FAILURES
Christine Diosdado, PE, and Paul Cordova, PhD, SE
Recent collapses of exterior elevated elements (EEEs) like balconies, walkways, and stairs have caused injuries and fatalities, leading to increased scrutiny of these features. Based on decades of experience inspecting and repairing these structures, stucco-clad EEEs on weather-exposed elevations are particularly susceptible to structural damage and catastrophic failure than unclad EEEs. Structural wood members encased in stucco with no provision to drain or vent, commonly referred to as stucco buckets, pose the greatest risk since they tend to collect and hold water against the surface of the structural member for prolonged periods following rain events. This repeated and sustained moisture exposure can cause deterioration of weather-resistive barriers (WRBs), wood decay, and reduction in the structural framing capacity. Stucco buckets not only increase the likelihood of moisture-related damage, but also conceal severely decayed and structurally compromised framing members, often with no visible signs of distress.
Case in point
As part of an apartment complex condition assessment, the authors visually surveyed stucco-clad EEEs and removed small areas of stucco and associated waterproofing elements at a select number of weather-exposed locations. Through this process, they were able to identify significant decay of structural beams made of glue-laminated (glulam) timber. The decay extended through multiple bottom laminations at the beam’s mid-span, requiring shoring and immediate repairs. Typical remediation requires removal and replacement of the decayed glulam with a new one of similar size. This approach was not possible because the beams framed the primary access and egress to the apartment units. To minimize disruption and to avoid the cost of relocating tenants, in-situ glulam repairs were developed.
Photo courtesy Simpson Gumpertz & Heger Inc.
Repair approach
The repair required removal and replacement of deteriorated laminations with new 2x Douglas Fir sawn lumber that are fully adhered and mechanically fastened to the remaining laminations. The span and loading determined both fastener size and spacing necessary to develop the required beam capacity. The minimum allowable fastener spacing, stagger, and end/edge distance limited the maximum force per length that can be developed in each lamination. The minimum allowable spacing, combined with the actual beam demands, limited the repair to no more than four lams out of 10 in a single 381-mm (15-in.) deep glulam. The adhesive was conservatively ignored when calculating repair strength, eliminating significant installation challenges.
Drainage and venting features were added to the stucco assembly to minimize moisture accumulation within the system and to improve the long-term integrity of the WRBs and the repaired glulam. Drip screeds were added to the stucco assembly at the vertical to horizontal transitions to direct bulk and incidental surficial water to the exterior. The final design also included furring strips and vents with an intumescent coating to provide air circulation and to promote drying while maintaining the stucco’s first-resistance properties.
The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect those of The Construction Specifier or CSI.
Christine Diosdado, PE, is an associate principal at Simpson Gumpertz & Heger Inc. Diosdado has more than 25 years of experience as a forensic engineer investigating and repairing stucco-clad structures. She can be reached at cdiosdado@sgh.com.
Paul Cordova, PhD, SE, is a senior project manager at Simpson Gumpertz & Heger Inc. Cordova has 20 years of experience as a researcher and structural engineer and has been involved in repair projects related to deterioration and failure of stucco-clad structures. He can be reached at ppcordova@sgh.com.
