Concept: Structure and Deformation

Based on a decision to span the subsurface roadway, the proposed structure will have to be lightweight and high-strength.  As a design goal, I want to avoid simply spanning the gap with a slab-on-truss structure.  Since the building will be seen from below, I want to employ a structural system which allows the bottom façade to relate to the other elevations.  As I mentioned in studio, I am proceeding with the courtyard form since it allows (besides program considerations) a concentration of structure around the site perimeter which (I hope) would allow me to combine structure and form into a coherent façade.

To address these requirements, I have been researching a hybrid of lightweight, post-tensioned concrete shell construction and typical high-strength concrete slab construction.  Capable of spanning large spans, a concrete slab of varied thickness is formed to create shallow arches (which will be evident in the interior program).  This undulating structural shell would touch down at foundations anchored to existing structural framework and some new shear walls or columns along the perimeter.

I have been exploring this form and concept in a series of digital models subjected to stress and deformation analysis.  The attached animation shows clearly the main areas of instability in the structural slab (high-strength H50/60 concrete) based on my preliminary design.  The model was subjected to gravity loads as well as typical live/dead loads (150 psf LL for libraries).  The animation exaggerates the combined loads to 210x allowable deformation (way beyond failure).  Since I can’t provide a structural engineering solution to these areas of instability, the analysis tells me where to focus on trimming back variables (perhaps reduce the live load, make the floor plan narrow to trim back the slab, or adjust the areas of slab lifting). 

[Click to animate] : Deformation of preliminary concrete shell design.

The second component of the proposed structural system is the wall/roof assembly.  I am still exploring options for this component but have narrowed it down to two types: a structural façade or a post and beam system.  Both options would be fixed to the structural slab.  The structural façade has the advantages of allowing a column-free interior and a rigidity that could reinforce the floor slab.  Since the building will be one-story, the post and beam strategy could be a light-weight alternative to reduce floor loads.  I will continue to work on some diagrams which illustrate these options.