TSA was contacted to determine the cause of floor distress in an imported limestone floor at a retail center. A reason for the distress that was proposed by others related to excessive structural deflection. To evaluate this claim, N-J performed an assessment of the structural framing system and reviewed the relevant technical standards for deflection requirements.
A problem with the different standards is that they appear to use similar language, but they often associate their language with slightly different definitions to suit their need. For example, the Marble Institute of America (MIA) recommends that the floor deflection limit be L/720 for wood floors and L/360 for concrete floors. The Tile Council of North America (TCA) recommends a deflection limit of L/360. The International Building Code (IBC) has several different deflection limits that apply depending on the structural material, but for a concrete floor with would be L/360 for live load or L/240 for total load, regardless of the floor finish. The problem is that these standards calculate the load used to determine the deflection in different ways because they have different purposes for their deflection limits.
The IBC specifies a uniform load of 100 pounds per square foot. The TCA uses a rotating 300-pound load instead of a uniform load, which produces much different results than the Building Code analysis.
Even though they reference deflection in the TCA and MIA standards, cracking of tile is only indirectly related to floor deflection. It is actually related to floor curvature under load. The difference is subtle, but substantive. In a structure with long spans, it becomes nearly impossible to come up with a scenario that causes cracking, while a short span structure with a similar deflection ratio could have cracking.
In this case, the span was quite long, so structural deflection was ruled out as a source of the distress. After removing selected tile, it was determined that the floor was not correctly installed. The cracking was related to use of a topping material that was improperly place on top of instead an anti-crack membrane instead of below the membrane as required by the manufacturer. The contractor also used screed pins to set the topping mortar depth; cracks in the tile were strongly associated with the screed pins.
