How to measure minimum bar spacing for Bundle Bars in Cast-in Place Concrete as per AASHTO provisions

Minimum Bar Spacing: As per AASHTO LRFD, section 5.10, the minimum bar spacing for bundled bars should be measured on the basis of the acquired diameter rather than computing from individual diameters.

As for instance, if there are 2 nos. 32 mm dia bars, the similar diameter in accordance with 2×32 is 45mm. Therefore, 45 should be employed to utilize the following clauses from AASHTO.

For 2×32 dia bars, clear spacing should be provided as ~68mm for Cast-in Place concrete and 60mm for precast concrete.

Refer following Clauses from AASHTO LRFD 5th Edition 2010

5.10.3.1.1—Cast-in-Place Concrete

For cast-in-place concrete, the clear distance among parallel bars in a layer should not be under :

5 times the minimal diameter of the bars,
5 times the maximum size of the coarse aggregate, or 1.5 in.

5.10.3.1.2—Precast Concrete

For precast concrete fabricated under plant control conditions, the clear distance among parallel bars in a layer should not be under :

The minimal diameter of the bars,
33 times the maximum size of the coarse aggregate, or 1.0 in.

5.10.3.1.5—Bundled Bars

The number of parallel reinforcing bars bundled in contact to function as a unit should not go beyond four in any one bundle, apart from flexural members, the number of bars greater than No. 11 should not go beyond two in any one bundle.

Bundled bars should be attached within stirrups or ties. Individual bars in a bundle, cut off inside the extent of a member, should be abandoned at several points with minimum a 40-bar diameter stagger. Where spacing limitations are dependent on bar size, a unit of bundled bars should be considered as a single bar of a diameter taken from the corresponding total area.

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Published By
Rajib Dey
www.constructioncost.co
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Method of constructing post tension RCC slab

In this construction video tutorial, you will learn how post tension RCC slabs are built up as well as how post tensioning is performed and benefits of post tensioning process.

POST TENSION SLAB: It refers to the slab that is tensioned as soon as the slab is developed. Reinforcement is arranged to avoid the compression.

In Post tension slab, cables or steel tendons are utilized to substitute the reinforcement. Post-tensioning offers a solution to get rid of the natural weakness of concrete in tension as well as optimize its strength in compression.

In concrete structures, this is obtained by arranging high-tensile steel tendons/cables in the element prior to start the casting. If the concrete attains the required strength, the tendons are pulled with special hydraulic jacks and retained in tension with specially designed anchorages which are attached at each end of the tendon.

It creates compression at the edge of the structural member that increases the strength of the concrete for resisting tension stresses.

If tendons are correctly curved to a specific profile, they will exert, other than compression at the perimeter, a useful ascendant set of forces (load balancing forces) that will neutralize applied loads, alleviating the structure from a portion of gravity effects.

In this type of concrete slab, cables are affixed in place of reinforcement. In Steel reinforcement the gapping among bars is 4inch to 6inch while in Post tension slab the gapping is in excess of 2m.

Go through the following video tutorial, to get more details on post tension slab.

Video Source: F&U-FORYOU

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Published By
Rajib Dey
www.constructioncost.co
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Basic differences between Development length and Lap Length

Development length is treated as specific minimum length of the bar that is needed on either side of a point of maximum steel stress, so as to transmit the bar force to encompassing concrete through bond devoid of slip, so that the bar is not pulled out under tension. Hooks, bends, mechanical anchorages are applied to supplement.

Lap length: When the steel is arranged in RC structure, if the necessary length of a bar is not suitably obtainable to produce a design length, then lapping is highly recommended.

Lap length stands for the least length that should have been arranged if two bars are attached together in order that forces are transmitted securely.

As for instance, while going to construct a tall column with height 100 feet. But, it is not possible to find a 100 ft long bar practically for caging. So, it is useful to slice the bars in each second story. Now, the tension forces are transmitted from one bar to the other at the location of discontinuity of bar.

Therefore, it is required to arrange the second bar nearer to the first bar that is suspended and overlapping should be provided. The amount of overlapping among two bars is defined as lap length.

LAP LENGTH IN TENSION.

1. For flexural tension – Ld or 30d either is larger.
2. For direct tension – 2Ld or 30d either is larger.

The straight length of lapping should not remain under 15d or 20 cm.

LAP LENGTH IN COMPRESSION: The lap length in compression should be similar to the development length in compression already worked out but not under 24d.

FOR DIFFERENT DIAMETER BARS: In case of bars with various diameter should have been cut off, the lap length is computed based on shorter diameter bar.

Article Source: quora.com

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Published By
Rajib Dey
www.constructioncost.co
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