Conflicts Between The Frame And
Other Trades
Other trades, particularly the
finish trades, must build their product to meet or surround the concrete elements. Their
products also have a set of industry established tolerances. These products are often
plant manufactured with significantly less variation in dimensions and are also often
exposed to public view and have erection tolerances that are much more restrictive than
the hidden concrete elements. It becomes the responsibility of the Architect/Engineer to
consider the differences in tolerances between the various elements of the finished
structure and to develop connection details and clearances to accommodate these
differences.
 |
CHANGE IN
WIDTH CROSS
SECTION DIMENSIONS = 5/8" |
CHANGE IN
WIDTH FORM OFFSET:
CLASS C =1"
CLASS D = 2" |
Fig. 3 Joist
Construction With Lapped Forms
Exterior Conditions
The ACI tolerance for the edge of a
concrete slab is +/- 1 in. The typical 5-in. brick ledge angle leaves about 1/2 in. clear
between the bolt and the brick, Figure 6. Any movement of the building edge outward
encroaches on the insulation and air gap and may require chipping of the brick or removal
of concrete. Chipping the brick weakens the brick and removal of concrete may destroy the
anchorage for the brick angle and reduce the cover on the reinforcing bars at the beam
edge. The Architect/Engineer must develop connection details that would accommodate the
potential 2 in. movement of the slab edge.
The installation of window units to the underside of concrete beams or
slabs, Figure 7, is frequently a cause of tolerance conflict. The tolerance for the
location of the soffit is +/- 3/4 in. The same tolerance applies to the top surface of the
beam which allows for an opening size variance of +/- 1-1/2 in. The window unit
manufacturer anticipating a 1/4 in. sealant joint top and bottom builds the unit to a
tolerance of +/- 1/16 in. The result is 1 in. wide sealant joints or perhaps a unit that
is 1 in. taller than the opening. The solution to this problem may be easier than some. An
expansion channel allowing for +/- 3/4 in. movement was a solution employed 20 years ago
that is still valid today.
Interior Conditions
The installation of floor-to-ceiling partitions presents conflicts
similar to the exterior opening. Attachment of partitions or other elements against the
underside of concrete joist construction must be designed to accommodate the likely 1/2
in. to 1 in. vertical offsets that may occur along the joist soffits.
Fig. 4 Clearance to Slab Below Sloping Surface
OFFSET DUE TO P/T VARIES FROM 0 TO 3/4" PER LEVEL
Fig. 5
Post-Tensioning Movement
