MIN. ALLOWED COVER = 1 1/2"- 1/2" = 1"
ACTUAL COVER = 5/8" < 1" MIN.
Fig. 1 Beam Stirrup
Cover
Formwork And Member Profile
Widely used concrete joist
construction is formed with thin gage inverted "U" shaped metal forms which lap
at joints and have flanges which are nailed to framing material forming the joist soffit.
The lap joints and soffit connection typically have offsets of up to 1/2 in. with
occasional 1 in. offsets (Figure 3). While not meeting the ACI cross-section tolerance of
-1/4 in., +3/8 in. for members up to 12 in. in width, the offsets do meet ACI 117 Class C
(1/2 in.) and Class D (1 in.) tolerances for offsets between adjacent pieces of formwork.
Fortunately, the offsets usually either make the member thicker or do not occur at
locations which would affect the structural integrity of the concrete joist. The end 3 to
6 ft of the joist rib is structurally most vulnerable to thickness deficiency.
This conflict can best be resolved by recognizing that the tolerance
range required with this method of construction is greater than with more costly
alternatives. Class "C" or "D" tolerances should be specified and,
except in areas affecting structural integrity, the cross-sectional tolerances should not
be strictly applied to this type of construction.
Conflicts Between Members
In a long span beam and slab parking
structure, the floor-to-floor height is often 10 ft 0 in. with a 3-ft deep beam. The
minimum clearance specified is 7 ft 0 in. which leaves zero tolerance. The ACI tolerances
on the forming would allow +/- in. for the soffit and top elevation. This could reduce the
clearance by 1-1/2 in.
The problem worsens in the ramp areas, Figure 4, where the slope of the
ramp at 5 or 6 percent grade makes the clearance on the uphill side of the beam 3/8 in.
less than at the centerline. This occurs because the beam soffits are placed level rather
than parallel to the slope of the ramp.
FOR DEPTH A, STIRRUP HEIGHT = 26 3/8"
FOR DEPTH B, STIRRUP HEIGHT = 27"
Fig. 2 Measuring
Beam Depth
Deciding where the beam depth is to be measured
presents a third problem on sloping or ramped surfaces. If the beam depth is measured on
the downhill side, the soffit of the beam is lowered by 3/8 in. which reduces the
clearance by 3/8 in.
The simultaneous occurrence of these three conditions is quite normal.
What was intended to be 7-ft clear becomes 6 ft 9-3/4 in. This is 2-1/4 in. less than
planned. The story height should be increased by 2-1/4 in. to allow for the construction
tolerances.
The use of post-tensioning sometimes compounds the problems of trying
to keep the edge of the building within acceptable tolerance (Figure 5). The usual
procedure would be to cast the first lift columns, then cast the floor slab. Before
tensioning the floor beams and slabs, it is likely that the 2nd lift of columns has been
cast. The tensioning operation compresses the concrete and tends to pull the edges of the
building toward the center or other stable location. The result may be the movement of the
slab by as much as 3/4 in., pulling the first floor columns out of plumb and moving the
2nd lift of columns laterally by this amount. The casting of the 2nd slab and the 3rd lift
of columns followed by the tensioning of the 2nd floor system moves the top of the 2nd
lift of columns an additional 3/4 in. inward for a total movement of 1-1/2 in. As the
building progresses, the offsets progress. A 10-story building could be off by as much as
7-1/2 in. unless corrective measures were taken.
The common remedy is to anticipate the movement and lean the columns
outward and build the slab edge beyond its desired location. Unfortunately, the movement
does not always occur due to the level of the post-tensioning force or constraints in the
building geometry. This is a condition where the Architect/Engineer should advise the
Contractor of the anticipated movement so the Contractor can act accordingly.
