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 supporting tensile forces by up to 25 percent. These Buildings constructed prior to the mid- masonry walls, resulting in bowing of the force increases are significant, 1960s will frequently be found to not walls and, if left unchecked, complete particularly in conjunction with the comply with present building code collapse of the roof. previously discussed bottom chord requirements due to the unrealistically tensile strength deficiencies inherent high tensile strength values assumed in Early wood research indicated that the the bottom chord strength of timber truss members would member designs. not be significantly affected unless Unbalanced, exposed to prolonged temperatures drifting snow above 150° F. More recent research and concentrated indicates that reductions in strength loads from begin at temperatures between 100° and mechanical units 125° F, which are readily achieved in may result in many attic spaces, particularly in spaces overloaded truss with poor ventilation. Poor ventilation of members and attic spaces in facilities that contain connections in manufacturing processes involving earlier designs. certain chemicals can also result in a Later building long-term reduction in strength of timber modifications, truss components from fume exposure. such as Figure 5 – The end shoe connection on this bowstring truss has construction of Self weight and roof loads cause failed by splitting the bottom chord members. The truss end has additions having immediate and predictable deflections in spread over two inches, pushing the masonry wall outward. higher roofs or all roof trusses, regardless of installation of construction material. Timber trusses in all timber trusses built prior to the updated mechanical equipment, may undergo additional deflections over time, late 1960s. create conditions of drifting snow or new due to a material property known as loads where none occurred before. creep. Creep deformation is a natural Truss failure evaluation phenomenon in which wood components Investigators tasked with evaluating the A proper evaluation of vintage, heavy undergo gradual, long-term length existing condition or cause of failure timber roof truss roofs must include a changes under prolonged loadings. of heavy timber roof trusses face a comprehensive inspection of each truss These length changes (elongation from formidable task. These structures are to determine its condition. Actual truss tension forces and shortening from unique, specialty products that require member sizes, grades of wood and the compression forces) result in additional the expertise of a licensed engineer physical condition of truss components, timber truss geometry changes over time. having specialized experience in including defects, distress, abnormal These long-term truss deformations are evaluating their structural integrity. behavior, rot and evidence of previous in addition to the previously mentioned One of the first steps in performing an repairs, all should be documented. immediate deflections at the time of evaluation is to locate all available plans construction. showing the original construction and Overall observations of sagging roof or later building modifications and truss ceiling lines, which may be accompanied Loosening of member connectors repairs. These drawings may provide by the presence of owner-attempted subjected to sustained service loads and information regarding the date of remedies (such as support posts), usually restraint forces from drying shrinkage of construction, the roof loads considered indicate structural distress that must be the timber components will further in the truss design and the roof investigated immediately. Outward

Page 4 members are indications of timber roof structure can be used to member or connection assess the necessity and feasibility of distress. Trusses with multiple repairs. Properly executed, a timber truss fasteners per connection evaluation should provide sufficient deserve special attention information to understand the present because of the propensity for roof condition and to make rational splitting caused by restraint decisions regarding roof structure repair from shrinkage, resulting or replacement, considering project from drying. economics and future use.

Web members must be Vintage timber roof trusses are unique examined at all visible products with a long history of failure. surfaces, including ends that Design problems combined with can be seen from between the susceptibility to long-term deterioration bottom chord members. warrant paying particular attention when Bottom chord timber splice these roof structural systems are members often conceal the encountered. Prudent risk assessment sides of the primary members starts with an understanding of their at the connection bolt behavior and the typical failure causes. locations. Visible ends and A licensed engineer with experience in edges must be carefully old timber truss systems should be examined for signs of engaged to properly evaluate the splitting or slippage of bolts condition of each truss by physically in elongated holes. Members inspecting each truss for signs of with large knots or unusual previous repairs, sagging, distress and wood grain should also be decay. Concealed truss components must Figure 6 – The bowstring truss end has been severely carefully evaluated, because be exposed to properly assess their rotted from long-term roof leakage. Water stains, condition, particularly at truss ends, black rot, top chord reverse curvature and ceiling grain variations greater than connections and along the bottom chord. water damage are all present in this view. 15 degrees can significantly reduce wood tensile capacity. When timber trusses are found to require bowing of masonry bearing walls near Trusses that exhibit excessive sag, strengthening repairs, building officials the location of roof truss end supports deterioration and rot, or have failed and may require the costly upgrading of all typically is due to the outward thrust of split members, must be immediately of the trusses in the facility to meet a failing truss. Trusses accompanied by evaluated for safety and to determine the current building code strength supplemental steel tie bars parallel to need for repair or replacement. A requirements. Local requirements for the bottom chord members require licensed engineer must be the sole judge upgrading old trusses should be special review to assess the benefits in determining the appropriate actions discussed with a local building official derived from the added steel, which must for protecting the public in the event that prior to undertaking repair work so that act in concert with the wood bottom the evaluation reveals significant safety life-cycle repair costs for strengthening chord members. issues. Temporary, emergency shoring existing trusses can be compared with may be required to provide a safe replacement costs using more modern, Bottom chord members, splices and environment for building occupants and and frequently less costly, structural connections must be visually examined limit further damage to the structure. systems. along the truss length to determine the presence of failed or split members. Information drawn from an evaluation – David B. Tigue, S.E. and Kinks in the curvature of the top chord and structural analysis of a vintage Kurt R. Hoigard, P.E.

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