction expanded to include the load-bearing slab and most recently, the load bearing surface which is essentially its origin."
But before we delve into the load-bearing surface structures, it is necessary to return once again to the turn of the century. A number of similar events occurred which helped to usher in the Concrete Age, all of which were large building fires. More th an 1600 people died in fires in theaters between 1882 and 1895 in only six European cities(5). Between 1882 and 1887 alone there were a total of 91 theater fires. The catastrophic destruction of the Ringtheater in Vienna in 1882 was so significant that the encyclopedic description of theaters in the Baukunde des Architekten was divided into two categories: pre- and post-Ringtheater fire. This and many other similar tragedies resulted in the first P russian Fire Codes being published in October of 1889. These regulated issues from the size and number of entrances into the theater to the materials to be used for the stage curtains and construction. It was stated within the Code that auditorium floors and seating were to be constructed of non-combustible materials. Reinforced concrete construction immediately became the chosen material for those areas of auditorium construction which were to be fire proof. Often a concrete core was surrounded by less expensive traditional construction techniques.
There were many different ways in which structural and architectural designers attempted to fire-proof buildings. The most common method was to apply a layer of fire-proof material around the load-bearing cast iron columns and beams. Two importan t phenomena were observed in the extinguishing of the great city fires in Chicago and Boston: first, cast and wrought iron elements melted after the fire reached a certain temperature; and second, that cast iron columns suddenly collapsed when one side wa s cooled by the extinguishing water. The first observation was somewhat obvious, but the second was much more disturbing due the loss of life of the rescue workers. One result of these observations was the banning of all cast iron load-bearing facade e lements. The immediate effect of this ban was the use of wrought iron load-bearing profiles with cast iron cladding. The 4 to 5 cm between these two was either left open or filled with various massive materials. After a short period of time, the stylis tic cast-iron cladding was replaced by the less expensive Terra-cotta with a plaster coating. An article(6) in the Deutsche Bauzeitung from 1884 illustrated a number of variations of this type of fire- proofing system. In the accompanying text, it was stated that at that time there was only one steel mill which even produced wrought iron profiles for columns in Germany.
A somewhat parallel development took place in the fire-proof floor slab: initially existing materials were adapted to the stricter codes, then new materials were added to increase the fire protection of the construction, and new systems were designed whic h accounted for the advantageous properties of the emerging materials. This resulted in three types of floor systems: reinforced brick, load-bearing steel beams with brick or concrete infill, and reinforced concrete slabs. The attempts which are of inter est when considering the development of reinforced concrete, are the two combinations of steel and concrete.
The first type of massive fire-proof floor systems consisted of burnt clay tiles spanning between two wooden or steel beams. The variation of tile shapes and forms was almost endless. The basic structural concept of these systems was either to create a compression arch between the two horizontal beams with a small number of tiles (see the Pioneer Fire Proof Construction Company Type 1 System) or simply to lay the wood or steel beams close enough to be able to span tiles between them (System Homan). In most cases a layer of mortar or concrete was applied upon these tiles in order to create a level surface for the finished flooring. As in most evolutionary processes, these types diversified, multiplied, and eventually disappeared.
The second type was found in placing an element, or elements, between steel beams. These elements functioned as formwork for the concrete which was added until it reached the proper level. These forms were pressed sheet metal, expanded metal with an ap plied layer of gypsum or cement plaster or prefabricated cement forms. In 1900 Professor F.W. Busing(7) spoke of "a certain domination of Cement in the construction of floor-slabs."
The third method of fire-proof floor slab construction was a reinforced concrete slab of a form somewhat similar to that known today. Two directions were chosen in the design of these systems: the traditional post and lintel method propagated by those su ch as Hennebique, and the new mushroom slab system. The traditional post and lintel systems also looked to take advantage of the continuity between the slabs and the beams. These systems, developed simultaneously in the USA (C.A.P. Turner) and Sw itzerland (Robert Maillart), found one venue of exploration in the prefabrication of the reinforcing cages. This was especially seen in the United States where the emphasis was continually on the development of rapid construction techniques that did not require a highly developed work force. The mushroom slab system represented the culmination of the development of massive fire-proof floor slabs. It utilized the unique two-dimensional load carrying capabilities of monolithic reinforced concrete. The m ajor impetus of this development was financial. Industry required the maximum possible floor to ceiling height to be clear of obstacles. The construction of the traditional beam and column construction systems resulted loss of space due to the height of the beams. By eliminating the beams, reinforced concrete was a better candidate to be chosen as the material to build the new factories.
The mushroom flat-slab method of construction also underwent a metamorphosis. There was a great diversification in the shape and size of the shear reducing mushroom. The original elegant curves were steadily reduced to more linear and angular sha pes and then boxes stepped atop one another to reduce the effort necessary for prodution. The flat-slab experienced an explosion in patented reinforcing systems similar to that seen with the concrete beam. There were many differing theories as to the be st and most efficient method to reinforce a slab. Some systems looked to structural efficiency as the answer, while others to the ease with which the slab could be built. In any case, between 1915 and 1920 there was an overabundance of articles discussi ng this in the professional journals. In addition, the issue of properly calculating the stresses in the section of such a construction was also an issue of contention. There was very little theoretical or experimental documentation to support the appli cation of this new structural system in the public domain. The structural designers and contractors sponsored experiments to gain city building department's approval for certain patented systems.
The patented systems did not vary significantly in their concepts. The differences were mostly in the method of reinforcement or the shape of the formwork: nothing very original, just different. These systems were patented for the protection of the indiv idual contractors. That is, most were not interested in developing a new or original method of reinforced concrete construction, rather to protect themselves from being sued for patent infringement by a competitor.
System Abramoff prefab table. =====
System Abramoff cross section