Howard L. Zimmerman Architects, P.C. was retained to conduct a forensic investigation following the collapse of recently installed courtyard curtain wall during conversion of an industrial building for residential use. The investigative work included on-site inspections of the failed conditions at the building and review of structural and architectural drawings. Subsequent to the investigation, HLZA prepared an Evaluation Report incorporating observations, possible causes and providing recommendations for corrective work.

Observations
A 4-story-high curtain wall in the courtyard had separated from its supporting elements and collapsed onto courtyard roof below. During our investigation we located wall-supporting elements that had failed. The supports were fabricated with light gauge stainless steel clips that were attached with screws to both the steel beam and the curtain wall metal studs. It was noted that the screws had pulled away from the clips causing the curtain wall to break up from the building's structure. In addition, it was determined that too few clips were installed. The means and quantity of attachments utilized appeared inadequate and failed to withstand the "self-weight" of the curtain wall and lateral (wind) forces acting upon this structure.

Further, it was determined that additional wind pressure through large window openings (over 50% of floor wall area) left unprotected during window replacement was acting upon this wall, effectively doubling lateral load. Finally, due to lack of provision to accommodate vertical deflection of the steel frame, it was concluded that part of the floor load was transferred to the stud wall and contributed to connection failure. Investigation of the adjoining walls with similar type of construction confirmed that they too lacked adequate connections and could potentially result in future damage to building elements and finishes.

Recommendations
HLZA concluded that the curtain walls should be rebuilt and reinforced with proper attachment to supporting structural elements providing lateral stability and accommodating vertical movement of steel frame. It was stipulated that this work was to be inspected before fireproofing and finishes were applied to the interior of the new wall.

Corrective Work
The collapsed curtain wall was rebuilt using a new means of attachment. The new steel angles were tack welded to the steel studs at each floor and bolted to the existing steel spandrel beam through slotted holes to provide for differential movement.

The other courtyard walls have also been reinforced with additional steel angle attached to the existing steel spandrel beams at each floor and tack welded to the metal studs.