CAUSE AND ORIGIN INVESTIGATION OF BOWSTRING TRUSS ROOF FAILURE

CAUSE AND ORIGIN INVESTIGATION OF BOWSTRING TRUSS ROOF FAILURE

https://sites.google.com/site/metropolitanforensics/cause-and-origin-c-o-of-bowstring-truss-roof-and-other-structural-failures

 

Many roofs constructed of wood bowstring trusses have failed this winter.  This type of roof is historically prone to failure.  They were very popular in the 1940-1960 time periods because it gave the owners the ability to build attractive structures with open floor spaces and no load bearing columns.  A lot of warehouses and hangers still have these wooden bowstring trusses. 

Unfortunately, there are many reasons why these trusses fail.  Based on our inspections of failed truss roofs, we can say that one of the most common reasons for failure is the deterioration of the truss ends where it is mounted on the load bearing wall.

In the course of an investigation of a recent bowstring truss roof failure we made the following pertinent observations:

• Inspections found water stains along the left and right masonry walls.  Inspections also found deteriorated mortar joints on these walls and in particular the failed left masonry wall; the significant long-term water staining to the left foundation wall, as well as the degradation of the wall components, is consistent with long-term chronic water infiltration into the wall structure originating from the roof area above.  Based on the conditions to the masonry wall components, it is not unreasonable to believe that the roof was leaking at the bearing point above over an extended period of time, which would have caused rot and deterioration to the roof framing at this location.

• Inspections of the left masonry wall did not find signs of adequate roof drainage; inspection found no roof drain(s) to be located at the left side of the building;  a roof drain was found to be located on the right side of the building; that drain was found to discharge a small amount of water;

• Inspections found that the top wall plate (or sill or ledger as is otherwise known) along the left exterior masonry wall was either rotted or had long term decay or water damage;

• Inspections found that the top chord was connected to the bottom chord using bolted split-ring connections;  this assembly was then toe-nailed to the top wall plate;

• Inspections found that the top plate at the left wall was not made of pressure treated wood and that a moisture barrier had not been provided;  the top plate was anchored to the concrete block wall using anchor bolts;  inspections found that a moisture barrier was not provided at the right exterior masonry wall;

• Inspections found many truss ends, top wall plates and other roofing material that were severely rotted from long-term roof leakage. 

Why the Bowstring Truss Failed

The end connections on these bowstring trusses were critical to the overall truss performance.  The bolted, split-ring end connections at these locations transfer large thrust loads from the top chord members into the bottom chord.  As a result of the long-term moisture and water impacts, rot and decay caused the wood to soften and lose strength, resulting in localized sagging of the bottom chord, and elongation or splitting of the end bolted split-ring holes.  We observed many truss ends that were rotted or decayed and where one of the truss members (either the top or the bottom chord) had separated from the split-ring connection.

Long-term deterioration of these truss ends resulted in failure of these connections and consequent loss of truss action.  When the end connections deteriorated or failed, the outward thrust from the top chord (that is under compression) when the truss is properly functioning, was no longer resisted by the bottom chord.  Instead, thrust loads pushed outward against the supporting masonry walls, resulting in bowing of the walls and, since were left unchecked, resulted in complete failure of the roof. 

Some deterioration to the joints of the masonry walls was found.  Also pre-existing repairs to the roof trusses were found consistent with prior knowledge of distress to the roof structure.  The above conditions clearly document that the roof structure has been subjected to chronic water infiltration resulting in degradation to the framing components causing a loss of structural capacity.  Therefore the root cause of the failure is attributed to long-term deterioration to the roof structure rather than the weight of ice and snow.  Although the weight of ice and snow on the roof was part of the loading that caused the roof to fail it cannot be considered the root cause of the roof failure.

Other Structural Failures

Metropolitan services range from determining the cause and origin (C&O) to evaluating the damage extent due to the snow load or tree fall.  Construction defects are commonly faulted when failures occur.  Frequently Metropolitan finds design defect, construction defects and decay as contributing causes of a failures.

Metropolitan is often asked to assess historical structures.  Often, remodeling that occurred decades earlier begin to show deformations and structural responses.  A recent claim involved a home where ten years ago a property owner wanted to create an “open space feeling”.  The architect then decided to remove a load bearing wall to create the open space and designed a flush flitch beam to take the load.  Metropolitan discovered that the flitch beam was not properly supported on both ends by a load bearing wall and was instead supported at one end by the weakened wall!  That is a major blunder folks.  The punishing winter we are having is stressing the structures to the point that these design defects are now showing up.

It is Metropolitan’s experience that there is no substitute for a thorough and comprehensive inspection of the structures to determine the number of conditions that caused the loss and whether the conditions worked independently, concurrently, or in a sequence to cause the damage. 

A comprehensive investigation is crucial to good claims handling.  Our clients require prompt and thorough claims investigation and fact finding and Metropolitan delivers high quality services at a highly competitive price. 

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METROPOLITAN PROFESSIONAL ENGINEERING AND FORENSIC SERVICES

P.O. Box 520

Tenafly, NJ 07670-0520

Tel.:  (973) 897-8162
Fax:  (973) 810-0440
E-mail: metroforensics@gmail.com