Desain Pelat 2 Arah Beton Bertulang - Part 2 - Pehitungan dengan Metode Koefisien Momen

Summary

The video demonstrates the design and detailing of a two-way reinforced-concrete slab using the moment-coefficient method.

Design example and assumptions

• The interior slab panel measures 6 m × 4 m and is supported by concrete beams on all four sides.
• The beams measure 300 mm × 500 mm and are cast monolithically with the slab.
• Material strengths are (f’_c = 25) MPa for concrete and (f_y = 410) MPa for reinforcement.
• The initial slab-thickness assumption is 120 mm, and the reinforcement bar diameter is 10 mm.
• Using the beam widths to determine the clear spans, the video takes the effective panel dimensions as approximately 5.7 m × 3.7 m.
• The loading is taken from the one-way slab example discussed in the previous video.

Methodology

1. Check the slab thickness

Because the beams and slab are monolithic, the beam is treated as a flanged section when calculating its moment of inertia. The presenter divides the beam section into parts, finds its centroid, and calculates its inertia. This is compared with the slab inertia in each direction.

The resulting average stiffness ratio, (\alpha_{fm}), is approximately 7, which is greater than 2. With a span ratio of approximately (5.7/3.7 = 1.54), the minimum-thickness calculation gives about 124.1 mm. Therefore, the assumed 120 mm slab is insufficient, and the design thickness is increased to 125 mm.

2. Calculate design moments

The slab is treated as a two-way panel supported on all four sides and fully clamped. The presenter selects the relevant Type 2 moment coefficients:

• X-direction support: 76
• Y-direction support: 57
• X-direction span/field: 36
• Y-direction span/field: 17

The design load is stated as 11.48 kN/m². Moments are calculated using:

[ M = 0.001 \times q \times L_x^2 \times \text{coefficient} ]

Support moments are negative, while span or field moments are positive.

3. Design reinforcement for a sample moment

The presenter works through one X-direction support moment as an example; the other locations are designed using the same procedure. For the 125 mm slab, the effective depth is taken as approximately 100 mm.

The calculation balances the tensile force in the reinforcement against the compression force in the concrete, using the equivalent compression block. The required steel area for the worked example is approximately 403 mm²/m, greater than the stated minimum of 250 mm²/m.

With 10 mm bars, the calculated spacing is about 195 mm, so the presenter selects D10 at 175 mm. The resulting installed steel area is approximately 449 mm²/m. The corresponding nominal moment capacity is approximately 15.47 kN·m, exceeding the required moment of approximately 13.96 kN·m. The selected spacing is also checked against the maximum spacing limits cited in the video.

4. Reinforcement schedule

Location Reinforcement X-direction support D10 at 175 mm X-direction field D10 at 250 mm Y-direction support D10 at 250 mm Y-direction field D10 at 250 mm

5. Reinforcement placement

Negative-moment reinforcement is placed near the top over supports, while positive-moment reinforcement is placed near the bottom in the slab field. The drawings use different colors for the X- and Y-direction bars and triangles to indicate whether bars are at the top or bottom.

The video also discusses bent-up bars, which can serve as top reinforcement over a support and bottom reinforcement in the span. Where alternating bars are shown, the drawing may specify D10 at 500 mm for one bar type, producing an effective alternating spacing of 250 mm.

Speakers and sources

• Speaker: One presenter delivers the design walkthrough. The subtitles do not identify the presenter by name; the video is from the Jafar Iskandar channel.
• Technical sources referenced: SNI 2847:2019, a previous video’s loading/design example, and course notes on stress analysis.