Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.25
dropped at one spot, is not considered and might exceed designs based on 50- or
75-psf live load. Formwork should be designed alternatively, with continuity, to
accept such spot overloads and distribute them to various unloaded areas, or with
independently braced units to restrict a spot overload to a spot failure. The first
alternative is preferable.
9.18.3 Lateral Loads for Shoring
Most failures of large formwork are ‘‘progressive,’’ vertically through several floors,
or horizontally, as each successive line of shoring collapses like a house of cards.
To eliminate all possibility of a large costly failure, the overall formwork shoring
system should be reviewed before construction to avoid the usual ‘‘house-of-cards’’
design for vertical loads only. Although it is not always possible to foresee exact
sources or magnitudes of lateral forces, shoring for a floor system should be braced
to resist at least 100 lb/lin ft acting horizontally upon any of the edges, or a total
lateral force on any edge equal to 2% of the total dead loads on the floor, whichever
is larger.
Wall forms should be braced to resist local building-code wind pressures, plus
at least 100 lb/lin ft at the top in either direction. The recommendation applies to
basement wall forms even though wind may be less, because of the high risk of
personal injury in the usual restricted areas for form watchers and other workers.
9.19 FORM REMOVAL AND RESHORING
Much friction between contractors’ and owners’ representatives is created because
of misunderstanding of the requirements for form removal and reshoring. The contractor
is concerned with a fast turnover of form reuse for economy (with safety),
whereas the owner wants quality, continued curing for maximum in-place strength,
and an adequate strength and modulus of elasticity to minimize initial deflection
and cracking. Both want a satisfactory surface.
Satisfactory solutions for all concerned consist of the use of high-early-strength
concrete or accelerated curing, or substitution of a means of curing protection other
than formwork. The use of field-cured cylinders (Arts. 9.7 and 9.14) in conjunction
with appropriate nondestructive in-place strength tests (Art. 9.14) enables owner
and contractor representatives to measure the rate of curing to determine the earliest
time for safe form removal.
Reshoring or ingenious formwork design that keeps shores separate from surface
forms, such as ‘‘flying forms’’ that are attached to the concrete columns, permits
early stripping without premature stress on the concrete. Properly performed, reshoring
is ideal from the contractors’ viewpoint. But the design of reshores several
stories in depth becomes very complex. The loads delivered to supporting floors
are very difficult to predict and often require a higher order of structural analysis
than that of the original design of the finished structure. To evaluate these loads,
knowledge is required of the modulus of elasticity E c of each floor (different),
properties of the shores (complicated in some systems by splices), and the initial
stress in the shores, where is dependent on how hard the wedges are driven or the
number of turns of screw jacks, etc. (‘‘Formwork for Concrete,’’ ACI SP-4). When
stay-in-place shores are used, reshoring is simpler (because variations in initial