Choosing a Structural Building Frame
The architects and developers of Kansas City Vine Street Lofts wanted reasonably accurate construction costs prior to committing to a particular structural building frame. In order to narrow the frame options, CEO Structural Engineers were engaged to design multiple frame system schematics that could each be evaluated by the general contractor for unit pricing. The scope of work required the design of seven structural frame variations including, structural steel, post-tensioned concrete, pre-cast concrete, waffle formed concrete, composite steel, two-way flat plate and even wood framing. All of the schematic designs were submitted to the GC and when the unit pricing was returned, the developer and contractor decided that the most efficient structural frame for the loft project was two-way cast-in-place (CIP) concrete. After the structural frame system had been chosen, CEO engineers determined further value engineering was possible, so they set out to refine the conventional CIP system for cost efficiency and constructability.
Optimized System Depth. Since the thickness of the floor plate has a big affect the overall height of the building, CEO decided the first order of business was to reduce the floor plate system-depth. The system-depth of a structural building frame is the minimum floor-plate thickness package including the deck and structural support beams and girders. In other words, assuming identical floor-to-ceiling heights, buildings with thinner floor system depths will be shorter than buildings with thicker system depths. The system thickness will have even greater labor and material impact on height as a building becomes taller.
The minimum system depth required for a floor-plate with a span of 28 feet often ranges from 24 to 30 inches. Through clever design of the steel reinforcement in conjunction with implementing a “step-beam” concept, CEO engineers were able to limit the system depth of the loft building to 15 inches, shaving off approximately 9 inches in overall floor-plate thickness over a thicker CIP or steel floor system. This design evolution became the CEO Optimized Pan Beam, a surprisingly thin, flat-profile floor-plate with reduced bounce and deflection.
This evolution ultimately reduced the height of the 4-story loft building by 27 inches. For a 4-story building, 27 inches may not seem like a lot, but the reduced height actually saved the developer an estimated $180K in labor and material costs related to concrete, exterior sheathing, glazing and interior finish required for the thicker system. Reducing the system depth benefited the project in many other tangible ways because the thin, flat profile of optimized pan-beam allowed much greater flexibility for architectural partition placement and horizontal service distribution such as HVAC, electrical and plumbing.
CEO Structural Engineers, Inc.