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US-15: Failure of existing long-span wood roof truss following installation of new roof-top HVAC units

Report ID: 949

Published: CROSS-US Newsletter 2 - September 2020

Report Overview

Failure of an existing long-span wood roof truss following installation of new roof-top HVAC units offers lessons in the special challenges of renovations to existing buildings.

Report Content

An existing wood roof truss failed after several new HVAC (Heating, Ventilating, and Air Conditioning) units and associated piping and ductwork were added on the roof of an existing building during a renovation project.

The renovation project team had proceeded without the involvement of a structural engineer (SE) until the building department required that an SE evaluate the structure and design modifications, if needed.  The late engagement of the SE in the process caused significant schedule pressures.  The SE visited the project site to understand the existing conditions and to discuss with the owner's representative the intended renovations.  Significant in this visit was an observation that ceilings were self-supporting on bearing walls, and there was minimal load from ceilings and MEP (Mechanical, Electrical, and Plumbing) equipment suspended from the roof.  The SE noted no deterioration of the trusses. The owner's agent at the site represented that in the renovations, ceilings would continue to be independently supported (not off the trusses), and that the minimal suspended load on the trusses would not be increased.  The mechanical engineer provided the weights of new mechanical equipment to the SE.  The SE was never provided final design documents from the architect or the MEP consultant.  The SE determined that the increased load from the new HVAC units represented a less than 2% increase in the demand/capacity ratio of the wood trusses, and so concluded that no remedial work to the trusses was required.  The SE prepared and submitted a structural design drawing indicating subframing support for the new HVAC units. 

During construction administration, the SE received a submittal from the mechanical contractor corroborating the HVAC unit weights provided to the SE orally.  The SE made one construction observation site visit.  While the HVAC subframing appeared to generally conform with the SE's drawings, they noted that several studs from pony walls atop the trusses in the original construction had been removed (Figure US-15-1).  The trusses were of "bowstring" configuration, and since the arc of the top chord is below the flat roof deck, "pony" stud walls supported the roof deck off of the trusses and special lateral bracing of the trusses' top chords were required. The SE provided a sketch to remediate the pony walls.

Several months later, the SE received a phone call indicating that the trusses were failing, and the roof was collapsing. The SE went immediately to the site and noted two important facts.  First, several lateral braces of the pony wall above the trusses near the truss ends had been removed during the recent construction and had not been reinstalled.  No notification of the removal of these braces was ever provided to the structural engineer.  These lateral braces were required because near the ends of the trusses, the top chords were not directly connected to the roof diaphragm and therefore the top chords were not laterally braced without the extra lateral brace members (Figure US-15-1).  Second, although the SE had asked and been told that the ceiling would not be supported by the roof structure, it had, in fact, been hung directly from the roof.  The amount of ductwork and piping was significantly more than was expected by the SE during the evaluation and design.


This case study highlights many challenges of renovating existing structures. Such work must consider (1) the existing loads on the structure and how the renovations may change those loads (2) any changes in design loading required by code updates, and (3) the existing capacity of the structure, which involves understanding the in-place structural conditions, including possible deterioration and structural changes brought about by the renovation. The contractor may also inadvertently change existing conditions during construction it considers structurally insignificant, but that are critical.  These and other factors are discussed below.

Existing conditions: During the "design" phase of the project the structural engineer made one site visit to observe the existing construction and to understand, based on a verbal report from the building owner's representative, the intended scope of renovation.  There were apparently no original design drawings available. Lack of original design drawings increases the need for documentation of the existing construction.  Visibility and close-up access to existing conditions can be a challenge.  The structural engineer's observation that bracing of the trusses' top chords required special care in this configuration is to be commended.

Unfortunately, the structural engineer was never provided renovation design drawings. The entire process relied on verbal communication of design intent that appears not to have been documented in detail.  This lack of documentation in conjunction with an apparent lack of final inspection and signoff at the completion of construction (see below) were two critical factors in this failure.  Of note is that the ceiling and mechanical equipment hung loads were significantly greater than the SE understood they would be.

The applicable building code for this project did not require that the renovated existing building comply in all respects with current codes for new construction, as is typical for minor renovations. The structural engineer's threshold check that if loads were not increased more than 2% then the renovation had negligible load impact on the trusses appears reasonable. 

The contractor made debilitating changes to structural conditions without SE's knowledge, most significantly removal of studs from the pony wall and removal of truss chord bracing.  It is, unfortunately, not uncommon that contractors take liberties with structure during MEP work.  To prevent this for wood construction the International Building Code1states:

2303.4.5 Alterations to trusses.

"Truss members and components shall not be cut, notched, drilled, spliced or other altered in any way without written concurrence and approval of a registered design professional.  Alterations resulting in the addition of loads to any member (for example, HVAC equipment, piping, additional roofing or insulation) shall not be permitted without verification that the truss is capable of supporting such additional loading."

Construction observation and signoff:  A final safeguard against the failure reported would be inspections by the SE and the building department at the completion of construction.  While the SE did make one site inspection during construction, critical changes to the structure and the loads imposed on it were made after the SE's visit.

Time pressures and scope of the SE's involvement:  The project's overarching conditions were an invitation for problems. That the SE was introduced to the project late in the process (1) shows a lack of appreciation amongst other members of the project team for structural implications of the work and (2) caused the work to be hurried.  The report does not indicate that a single prime design consultant was in charge.  Communication between project team members was not clear.  Documentation was poor.  While it is not uncommon for top-down budget considerations to limit an SE's involvement on projects where the apparent structural component to the work seems small, where structural safety is concerned the project structural considerations must not be compromised.

There was a catastrophic collapse of the Sampoong Department store in South Korea in 1995 due to a variety of causes and 500 persons died. There were several deficiencies, one of which was the re-positioning of heavy HVAC units on the roof2.

1. International Building Code, 2018 Edition, International Code Council.
2. https://safety.productions/2019/02/17/sampoong-department-store-collapse-south-korea/.


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