Editor’s Note

If not a record, this issue must approach a record with 13 new articles as well as discussion on past articles. It requires rigorous self-discipline for the editors to always focus on the basic intent of this periodical when reviewing submissions. The papers should not be too long and must be addressed to problems that designers and constructors may meet in their practice at some time. We want to present solutions to real problems, not theoretical solutions looking for a problem. Long mathematical derivations are unacceptable.

The Structural Forum, prepared this quarter by Andrew Longinow, addresses the problem of floor vibrations excited by occupants of a building. This is not the frequently seen article on the dynamic theory involved in floor vibration, but instead a brief description of six actual problems investigated and the solutions developed. It is a very good introduction to the problem. This writer has experienced vibrations, so in examining a building floor he invariably tries a heel drop test first to determine if there is likely to be a problem of this kind. In an impressive new head office building built for an American corporation, the receptionist complained of floor vibrations when anyone walked by. Upon investigation, it was found she was sitting at a desk in the center of a large room, unfurnished save only for the carpet and desk. Almost certainly, several heavy chairs or sofas together with a heavy bookcase or two would have rendered the vibration unnoticeable.

Although the incidents are not widely reported, many cities have experienced the problem of parts of the exterior facade breaking free from a building and falling on what was below. Obviously, this can be a great danger to anyone in the vicinity. Only last month, a couple dining outside a restaurant in Montreal were struck by a concrete slab that had fallen from the building above. The woman was killed and the man injured. Just two weeks later, yet another innocent bystander was killed by a component that had fallen from a building. Such occurrences in Chicago resulted in local legislation, commencing in the 1980s, requiring facade inspections at specified intervals. Kurzydlo has documented the developments in this legislation as the city tried to make the requirements more effective and building owners fought back to curtail the expense. The structural engineers who had to perform or direct the inspections and take responsibility for the veracity of their assessment were caught in the middle. The author’s documentation of the developments may not be of great interest to structural engineers but will be invaluable to engineering groups and city officials involved in setting up similar legislation in other cities.

When several different remote systems are available for either the strengthening of a structure or to determine the extent of deterioration or decay, selecting one can be challenging. Agrawal and his colleagues have provided an invaluable review of available methods or techniques. Case studies of actual applications can be invaluable. This paper addresses not only the continuing rate of corrosion, if any, but also the relevance of the repair technique used.

The significance of the paper by Griggs on the preservation and restoration of nineteenth-century cast-iron and wrought-iron bridges stems from the very different characteristics of these two materials and also from the differences between the two irons and the carbon steels used today. Cast iron, high in carbon, is appreciated by most engineers for its high compressive strength, weak tensile strength and consequent brittleness, ready machineability, and inability to be bent or formed in any way. Rather less known is its superior resistance to corrosion compared with mild steel. Castings not required in significant numbers are made by the traditional method of constructing a mold in sand from a wooden pattern then pouring molten iron into the mold. Making the patterns of wood laminations for complex shapes requires highly skilled tradesmen who are knowledgeable of venting to avoid air traps and cores to form voids in the casting.

Wrought iron is the original iron of the industrial revolution and is made by the puddling process developed before the invention of the blast furnace. Gray pig iron and millscale were heated in small furnaces on a bed of iron oxides. Pasty balls were withdrawn, hammered, and rolled into crude bars. These were then piled together, heated to welding temperature, and rolled into suitable sections. The rolling process elongated the slag into fibers, improving uniformity. Wrought iron’s resistance to corrosion is about 20% better than mild steel’s. The slag fibers divert crack formation caused by bending and shaping. Unfortunately, the term “wrought iron” is now applied to any ferrous metal that is heated and bent or hammered, that is, wrought, into a decorative form.

Design of wood shear walls may seem unusual to anyone except for specialists in low-rise multistory building construction. However, in wood design, most engineers have had to use tabulated values that appeared to be rounded or imprecise shear values for nails. The paper by Leichti and Kurtz refines the capacities, based not only on length and diameter but also on type of nail head. Importantly, it was found that one of the three types tested, the casing heads (CSE) nails, reached only 78% of the nominal values.

Megri addresses integrating the different fire protection/life safety elements into the building design process. Frequently, integrating these elements will upset the preferred system of one or more of the other specialist engineers. Furthermore, while nominally it may be the task of the HVAC engineers, fire protection is not likely their first priority. Indeed, anything that causes specialists to change their favored scheme will be borne with reluctance. Thus, any discussion on how to address fire protection rationally before other trade solutions are set should be welcomed.

The paper by Al-Gahtani discusses optimum design of pipeline (concrete) block anchors. The writer’s immediate understanding, which may be erroneous, was the thrust blocks we used at bends in buried water mains to prevent the spigot from sliding out of the socket under water pressure. We backfilled the void between the bend and undisturbed soil with concrete without much thought as to whether it was entirely adequate.

Edgar Muñoz Diaz et al. have evaluated bridge failures in Colombia since 1986. Approximately one-third of the failures were of steel bridges and were owing to structural deficiencies, both in design and resulting from other causes. The remaining two-thirds were concrete bridges, and these failures were caused primarily by scour at the supporting structure or by inadequate design load. A rigorous program of bridge investigation is recommended, accompanied by development of a strengthening technique for specific types of deficiency.

Review of regulations and guides for excavation and trenches in several countries is used by Lan and Daigle to highlight shortcomings in the current requirements of the province of Quebec. Apparently, the requirements, which are unlikely to be unique to Quebec, do not take into consideration the type of soil and its characteristics. The writers’ observation in Ontario is that for depths of about $1.5\mathrm{m}$ or greater, prefabricated shoring is normally provided. However, recently it was reported that a worker was trapped, but not covered, in a failed trench.

Strohman and Liepins report on a finite element investigation into the snap-through, antisymmetrical buckling of a $26\mathrm{m}$ $\left(85\mathrm{ft}\right)$ diameter shallow, i.e., a low-rise concrete dome. The subject is very clearly presented in the abstract. The critical finding is that failure would have occurred at a load significantly less than that predicted by the procedure prescribed in the American Concrete Institute document, ACI 350.

Haddock and Razzaq have developed a solution to the flexure of an asymmetric built-up wood beam composed of materials of different elastic moduli. Supposedly, such beams are found in some house construction. The elastic solution to this problem is derived by working though 60 equations covering 8 pages. In reality, connecting the three pieces of wood monolithically so that the beam acts as assumed in the elastic solution would be a challenge. This technique would not be used on a routine basis. Rather it would be useful in strengthening an existing inadequate condition, in which the basic material is already in place but has not been connected to act compositely as this solution requires or in which an additional member can be installed readily. The writer suspects that connecting by toe-nailing would not be workable. Nail plates and nail angles would be more appropriate.

The article by Beguin recounts the development of an unusual sheet steel channel section that, laid with the axis of symmetry vertical, provides the floor structure. From below, the soffit appears as a continuous sheet metal surface, with butt joints between elements. A permanent wood form resting on the upper horizontal lips of the channels provides for a thin concrete slab floor to which any suitable finish can be applied. The void below allows hidden distribution of services but does not appear to be readily accessible for future modification. This system will likely not gain widespread acceptance in North America. Not only is the wood industry entrenched in house construction, it continues to develop such products as the particle board web wood joist for longer spans, with almost shrinkage-free performance. Europe, however, does not follow the wood construction used in North America, although it is penetrating Britain to the extent of 20%. The writer’s son’s $25\text{-}\text{year}$ -old house on the outskirts of Paris has concrete basement walls and a precast concrete floor with concrete topping and hardwood or ceramic tile finish. Being one story, it has light steel roof trusses supporting spaced laths carrying the clay tile roof. The walls are concrete load bearing block with stucco, so there is scarcely any structural wood. In such a building environment, the proposed floor system might have a place.

The paper by Abdelhamid and colleagues is about lessons learned from the rather special formwork for part of the Central Artery/Tunnel project in Boston. While it does not address circular or elliptical forms for bored tunnels, the selection of forms was also key to production for trades other than those constructing the forms. Papers on formwork are rarely submitted, so this paper fills a void in the construction knowledge of many of us.

Mohammed and Kennedy have written on the fatigue resistance of corrugated steel sheets’ bolted lap joints under flexure. In most applications, such as conduits under highways, it is a case of “out of sight, out of mind.” Only those directly involved appreciate the repeated loading effect to which these common structures are subjected. This then is one of the low-profile mundane topics that one reads out of interest without having heard of it before.