When evaluating horizontal surfaces for differential displacement or excessive deflection, non-destructive diagnostic techniques help to minimize disruption to both the structure and its users. In most cases such techniques are capable of yielding sufficient information for a qualitative evaluation without the need for destructive testing.

The most common and most reliable form of non-destructive diagnostic test is the Horizontal Surface Mapping. Such mapping is typically performed with the aid of a water or gas level and is capable of identifying and quantifying which areas have displaced and by how much.

According to the "ACI Manual of Concrete Practice, Part 2", as published by the American Concrete Institute, construction standards allow for finishing defects within cast-in-place concrete slabs of up to a 1/2" vertical variance within a 10' horizontal distance. Therefore, fluctuations of up to 0.5 may be attributable to initial construction.

Horizontal surface mapping, often referred to as a floor survey, can be performed on different types of construction including but not limited to, concrete slabs, elevated floor systems, flat roof systems and ceilings. When a horizontal surface survey is used on an elevated floor system and/or roof system, it can be used to evaluate potential deflection of floor/roof members or used to evaluate roof pitch on existing flat roof systems for roof drainage purposes.

According to the "Residential Construction Performance Guidelines - Homeowner's Reference", as published by the National Association of Homebuilders construction standards allow for finishing defects within wood framed floors of up to a 1/4" vertical variance within a 32" horizontal distance. Therefore, fluctuations of up to 0.3 may be attributable to initial construction.

FBC 1507.10.1 Slope. Built-up roofs shall have a design slope of a minimum of one-fourth unit vertical in 12 units horizontal (2-percent slope) for drainage, except for coal-tar built-up roofs that shall have a design slope of a minimum one-eighth unit vertical in 12 units horizontal (1-percent slope).

Once the data is collected and processed it is then used to develop a graphical presentation. One way in which to display the data is to develop a topographic map of the surveyed area. The topographic map can then be superimposed on to an existing floor plan. This map illustrates for the user a clear representation of the areas of the structure that have evidenced differential displacement.

Figure 1: Topographic mapping of a structure with data point locations

Because Horizontal Surface Mapping is only as valuable as the methods and procedures employed, a recognized standard must be followed. The standardization process began when the American Society of Civil Engineers (ASCE) 3rd Forensic Congress published a peer reviewed paper on guidelines to performing a Horizontal Surface Mapping. This paper can be found at http://cedb.asce.org/cgi/WWWdisplay.cgi?0303233. Most recently, the Florida Association of Sinkhole Stabilization Specialists (FAS3) www.FAS3.org is preparing to release an industry standard on how to perform Horizontal Surface Mapping.

If you would like information on horizontal surface mapping, our Structures Group is ready to assist you. Please contact Jose Busquets at jbusquets@brackenengineering.com.

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