Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

182 Chapter Six
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Shoring Formwork for Multistory Structures
When concrete is placed for the beams or slabs for a multistory building,
the concrete is supported by decking, joists, stringers, and shores, which
in turn must be supported by the concrete previously placed for the
lower floors, some of which has not gained sufficient strength to support
its floors plus the additional load from the shores resting on it. Thus, it is
necessary to provide a safe method of support that will not endanger the
lower concrete floors. If vertical posts or steel frames are used to support
the decking system, they usually rest on a lower floor, and that floor
should be supported by shores correctly located on the floors below it.
When constructing a multistory building, it is desirable to know
how many lower floors should be supported by shores that are left in
place or are placed there by a reshoring operation. Frequently, this
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decision is left to the superintendent, who may, in his or her desire to
economize by reusing the lower forms before it is safe to remove
them, cause a failure in one or more of the floor slabs.
The question of how long the original shores should be left in place,
how many reshores are required, and how long the reshores should be
left in place, varies with each project. Each structure must be evaluated
by the form designer to ensure safety of workers and compliance with
code requirements. This topic is discussed in “Formwork for Concrete,”
published by the American Concrete Institute [1]. The following are
some of the factors that should be considered in determining the number
of floors that should be shored to support the construction loads:
1. Structural design load of slab or members, including live
load, partition loads, and other loads for which the engineer
designed the slab. Also, any variations in live load that the
engineer may have used in design
2. Dead load weight of concrete and formwork
3. Construction live loads such as placing crews, equipment,
and materials storage
4. Design strength of concrete specified
5. Cycle time between placements of successive floors
6. Strength of concrete at time it is required to support shoring
loads from above
7. Distribution of loads among floor slabs, shores, and reshores
or backshores at the time of placing concrete, stripping formwork,
and removal of reshoring and backshoring
8. Span of slab or structural member between permanent supports
9. Type of formwork systems; that is, span of horizontal formwork
components, individual shore loads, etc.
10. Minimum age where appropriate
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