18-07-2012, 11:48 AM
Temporary Structures
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Temporary Structures (TS)
Definition
Any means or methods which provide temporary support, access, enhancement, or otherwise facilitate the construction of permanent structures.
Necessity
TS form the interface of design and construction. Most permanent structures simply could not be built without TS.
Temporary Structures
Impact on Schedule, Cost, and Quality
Losses in time and money will occur if the TS are not planned and coordinated with the same degree of thoroughness as the permanent structures.
Safety
Failure of TS have been responsible for hundreds of deaths on construction sites. Safety should be the overriding priority of contractors and designers responsible for implementing TS.
Categorizing Temporary Structures
Several common types of work which occur on construction sites are:
Concrete Formwork
Scaffolding
False work/Shoring
Rigging
Excavation support
Miscellaneous (site specific)
Design Consideration
Safety
Designers must place the first priority on safety. OSHA (Occupational Safety and Health Administration)
codes, as well as other codes in the industry, provides performance specifications regarding TS.
Cost
TS can be the most expensive part of some construction projects. The designer must have knowledge of all the options which will sufficiently solve the TS problem.
Concrete Formwork (FW)
Forms are TS that support the fresh concrete until it can support itself.
Forms mold the concrete to desired size and shape and control its position and alignment.
The cost of FW is between 40% to 60% of the cost of concrete structure.
Concrete Formwork (FW)
But formwork is more than a mold; it is a
temporary structure that supports:
Its own weight
The freshly placed concrete
Construction live loads (including materials, equipment, and personnel).
Formwork Requirements - 1
Safety – FW must be:
Strong ( to carry the full load and side pressure from freshly placed concrete, together with construction traffic and equipment).
Sound (made of good quality, durable materials).
Formwork Requirements - 2
Quality – FW must be:
Accurate (within specified tolerances for form dimension)
Rigid (adequately braced and tied to prevent movement, bulging, or sagging during construction).
Tight Jointed (to prevent cement paste leakage which disfigures the surface of concrete).
Properly Finished (to provide a concrete surface of good appearance).
Formwork Requirements - 3
Economy – FW must be:
Simple (simple to erect and dismantle)
Easily handled (the sizes of units should not be too heavy to handle)
Standardized (ease of assembly and possibility of reuse)
Scaffolding
Scaffolding has been used to provide access areas for building and decorating structures taller than people who work on them.
The word “scaffolding” refers to any raised platform or ramp used for ingress and egress for pedestrian movement and/or the passage of building materials.
Scaffolding
It is a common practice to use a minimum factor of safety of four in the design of all scaffoldings.
Design Loads:
According to OSHA and ANSI criteria,design load ratings for scaffold platforms are as follows:
Light-Duty Loading. 122kg/m2 maximum working load for support of people and tools
Medium-Duty Loading. 244kg/m2 maximum working load for people and material often
Heavy-Duty Loading. 366kg/m2 maximum working load for people and stored material
Shoring/False work
The shoring which supports freshly placed concrete is necessarily supported by lower floors which may not yet have attained their full strength.
Therefore shoring must be provided for enough floors to develop the needed capacity to support the imposed loads without excessive stress or deflection.