Desain Pelat 2 Arah Beton Bertulang - Part 1 Konsep SNI 2847-2019

Summary

The video introduces the concepts and preliminary design rules for two-way reinforced-concrete slabs under SNI 2847:2019. Following an earlier lesson on one-way slabs, it covers slab types, thickness selection, reinforcement requirements, and design moments calculated using the moment-coefficient method.

What Makes a Slab Two-Way?

A two-way slab is supported on all four sides, and its long-span-to-short-span ratio is no greater than 2. Because it bends in both directions under load, it requires reinforcement in both directions.

Common Two-Way Slab Systems

• Beam-supported slab: Beams surround and support the slab.
• Flat plate: A slab supported directly by columns, without beams or local thickening. It is generally suited to shorter spans and lighter loads.
• Flat slab: Similar to a flat plate, but thickened near columns with drop panels. Column capitals may also be used. These features help address two-way (punching) shear.
• Waffle or ribbed slab: Ribs and voids reduce the slab’s self-weight while retaining stiffness, making this system useful for longer spans or heavier loads.

Minimum Slab Thickness

Thickness requirements depend on the slab system and support conditions.

Slabs Without Beams Between Supports

The video gives the following approximate minimum-thickness ratios:

Condition Exterior panel Interior panel Minimum thickness Without drop panels (l_n/30) (l_n/33) 125 mm With drop panels (l_n/33) (l_n/36) 100 mm

Slabs Supported by Beams

Thickness depends on (\alpha_{fm}), the ratio of beam flexural stiffness to slab flexural stiffness, and (\beta), the ratio of the longer clear span to the shorter clear span.

• If (\alpha_{fm} < 0.2), treat the system like a slab without beams and apply the corresponding provisions.
• If (0.2 < \alpha_{fm} < 2), use the code formula for that range, with a minimum thickness of 125 mm.
• If (\alpha_{fm} > 2), use the code formula for that range, with a minimum thickness of 90 mm.

The clear span, (l_n), is measured between the faces of the supporting beams. When the beam and slab concrete have the same elastic modulus, the stiffness comparison can be simplified to a comparison of their moments of inertia.

Minimum Reinforcement and Spacing

Slabs require minimum reinforcement, including reinforcement for shrinkage and temperature effects. The required reinforcement ratio depends on the reinforcing steel’s yield strength; for higher-strength reinforcement, the minimum ratio is determined from the applicable code limits.

The video gives these maximum reinforcement spacings:

• Main reinforcement: the smaller of (2h) or 450 mm.
• Shrinkage-and-temperature reinforcement: the smaller of (5h) or 450 mm.

Moment-Coefficient Design Method

The lesson focuses on the moment-coefficient method for determining ultimate design moments. Other methods—including direct design, equivalent-frame, and yield-line methods—are mentioned but not developed.

The basic calculation is presented as:

Design moment = 0.01 × ultimate load × (short clear span)² × moment coefficient

A panel requires moment calculations in both directions. For each direction, the design considers:

• The support moment
• The span (field) moment

Moment coefficients are selected from code tables based on the panel’s span ratio and the restraint conditions at its edges:

• Simply supported: The slab can rotate at the support.
• Elastically restrained: The slab and beam are cast monolithically, but the beam still allows some rotation.
• Fully restrained: The connection provides substantial restraint against rotation.

The design is illustrated for a 1 m-wide slab strip. Reinforcement designed for that strip is then repeated across the slab panel.

Speaker and Technical Source

• Speaker: One presenter/narrator, not named in the subtitles. The video is from the Jafar Iskandar channel.
• Technical source: SNI 2847:2019, including its slab-thickness and moment-coefficient provisions.