Compressive Failure of Web Members at Top Chord

Bowstring Truss Failures, Part 1: Deficiencies & Failure Causes

DonanEngineering Commercial & Residential Roofing, Forensic Engineering, Structural Damage & Collapse

Introduction

Wood bowstring trusses were used extensively in commercial buildings from the 1920s to the 1960s. Wood bowstring roof trusses provided long clear spans necessary in warehouse and industrial buildings. Many roofs supported by bowstring trusses are still in service today. Compared with other structural timber-truss types, bowstring trusses have exhibited more frequent structural problems. The structural deficiencies encountered are attributable to deficiencies in the original design of the trusses combined with damage due to exposure and age.

Three factors, including a series of code changes, refinements in analysis methods and revisions to allowable timber stresses, have resulted in these trusses now being determined to be significantly overstressed. The International Existing Building Code (IEBC) defines a dangerous member as one where, “the stress in a member or portion thereof due to all factored dead and live loads is more than one and one third the nominal strength allowed in the International Building Code…”
Increases in loading and decreases in allowable stresses alone render bowstring trusses dangerous. While these trusses have carried the roof load for decades, it is not uncommon for failures to occur particularly after large snow accumulation. Bowstring truss failures typically do not result in large scale collapse and repair is possible without replacement. However, a building with wood bowstring trusses presents the building owner with extra maintenance requirements to avoid failure. It should also be noted that fire fighters are reluctant to enter buildings with bowstring truss roofs due to the risk of collapse.

Description of Bowstring Trusses

Bowstring trusses are made with a curved (bow) top chord and a straight bottom chord (string). The top chords were initially constructed with multiple leaf chords where the curvature was cut into the solid sawn chord members. The members are lapped from side to side so that the splices in the top chord members do not occur at the same location from one side to another. These splices are also located so that they fall in between truss panel points. The bottom chords were made of two solid sawn members and were spliced between web members using wood splice blocks between the chord members and on the outside of the chords. Bolted connections were used to connect web members to top and bottom chord members. A second style of bowstring truss used glued-laminated chords where the top chord was curved in the laminating process.

Multiple Leaf Top Chord

Multiple Leaf Top Chord

Bowstring trusses arched in the shape of a parabola, and exposed to uniform loads, theoretically would experience uniform stress across the top and bottom chord cross sections with no bending in the chords. Because most structures must sustain some unbalanced load, web members are required. In addition, it is much easier to fabricate a circular arch than a parabolic one. Unbalanced loads result in compression loads in the web members and bending of the top chord.

Bowstring Truss Deficiencies and Causes of Failures

Two design deficiencies are often encountered when investigating bowstring roof trusses: overstress of bottom chords under uniform roof snow load, and overstress of web-member-to-chord connections due to inadequate allowance for accumulations of rain or drifting snow. The application of unbalanced snow loads as prescribed in ASCE 7 results in a localized increase in forces for many truss members and is significantly more severe than the original design loading.

The parapets of the roof make the roof prone to drifting snow resulting in unbalanced snow loads. Finally the allowable timber stresses were reduced in the late 1980s. The combination of higher loads and lower allowable stresses can result in design conditions that overstress the bowstring trusses. The use of computer-assisted frame analysis also identifies secondary moments that the original classical determinate truss analysis did not take into consideration. The National Design Standard (NDS) for Wood Structures provides several adjustment factors applicable to design that affect the allowable stress of wood members.

In addition to design deficiencies, wood truss members are compromised by water leaks, due to lack of maintenance, or by the construction methods used in erecting masonry supporting walls. Many times modern building modifications, such as the addition of roof-mounted mechanical equipment, suspended ceilings or other items was not included in the original design. Some trusses suffer impact damage from warehouse equipment or are weakened by cutting/notching of the wood members. Unbalanced loading causes reversal of loads resulting in alternating tension and compression in the web members which weakens the truss bolted connections. Weakening of the truss may also be caused by the subsequent addition of a load-bearing floor to create additional storage space along the bottom chord of the truss.

Bottom chord fractures and splits can be encountered on bowstring trusses. Partial fractures often initiate at defects such as knots with steep grain slope. Splits in bottom chord members or bottom chord splice members can initiate at bolt holes. Both partial fractures and splits in bottom chords at steep grains slope can lead to truss failures. Splits and checks in web members are common in bowstring roof trusses. Since web members carry only tension or compressive forces, minor checking is generally not a defect that requires repair. However major check or splits that run to the bolted connections can reduce capacity. Often splits are caused by large compressive web member forces.

Compressive Failure of Web Members at Top Chord

Compressive Failure of Web Members at Top Chord

Decay develops in bowstring trusses as a result of roof leaks. Since the top chords of bowstring trusses are mechanically laminated with several plies, there are numerous planar gaps where water is drawn by capillary action. Water that works its way between the top chord plies does not easily dry and can result in decay. Corrosion on the nails installed on the top chord plies is an indication of prolonged moisture exposure from roof leaks. While age of wood does not by itself result in a reduced capacity, exposure to moisture or extreme environmental conditions will result in age-related deterioration and reduced structural capacity.
Sometimes moisture damage is evident on bowstring trusses in the form of wood rot. Dry, crumbly rot is commonly called “rot” or “dry rot”, but the term is misleading because wood must be damp to decay, although it may become dry later. However, the absence of visible rot does not mean that the wood members are not deteriorated or weakened.

Repeated exposure to moisture also affects fastener capacity. Resistance of a nail to direct withdrawal from a piece of wood is intimately related to the density or specific gravity of the wood, the diameter of the nail, and the depth it has penetrated. If common smooth-shank nails are driven into green wood that is allowed to season or into seasoned wood that is subjected to cycles of wetting and drying before the nails are pulled, they lose a major part of their withdrawal resistance. The withdrawal loads for plain nails driven into wood that is subjected to wide alternating changes in moisture content may be as much as 75 percent below the normal withdrawal loads. Reduced nail withdrawal resistance can allow movement of the top chord plies and result in increased loads in the chord and web members.
Sagging and delamination of top chords are frequently observed on older trusses with mechanically laminated top chords. Sagging top chords, sometimes referred to as galloping top chords, develop when nails attaching laminations allow movement between the laminations. Sagging chords result in increased loading in the web members and can result in compressive failures.

 

[1] “Bowstring Trusses “Fail” to Meet Current Code Requirements”, Paul C. Gilham, P.E., Inc. Terry D. McKee, P.E., Western Wood Structures.
[2] “Roof Truss Design”, theconstructor.org/structural-engg/roof-truss-design/281/, The Constructor.
[3] “Wood as an Engineering Material, General Technical Report FPL-GTR-113”, United States Department of Agriculture
[4] “Nail-Withdrawal Resistance of American Woods”. FPL-093, Forest Products Laboratory, US Department of Agriculture.
[5] “Investigating and Repairing Wood Bowstring Trusses”, Richard J. Kristie and Arne P. Johnson. Practice Periodical on Structural Design and Construction, February 1996 (pages 25 to 30).

 

Read Bowstring Truss Failures, Part 2: Repair Methods