Structural failure Building Collapse Snow load

Catastrophic Collapse of a Log-Building Lodge: A Forensic Analysis for Civil Engineers

*The collapsed structure. Source: STRUCTURE magazine*

In the high alpine environment of the Rocky Mountains, a vacation resort faced an unexpected disaster. Despite adequate structural design and a less-than-design snow event, a commercial lodge structure experienced a catastrophic collapse just two years after its construction. This article delves into the detailed investigation that followed, providing critical insights for civil engineers and construction professionals alike.

The investigation began at the center load-bearing wall, using two tracked excavators to remove components. Source: STRUCTURE magazine.

Background and Collapse

The collapsed structure was part of a recreational ranch facility, a popular vacation resort offering activities like horseback riding and hiking. Located on leased federal land, the ranch constructed a 3,100-square-foot, one-story fourplex log building.

Construction began in early summer, but due to financial issues, the project halted before completing essential interior finishes. This incomplete state persisted through two harsh winters, with significant snow accumulations. On May 1, the building succumbed to a catastrophic failure, with witnesses reporting over 10 feet of snow and a thick ice layer on the roof at the time of collapse. Unified Building Sciences and Engineering, Inc. (UBSE) was hired to investigate.

UBSE's investigation revealed several critical oversights:

Unsheathed Load-Bearing Walls: The construction plans specified that interior load-bearing walls be sheathed with gypsum wallboard to meet structural standards. These walls were left unsheathed, making them vulnerable to buckling under snow loads.
Lack of Blocking and Missing Party Walls: The central load-bearing walls lacked blocking, and the design-included party walls were never installed, leaving the roof ridge unsupported.
Inadequate Understanding of Requirements: The general contractor and property owner did not recognize the importance of completing interior finishes to maintain structural integrity during the construction halt.

The center load-bearing stud wall was the critical structural element that failed. Source: STRUCTURE magazine

The evidence revealed that the load-bearing wall studs were not sheathed, failing to prevent weak axis buckling. The floor sheathing and double top plate, along with the studs, should have been 10 feet apart. However, the wall studs had all buckled and become widely displaced. Source: STRUCTURE magazine.

Technical Analysis

The collapse originated near the building's center, where the inadequately supported load-bearing walls buckled under heavy snow loads. The maximum roof load during the first winter was 20 to 30 pounds per square foot, increasing to 60 to 70 pounds per square foot by May 1st of the second winter. With proper sheathing, the 2×6 stud walls could have supported approximately 5,000 pounds per stud. Without sheathing, their load-bearing capacity dropped to around 400 pounds per stud, insufficient to withstand the accumulated snow and ice.

The collapse was due to the failure to follow construction specifications and understand the critical role of interior finishes in structural integrity. The structure was appropriately designed, but the lack of gypsum wallboard sheathing led to buckling under moderate loads, causing the roof structure to collapse and the perimeter log walls to lean outward.

Conclusion and Lessons Learned

The catastrophic collapse of the log-building lodge highlights several important lessons for civil engineers and construction professionals:

Adhere to Construction Specifications: Ensuring all construction specifications are followed, especially for load-bearing elements, is crucial for structural integrity.
Understand Impact of Incomplete Construction: Recognize the importance of completing essential interior finishes and the risks associated with halting construction in an unfinished state.
Communication and Awareness: Ensure that all parties involved in a project are aware of critical structural requirements and the implications of deviating from the plans.
By learning from this disaster, future projects can avoid similar failures, ensuring safety and structural integrity in challenging environments.

Sources: structuremag.org, issuu.com