One-Way Concrete Joist Construction - Page 3

However, 14-gauge pans with a depth of 16 inches or greater should always be installed with internal bracing. The soffits of all steel pans should be strengthened with some type of permanently attached internal brace, the most common of which is a welded sheet metal angle at least of the same gauge as the body of the pan. Because the steel lap pan system is characterized by offsets, fins and protrusions as well as chips and Clings that result from the removal of the pan sections, the contract documents should include guidance and information on acceptable tolerances for formed surfaces.

SHARP

45°

ROUNDED
Figure 2 Lap Pan Shapes

Figure 3 Setting Lap Pans

    It should be pointed out that care must be taken with the installation of any embedded items or mechanical inserts or fixtures. If the attachment of these items is not considered in relationship to the stripping of these forms they may act as anchors preventing the removal of the pan forms. Therefore, it is recommended that only center pin soft rivets be used. The center pin of the rivet will remain exposed on the underside of the pan form and can be removed prior to stripping. This will allow the body of the rivet to close as the pan is stripped, permitting the easy removal of the forms.

TOLERANCES
   Tolerance guidance can be found in several ACI standards and reports. ACI 117, "Standard Tolerances for Concrete Construction and Materials"; ACI 301, "Specifications for Structural Concrete"; and ACI 347R, "Guide to Formwork for Concrete," provide information on finished surfaces, but do not address pan joist surfaces specifically. ACI 117 and ACI 347R limit offsets and other irregularities based on "Class" of surface finish. See Table 2. The ACI 117 standard might be regarded as the most authoritative. The mandatory specification checklist in ACI 117 requires the Architect/Engineer to designate the intended Class of surface finish and thereby establish the tolerance for form offsets. ACI 301 addresses the finishing of formed surfaces in Chapter 2 and differentiates between rough form finishes (those not exposed to public view) and smooth finishes (exposed to public view). ACI 301 requires: "Patch tie holes and defects. Remove all fins completely." for smooth formed finishes, but permits up to 1/4 inch fins for rough finishes. As a default, Article 5.3.3.5 of ACI 301 calls for the finish to be based on exposure to public view where surface finish is not designated in the contract documents.

TABLE 2 Surface Finish Class

    ACI Committee 347 notes that revisions of the 347R report are in progress to change the limit for abrupt offsets within Class C finish to 112 inch, consistent with ACI 117. Although ACI 347R cautions against using pry bars directly against concrete to remove formwork, this is common practice in many areas of the country.

CONCLUSION
   Surface irregularities should be expected in pan joist construction. It is difficult to patch surface spalls successfully. The patch may be more noticeable than the spall. The continuing use of the steel lap pan joist form system remains a proven and excellent method of forming site cast reinforced concrete floor systems in non-critically exposed applications.