Structure may subject to adverse effect from local damage from severe local abnormal loads in addition to conventional design load, which are not considered to occur in design life. Such loads include explosion due to gas or industrial liquids, vehicle impact, impact of falling objects and local effect of very winds such as tornadoes. Improving redundancy and ductility of structures by providing minor changes in the detailing of the reinforcement , the overall integrity of reinforced concrete structure to withstand such abnormal loads can be enhanced substantially. This is achieved by providing, as a minimum, some continuity reinforcement or tie between horizontal framing members. In the event of damage to a major supporting element or an abnormal loading event, the integrity reinforcement is intended to continue any resulting damage to a relatively small area, thus improving stability.
General integrity to a structure
Improvement of integrity of a whole structure providing proper ties, creates certain differing observation among engineers for a particular framing system. So providing general structural integrity to a structure is a requirement that can not be stated in simple terms. The code, however , does set forth specific examples of certain reinforcing details for cast-in-situ joist, beams, and two-way slab construction.
Cast-in-place Joists and Beams
When a support is damaged, top reinforcement which is continuous over the support, but not confined by stirrups, will tend to tear out of the concrete and will not provide the catenary action needed to bridge the damaged support. Some catenary action can also be provided, providing a portion of the bottom reinforcement in beams continuous over the supports. By providing some continuous top and bottom reinforcement in edge or perimeter beams, an entire structure can be tied together; also, the continuous tie provided to perimeter beams of a structure will toughen the exterior portion of a structure, should an exterior column be severely damaged.
Fig-1 Continuity Reinforcement for Joist Construction.
Notes:1. Larger of 1/4(+As1) or 1/4(+As2) continuous or spliced with Class A splices
2. Larger of 1/6(-As1) or 1/6(-As2) continuous or spliced with Class A splices
Fig-2 Continuity Reinforcement for Perimeter Beams.
Notes:1. Larger of 1/4(+As1) or 1/4(+As2) continuous or spliced with Class A splices
2. Larger of 1/6(-As1) or 1/6(-As2) continuous or spliced with Class A splices
Fig-3 Continuity Reinforcement For Beams Without Closed Stirrups
The following specification should follow to improve integrity of the overall structure :
a) In one-way slab construction, at least one bottom bar shall be continuous or shall be spliced over the support with a class-A tension splice. At non-continuous supports, the bars may be terminated with a standard hook.
b) Beams at the perimeter of the structure shall have at least one-sixth of the tension reinforcement required for negative moment at the support and one-quarter of the positive moment reinforcement required at mid-span made continuous around the perimeter and tied with closed stirrups. Closed stirrups need not be extended through any joints. The required continuity may be provided with top reinforced spliced at mid-span and bottom reinforcement spliced at or near the support with class-A tension splices.
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