Wednesday, November 2nd, 2016

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Floor Levelness of Elevated Floors: Considerations in Design Part 2

by Jared O.W. Lewis

The built environment is ever evolving to meet the requirements and expectations of the final uses of the facilities.  Many facilities are eliminating floor finishes in favor of exposed or polished concrete, specifying rigid architectural finishes for base molding and soffits, and selecting exterior panelized systems requiring tight construction tolerances.  It is incumbent upon the structural engineer to understand the architectural requirements and owner’s desires early in the design process.   The structural aspects of the specifications are paramount in making construction successful.

Specifications for floor levelness and flatness have changed.  In the past, levelness was measured in terms of a vertical dimension over 10’ and relied upon the use of a rigid straightedge.  The American Concrete Institute no longer uses this technique.  We now specify floor levelness (F­­­L) and floor flatness (FF) in terms of a numerical F value.  FL refers to how much rise and fall occurs in the elevation of the floor surface over the floor area.  The integer value is merely an average of many surveyed points of the floor.  The higher the value, the more level the floor is.  FF refers to how many surface anomalies occur on the floor surface over the floor area.  This is a measure of the quality of the finish of the floor surface.  The higher the integer value, the less the anomalies and better the finish.  Without context of the value definition, they are meaningless and we run the risk of specifying unwarranted or unwanted levelness and flatness values.  Mis-specifying levelness and flatness can have a large impact on construction cost and cause considerable re-work.

How do we know what we should be specifying?  As stated in our last post, good design is part art and part science.  The success of any design comes from knowledge and experience.  At catena consulting engineers, we review our design and specification against the built product to gain better anecdotal knowledge for future projects.  In the context of F-number specifications, we use the following approximate correlation between the straightedge specification and the F-number specification:

jared-tableSource: American Concrete Institute, Specification for Tolerances for Concrete Construction and Materials (ACI 117-10) and Commentary. P. 117-39

While this is a useful baseline tool for specifications, what happens when the specification isn’t enough?  How do we compensate for known deflections? What deflection do we compensate for?  System self-weight?  Design live loads? And how do we ensure that the structure is positioned correctly to accommodate the deflection?  These are all pertinent and necessary questions for any project and the answers vary for every project.

Catena consulting engineers is currently working on the Patriot Hall redevelopment for Clatsop Community College.  To retain portions of the existing Patriot Hall building and provide constructability on a constrained site, the structure utilizes long span elements to support floor structures below.  The architectural finish requirements leave little room for construction tolerance and the structure is exposed.  In this case, we communicated early in the design phases how best to control the final elevations of the floor structure.  We had the benefit of the contractor being involved during design and were able to include their recommendations into the design documents.  In doing so, we were able to reduce the specified FL values within the specification ensuring that we did not spend resources in areas of construction we had made accommodations for elsewhere.  Instead, we communicated and anticipated vertical deflections and adjusted steel elevations prior to the installation of concrete.

Each project is unique and requires unique solutions.  Understanding what an F-number specification means is the first step in knowing whether your project needs a more robust specification or if a solution exists in another aspect of the design and construction process. Perfect answers only exist in the form of collaboration, communication, and trust.

***Photo Credit: Gary Danielson

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