Today we will discuss the most important topic, why we provide crank bars in slabs called bent-up bars. Generally, in the fixed R.C.C ended Slabs, means column at the ends of the supports. Those slabs face Two types of moments that develop, which are called.
- The positive moment called (Sagging) develops at mid-span.
- The negative moment called (Hogging) creates at the supports.
The continuous beam, overhanging beam, etc., all face two types of bending moments, positive and negative, so to resist these moments is beams. In slabs, we provide bars and give technical names, which we discuss in detail below.
1. Bent-up bars or known as Crank bars. We provide the ends on the top of the slab or on the top of the mid supports (if any type of mid-support is present) to counter the negative moment called (Hogging), which produces at that sections of the slabs and beams.
2. Different Angles on the crank bar (Means bends) are provided in the slabs. we just provide Straight Bars at the end on the top of the slab, and in some cases, we provide angles on the slab crank bars, which is 45°. In some cases, the 30° bent is provided when shallow beams are present at the end of the slabs.
3. Normally, the length of the crank bar is a minimum of 300 millimetres, and the slope or bend on the crank bars (Bent up bars) is 1: 10.
4. By providing crank bars in the R.C.C slabs, The Strength of the slab will be increased very much from those of the slabs in which crank bars are not present.
5. The spacing between the main bars with a 12 mm or 10 mm diameter should be 6 inches centre to center c/c, and if you are using 8 mm diameter bars, it should be 4-inch centre to centre c/c.
6. The spacing of distribution bars having a diameter of 10 mm should be 9 inches to 12 inches centre to centre c/c. For 8 mm diameter bar 7 inches to 9 inches centre to centre c/c. The spacing also depends on the span and thickness of the slab, but we usually go with this.
why are crank bars provided in slabs?
- To resist Hogging (Negative Bending Moment) at supports.
- To counter the risk of the failure of the slabs.
- To encounter the shear force. The sheer power is always more significant in the sections of support.
- By providing the crank with the strength of the slab increased.
The crank bars or bent-up bars are essential in R.C.C beam or slab construction because without countering the Hogging (Negative moment), the structure will fail, or the strength of the structure will reduce. The hogging bending moment will develop at the supports of the slabs and beams.
watch the video on YouTube for your better understanding.
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