Macarena Schachter Adaros

Use of Ring Beams for Progressive Collapse Retrofit

The renovation of an existing office building to meet the progressive collapse requirements according to UFC4-023-03 is presented. The original building was composed of two structurally independent structures, both of which housed three indoor tennis courts. Both structures were column free in the interior and were constructed of built up wide flange perimeter columns and 6ft deep roof trusses for lateral and gravity load. A first building renovation transformed the original structure into two 3-story office buildings by adding a 2-story steel frame structure inside the original space. A second renovation added steel framing between the two buildings to create a single structure and provided support for a new cladding system. The office building relies on stair masonry walls for lateral support. Existing structural drawings were available only for the second renovation. In order to meet the UFC4-023-03 Progressive collapse guidelines, a system of hangers and ring beams was designed. In most cases the steel columns were used as hangers and their connections strengthened accordingly; in some cases rod hangers were provided. Ring beams were located at various levels depending on the column layout and additional blast retrofit requirements. In all cases, the ring beams were designed as continuous beams supported by the existing columns who behave as pins. This assumption results in heavier beams but avoids additional retrofit for the columns and the use of brittle and expensive moment connections. This paper presents details of the implemented retrofit as well as some of the challenges encountered during the design phase: the complexity of the geometry of the different structural systems, the definition of how many columns to be simultaneously removed and the applicability of the linear elastic method to the design of multiple elements of different ductility.

Three Dimensional Formulation of Large Displacement Problems: The Zipper Frame Example

A zipper frame is a conventional braced frame with chevron type braces at all floors, connected through a zipper column linking their midpoints. Under lateral force, a 3 stories frame is expected to behave as follows: after the 1st floor braces buckle, the zipper column transmits the unbalance vertical force to the 2nd story, forcing them to buckle. The new unbalanced vertical force is then transmitted to the third story, which is designed to remain elastic. This is known as the zipper mechanism. Three shaking table tests were performed to evaluate the seismic response of a 3 story suspended zipper frame model. In every shaking table test performed, the braces buckled out of plane but the zipper mechanism did not always develop. Following the test results, a new analytical tool was developed to capture the 3 dimensional phenomena of buckling. It is based on the well known corotational formulation. Every element is described independently. The rigid body modes are removed by using a corotated coordinate system. Within the deformed shape, the element can undergo large rotations and large strains, which are evaluated respect to the corotated origin. The structural problem is solved in the state space, where displacements as well as velocities are variables. The final set of equations is a combination of differential and algebraic equations that is solved using the IDA software. This paper presents the main results of the shaking table tests as well as some general description of the proposed formulation and its capability to verify the shaking table findings.