Vintage Timber Truss Roofs: Collapses Waiting to Happen
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TM ISSUE NO. 16 INSIGHT A technical newsletter by Raths, Raths & Johnson, Inc. for the construction industry. “Insight: to see into and understand; an item of knowledge gained by this power.” From 1920 to the 1950s manufacturing facilities, warehouses, grocery stores, bowling alleys and roller rinks were commonly constructed with timber roof trusses forming large open spans, frequently longer than 70 feet. Currently, modern building codes prescribe larger roof loads than during the time of initial design based on a better understanding of timber behavior as a result of comprehensive timber testing during the 1960s. Couple this with long-term deterioration of by environmental influences that weaken and deteriorate the wood components and you have a recipe for failure. RRJ originally wrote this article for Claims Magazine and it appeared in the June 2000 issue. It is reprinted here with their permission. INSIDE Tech Tip Vintage Timber Truss Roofs: Collapses Waiting to Happen Figure 1 – Schematic of timber roof truss with drifted snow loads shown. Tech Tip Vintage Timber Truss Roofs: Collapses Waiting to Happen Buildings with heavy timber roof trusses be defined by bending can be found in many urban areas. tests of small, clear, Common in buildings constructed from the straight-grained wood 1920s through the 1950s, timber roof samples. Full-size trusses utilized large pieces of wood bolted lumber tests begun in together to form open spans, frequently the 1960s revealed that longer than 70 feet. Timber roof trusses construction-grade can be found in almost any kind of lumber, with natural building, but were mostly used for single- imperfections such as story industrial and commercial buildings knots, checks and requiring large, open floor spaces. irregular grain, Examples include manufacturing facilities, provides in-service warehouses, grocery stores, bowling alleys tensile strength and roller rinks. significantly less than Figure 2 – The web members on this bowstring truss are that predicted by the Today many of these buildings have been bolted to the dual laminated top chord members. earlier small scale, clear adapted for other uses as wide ranging as wood tests. car dealerships, restaurants and video curved shapes or laminating multiple smaller pieces bent over a jig to the desired stores. While different in their uses, By 1968, lumber industry standards shape. Bottom chord members were timber-truss-roofed buildings share several established a reduction factor of 0.55 to common attributes: large size; long, typically constructed with large, straight relate tensile strength to bending strength. uninterrupted roof spans; and a propensity lumber members joined with either wood Current building codes have increased this to sag or collapse under roof loads smaller or metal bolted splice plates, located near factor to 0.60, meaning the allowable than those prescribed by modern building mid-span, to achieve the required length. tensile strength design values are only codes. RRJ finds most timber roof truss The top and bottom chord members were about 40 percent of those listed in the early failures are predictable, and fall into two fastened together at the truss ends with codes. All trusses constructed prior to the general categories, those related to design U-shaped steel heels, or end shoes, bolted late 1960s have a common code issues and those related to long-term to both chord members (see Figure 3). deficiency; the bottom chord members deterioration. have inadequate tensile strength to support Design deficiency issues code-prescribed roof loads. A variety of timber roof truss The shortcomings in configurations have been used over the early heavy timber years. One of the most common types is truss designs are rarely the bowstring truss, so called because of its attributable to mistakes arched top chord profile (see Figure 1). by the designer. Roofs constructed with bowstring trusses Instead, they typically became popular in the late 1930s, and are involve inaccuracies in readily identified by their curved shape. the industry-accepted Although several systems were used in assumptions upon their manufacture, the most popular which the designs were method was to construct both the top and based. The most bottom chords with two parallel members. common deficiency in In this arrangement, the web members early truss designs were sandwiched between the chords and involves inadequate connected with bolts (see Figure 2). bottom chord tensile strength. Early truss Figure 3 – This bowstring truss end, complete with the bolted The curved top chord members were made designs assumed wood steel end shoe, was removed from the southwest corner of a either by sawing straight lumber into tensile strength could collapsed building investigated by the authors. Page 2 Another common truss design issue create semi-rigid connections. Semi-rigid weaken and deteriorate the wood involves snow loads. Early building codes connections have a low tolerance for joint components. These environmental assumed roof snow accumulations were of eccentricity, joint rotation and wood drying influences include prolonged exposure to uniform depth. In reality, wind frequently shrinkage, which can occur for several water from roof leaks, elevated forms snow into drifts that can be years after the large timber truss pieces temperatures in poorly ventilated roofs, significantly deeper than the average snow leave the saw mill. deleterious fumes from various depth. Snow drifts behind raised building manufacturing processes and long-term, parapets, adjacent to higher portions of Unfortunately, the analytical methods creep-induced distortion of the original the same building, and on the leeward side typically employed prior to the wide-spread truss geometry. of curved or sloping roofs can produce availability of computerized structural off-center or unbalanced roof loads far analysis programs in the 1980s did not fully Bowstring roof trusses in many buildings in excess of those predicted by the consider the effects of member continuity, were supported within pockets constructed early codes. connection eccentricity (the offset between integrally within exterior masonry bearing theoretical member center lines and the walls and pilasters. The exterior building This phenomenon was not specifically actual connection points) and the semi- walls were typically extended above the recognized in most building codes prior to rigid nature of multiple-fastener roof system to form a parapet, and drains to the mid-1970s. The additional load, and its connections. These conditions can combine collect and dispel water from the roof were off-center location, can pose significant to produce actual truss member stresses installed in the valleys formed at the problems for older trusses by changing the significantly higher than those anticipated interface of the curved roof surface and distribution of forces in truss member by the original designer, and frequently parapet wall. Over time deterioration of the components, resulting in their overload. result in connection splitting failures that roofing materials and improperly Bowstring trusses, which behave ultimately endanger the entire truss (see maintained drains can allow water to principally as a tied arch under uniform Figure 4). infiltrate into the masonry pockets loads, are particularly vulnerable to supporting the truss ends, unbalanced loadings. creating a moist, decay- promoting environment. Roof overload can also occur due to later addition of loads that were not considered The end connections on in the original design, such as an bowstring trusses are accumulation of roofing materials, heavier critical to the overall truss ceiling finishes and new mechanical performance. The bolted, equipment. RRJ has investigated timber steel, U-shaped end shoes truss roof collapses involving 12 or more at these locations transfer roofing layers, multiple ceiling levels, large thrust loads from the added sprinkler systems and roof-top top chord members into HVAC units. The added weight from these the bottom chord. Rot and items can exacerbate truss member decay cause the wood to overload conditions, particularly when Figure 4 – The bottom chord members on this timber roof soften and lose strength, combined with unbalanced snow loads and truss failed by splitting through the bolted web connections. resulting in top-chord inadequate bottom chord tensile strength. shortening from crushing against the steel shoe, localized sagging of Two other factors contributing to timber Long-term deterioration the bottom chord, and elongation or truss failures involve the connections Post-collapse review of vintage timber roof splitting of the end shoe bolt holes (see between the individual truss members. trusses reveals that design issues are not the Figure 5). Long-term deterioration of truss Truss joint design developments in the only risk factor associated with their ends often results in failure of these 1930s resulted in the use of multiple split- performance. Many collapses are the result connections and consequent loss of truss ring or bolted fasteners that inadvertently of long-term environmental influences that action. When the end connections Page 3 deteriorate or fail, the outward thrust increase truss deformations. The change construction materials supported by from the top chord is no longer resisted in truss geometry due to long-term the trusses. by the bottom chord. Instead, thrust loads deflections can increase bottom chord push outward against the