Behind the Walls: The Silent Collapse of Structural Integrity “A House of Cards: The Hidden Dangers of Concealed Damage”

Title: Behind the Walls: The Silent Collapse of Structural Integrity “A House of Cards: The Hidden Dangers of Concealed Damage”

Award Category: Exterior Wall

Project Address/Location

12 South Van Dorn Street

Alexandria, VA 22304
United States

Submitted By: Bahareh Forouzan

Company Info

Structural Rehabilitation Group, LLC

18310 Montgomery Village Avenue
Suite 530
Gaithersburg, MD 20879
United States

315-262-7100

[email protected]

Project Description and Background: The project focused on one structure within a residential community which included seven (7) four- and five-story precast concrete and masonry structures. Building enclosures featured precast concrete spandrel panels, an aluminum and glass window system supported by exposed precast columns and split face brick modules. The interior floor system consisted of precast pre-stressed double tees, while private balconies at the front and rear were made of precast and pre-stressed concrete flat slabs. Sealant joints serve to complete the enclosure to seal the barrier type system to protect the steel connections and embedded connection plates from contact with moisture. The precast and masonry clad structures were built approximately 50 years ago. The exterior precast concrete spandrel cladding panels, located below the window assemblies, were supported by exposed precast columns along the long elevations of the buildings. Exterior faces of these precast panels were coated with a cementitious coating. The need for consultant involvement emerged from significant concrete deterioration observed in the columns and precast enclosure, which posed serious risks to the building's structural integrity. Additionally, a resident on the first floor reported recurring cracks in the drywall and noticeable air leaks around the windows. Despite previous efforts by the maintenance crew to seal the window perimeter and the resident's recent drywall repairs, the issues persisted. This ongoing problem prompted the decision to engage a consultant for an evaluation and targeted solutions at the rear elevation of the subject building. The primary client constraint was the need to keep the units occupied during construction. According to the condominium documents, the financial responsibility for any relocation fell solely on the Unit Owners, which complicated project planning and execution. To address this challenge and mitigate financial impacts, the design was specifically developed to allow residents to remain in their homes throughout the repair process.

Scope: Initially, visible defects, such as cracks and concrete column spalling, prompted the condominium management to seek the advice of an engineering consultant. Our visual investigation revealed that these conditions were indicative of more severe underlying problems. The consultant played a vital role in the diagnostic phase of the project. Their expertise was essential in conducting a visual survey, along with intrusive sampling to assess the condition of the precast columns and spandrel panels. The consultants’ team, which included structural, shoring and geotechnical engineers, collaborated closely with contractors and project managers to develop a thorough understanding of the existing conditions.   Methodology and Diagnostic Testing The methodology employed during the investigation included several key steps: 1. Visual Survey: This initial assessment from various vantage points (ground level, units, and roof) provided a baseline understanding of the building's condition. It helped identify visible defects that warranted further exploration. 2. String Line Survey: This technique was used to measure and assess lateral movement in the precast columns and panels relative to plumb. The assessment confirmed that the precast spandrels were displacing laterally and were out of plumb at different locations. 3. Core Sampling: Core sampling of the precast panels was conducted to evaluate the mechanical properties of the concrete. The precast spandrel panel consists of two (2) distinct materials; a 1-inch exterior layer made of a white cementitious material (which contains fine aggregate but no coarse aggregate) and a 3-inch layer of porous concrete, similar to that used in molded CMUs, on the interior face of the panel. The matrix of the porous concrete displayed poor quality, indicating that it is a weaker material compared to conventional concrete. The testing revealed significant variations in compressive strength, highlighting the need for structural connection and repairs that were designed around significantly weak in-situ materials. 4. Destructive Sampling: Interior finish removal in living rooms and bedrooms was required to expose the precast panel-to-column connection locations which allowed for inspection of the connection angles. Field inspections documented severe corrosion, displacement and in some locations, disengagement of the connections. The survey revealed varying degrees of failure of the embedded connections cast into the back of the pre-cast panels. Design Adjustments and Innovations Based on the diagnostic findings, the consultant developed innovative design solutions to enhance the stability and functionality of the structures. Key design adjustments included: • Connection Design Improvements: The original connection designs did not accommodate thermal movement of the panels. New connections with horizontally slotted holes were introduced, enabling thermal movement while maintaining structural integrity. • Lateral Restraint Enhancement: Column lateral displacement of various column segments was due to failed connections. The restoration of lateral restraint restored the stability of the five-story columns by connections to the bearing walls. • Water Management Solutions: To address the corrosion distress identified in the connections, the barrier system was enhanced with water-resistant coatings and properly detailed sealant joints along all system transitions. This proactive measure aimed to prevent water intrusion and further deterioration of structural components.

Solution: The project not only focused on addressing immediate concerns but also aimed to enhance long-term functionality, sustainability and maintainability. Innovative Aspects of the Solution Advanced Shoring Design: A standout innovation was the development of a new shoring system that incorporated turnbuckles and articulating legs. This design allowed for precise adjustments to reposition out-of-plane precast spandrel panels, essential for stabilizing gravity loads. Adaptive Connection Design: The original rigid connections were redesigned with horizontally slotted holes to allow thermal movement while maintaining structural stability, addressing immediate issues and enhancing long-term resilience. Functionality: The original construction did not accommodate creep and deflections of the columns and spandrel panels. Significant shot circuits in the gravity load path led to compression of the wall system traveling thought the aluminum and glass window assemblies. Concrete spandrel spalls due to compression and rotation of the panels resulted in out-of-plane panel conditions. Modifications to the rough openings accommodated isolation of wall and floor loads from the window assemblies. Water Management Strategy: The project team addressed water intrusion, a key cause of corrosion, by implementing a water management solution using sealants and water-resistant coatings. This approach created a moisture barrier, reducing further deterioration and extending the longevity of the structural components, which in turn lowered future repair costs. Fire Stopping: The as-built construction predated fire stopping between floors, raising concerns about “like-kind” reconstruction. Careful detailing allowed for improvements to accommodate fire stopping along the perimeter of the work zone and along floor-to-floor separations. Variations from Proposed Solutions While the initial solutions were designed to address structural concerns, implementation evolved as field conditions were encountered. Vertical and horizontal displacement of various columns and precast panels required modifications to the shoring design to facilitate repositioning during construction. A critical discovery occurred when the sealant between the precast panels and windows was removed. It became evident that the panels and window assemblies were not adequately isolated, leading to their participation in the vertical load path. This resulted in compression of the panels against the window system mullions, causing crushing and spalling failures in the precast panels below. Furthermore, this compression load path contributed to the out-of-plumb conditions observed in both the precast spandrel panels and the aluminum and glass window systems. To address these issues, remedial detailing was developed to decouple the precast spandrel from the window assembly above. Value Engineering and Sustainability The project emphasized value engineering by minimizing disruption to residents and reducing relocation costs. A temporary partition wall isolated the work area, allowing residents to remain in their homes, while a five-foot-wide section of the bedroom was designated for repairs. Mechanical ductwork was relocated in some units to maintain heating and cooling. By shoring the precast panels and constructing dividing walls, residents could occupy their units, significantly lowering relocation expenses. The rehabilitation strategy considered both window removal and reuse, ultimately leading to a salvage-in-place approach. This reuse of materials and water-resistant features aligned with sustainable practices, resulting in cost savings and positive financial impacts for building users.

Value: The involvement of a consultant was crucial to the success of this project, particularly in navigating the unexpected challenges that arose during the demolition and repair phases. Discovery of Hidden Issues As the project progressed, the contractor began the demolition of spalled portions of the precast columns. Removal of the topical coatings revealed concealed cracks that had been patched over which were hidden beneath the coatings. The consultant’s initial assessments had identified some connections as being in fair condition, but the demolition uncovered severe corrosion in areas that had previously been visually inaccessible. Without consultant involvement, these hidden issues could have gone unnoticed, potentially compromising the structural integrity of the building in the future. The Surprising Journey of Existing Buildings Continues! Just when we thought the surprises were over, the team encountered another critical issue; a severely compromised footing due to the softening of clay caused by chronic drainage problems at the building’s rear corner column. Engaging a geotechnical consultant allowed the project team to address this issue effectively. The sub-consultant’s expertise was instrumental in working with us to develop an underpinning strategy to replace the damaged footing and installing a new, properly designed foundation drainage system. This proactive approach not only resolved immediate concerns but also ensured the long-term stability of the structure, demonstrating the value of specialized knowledge in complex situations. Technical Expertise and Innovative Solutions The consultant's ability to innovate was evident in the development of a shoring system that incorporated turnbuckles and articulating legs. This design not only stabilized out-of-plane spandrel panels but the consultant working with the contractor modified the shoring system to accommodate precise adjustments of the precast concrete spandrel panels to achieve plumb and horizontal alignment, that saved costs and minimized disruption to residents as well as time extensions for the project. Such innovative solutions showcased the added value that the consultant provided –transforming potential challenges into opportunities for enhanced structural integrity. Collaborative Framework The project highlighted the importance of collaboration among various stakeholders, including structural engineers, shoring engineers, geotechnical consultants, contractors, and condominium managers. The consultant facilitated this collaboration, ensuring that all parties were aligned in their objectives and strategies. Regular meetings and open lines of communication allowed for the sharing of insights and expertise. The consultant's involvement also extended to educating the project team and the condominium management about the complexities of structural rehabilitation. By demystifying technical concepts and outlining the rationale for certain design choices, the consultant empowered stakeholders to make informed decisions, ultimately leading to a smoother project execution. Importance of IIBEC Member Involvement The participation of IIBEC members further enriched the project by ensuring that best practices in building enclosure design and evaluation were adhered to. IIBEC members brought a wealth of knowledge about water and moisture management, thermal performance, and the longevity of materials, all of which were crucial given the extensive water damage and corrosion issues identified during the investigation.

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