Ganjil 2425 - Struktur Beton Bertulang: Perencanaan Geser

Main ideas and lessons (structural shear design in reinforced concrete)

The video explains reinforced concrete beam (and column) design progression:

1. Determine geometry (member sizes).
2. Determine reinforcement amount.
3. Ensure member capacity resists internal forces from statics—primarily bending moment and shear force (and also other actions like axial force and torsion depending on the member).

Bending (moment) vs shear (next “enemy”)

• Moment resistance was handled earlier by adding longitudinal reinforcement.
• The focus now shifts to shear resistance, which is typically more brittle and can lead to sudden collapse.

Design philosophy

Internal forces from statics (e.g., (M_u), (V_u)) must be resisted by nominal capacities:

• For moment (conceptually): ensure required (\le) capacity (often expressed as (M_u \le \phi M_n)).

• For shear: ensure shear strength capacity exceeds demand: [ \phi V_n \ge V_u ] where the lecture discusses (\phi) (typically 0.75 later).

---

Method / key steps for shear design (as presented)

1) Identify governing internal forces

Dominant internal forces considered:

• Shear and moment for beams.
• Columns can also involve axial force and torsion (depending on the member and loading conditions).

Shear design begins after you can draw shear/moment diagrams from the loading.

2) Understand equilibrium relationships (statics basis)

Shear design uses:

• moment equilibrium
• force equilibrium over a segment

The video repeatedly connects changes in shear/moment along distance (dx) to statics equilibrium, forming differential expressions that lead to shear stress/transfer mechanisms.

3) Use concrete shear stress concept (pre-cracking behavior)

Before cracking/oblique cracking:

• shear stress in concrete can be modeled using classical shear mechanics ideas, typically involving forms like: [ \tau \sim \frac{VQ}{Ib} ] (variables may vary by textbook)

Then the video transitions from “elastic, uncracked” behavior to reinforced concrete cracked behavior.

4) Crack mechanics: principal stresses and stress trajectories

Cracking is explained via:

• principal stresses (maximum/minimum normal stresses)
• Mohr circle / principal stress direction concepts
• stress trajectories and their relation to crack patterns

Crack types discussed:

• Vertical flexural cracks (bending-dominated)
• Diagonal/shear cracks (bending + shear; typically diagonal tensile cracking)

Shear failure is described as generally more brittle than flexural failure.

5) Shear transfer mechanism between cracked planes (cracked beam)

After cracking, shear transfer includes contributions such as:

• concrete (shear carried by the compression/resisting concrete)
• dowel/tension reinforcement action
• aggregate interlock
• compression zone contributions

The lecture introduces “flow-like” concepts (shear flow) and notes that:

• when transfer via flow is blocked (e.g., loss of bond or oblique cracking),
• load may transfer more through an alternative mechanism described as curved action (inclined strut + tensile reinforcement).

6) Role of reinforcement: longitudinal + transverse (stirrups)

Longitudinal reinforcement

• Helps resist vertical crack opening/gaping perpendicular to crack planes.
• Effectiveness depends on crack orientation relative to reinforcement direction.

Shear reinforcement (stirrups)

• Required to resist diagonal/shear cracking and prevent dangerous brittle shear failure.
• Can be:
  • vertical (common), or
  • oblique in some cases (e.g., earthquake/practical conditions discussed).

7) Shear strength without shear reinforcement: (V_c)

The video warns that relying too much on concrete shear strength (V_c) increases the risk of brittle failure.

It presents ACI/SNI-style forms of (V_c) with modifiers for:

• normal vs lightweight concrete (a (\lambda) factor)
• influence of axial load (compression increases resistance; tension reduces it)
• aggregate size and roughness/interlocking

8) Shear strength with shear reinforcement: (V_s) and total (V_n)

Total nominal shear capacity: [ V_n = V_c + V_s ]

Key ideas for stirrups:

• Before oblique cracking, stirrup strain may be small (stirrups can be “idle”).
• After cracking opens, stirrups begin to carry shear more effectively.
• Capacity increases with stirrup contribution until ultimate behavior trends toward splitting/dowel-type failure modes.

9) Apply strength reduction factor and compare to demand

Design target: [ \phi V_n \ge V_u ] Strength reduction factor discussed: [ \phi = 0.75 ]

The “enemy” is sliding/shear failure that should not govern prematurely or unexpectedly.

10) Minimum and maximum shear reinforcement requirements (SNI 2847 references)

Minimum shear reinforcement

• Used when shear demand is high compared to concrete capacity.

• Lecture phrasing references a threshold like: [ V_u > 0.5\,\phi V_c ]

• Includes a minimum stirrup area (A_v) expression (with terms such as (\sqrt{f’_c}), spacing (s), effective depth (d), (f_y), plus lower bounds).

Maximum spacing limits

• Upright stirrups: maximum spacing often related to (d/2) (and sometimes tighter limits like (d/4) for earthquake design later).
• Inclined stirrups: spacing limit discussed up to (d) (wording varies by case).

11) Special provisions: shear at distance (d) from the support / shifting (V_u)

SNI rules include evaluating shear demand and allowing discounting/shifting:

• Shear demand may be reduced by evaluating (V_u) at a location relative to the support face (in terms of distance relationships such as (D) vs (d)).
• Discounting is allowed only under specific conditions, such as:
  • loading applied near top,
  • compressive reaction condition,
  • absence of concentrated load at a specified distance,
  • assumptions consistent with cracking/compression zone behavior.

---

Concrete cover (durability requirement related to shear design context)

The video also includes an SNI reminder about minimum concrete cover, which affects effective depth and the moment arm:

• Cover thickness depends on exposure conditions:
  • Normal conditions: commonly around ~40 mm for beams/typical members
  • Some special cases (e.g., certain plate/blind conditions): may use smaller cover (e.g., ~20 mm)
  • Exposure to soil/groundwater/submersion: cover increases significantly (examples: up to ~75 mm)
• Reason: corrosion protection (water/salt water ingress)

Effect on strength: increasing cover reduces effective depth and changes compression/tension arm length, generally reducing moment capacity, so it must comply with code limits.

---

Key “formulas / relationships” explicitly referenced (as concepts)

• Shear stress concept: forms like [ \tau \sim \frac{VQ}{Ib} ]

• Shear design check: [ \phi V_n \ge V_u \quad\text{with}\quad \phi = 0.75 ]

• Concrete shear strength (V_c): includes factors for:

  • normal vs lightweight concrete (via (\lambda))
  • axial compression/tension effects
  • aggregate size/roughness and interlock

• Total nominal shear: [ V_n = V_c + V_s ]

• Shear reinforcement contribution (V_s): depends on (A_v), (f_y), effective depth (d), spacing (s), and stirrup geometry/angle (vertical vs inclined).

• Minimum shear reinforcement:
  • triggered when (V_u) exceeds a fraction of (V_c)
  • includes minimum (A_v) and strength reduction factor requirements (e.g., (\phi = 0.75))

---

Speakers / sources featured

Speakers (within the subtitles)

• Ma’am / the lecturer (main speaker; name not clearly stated)
• Juan
• Joselin
• Re Vin
• Stevun, Sibu
• Indra / Indra Mahisara Pratna
• Anggita / Anggita PR prasasty kusumaardani
• Ruben
• Kate
• Hello Mrs. Suar / Mrs. Suar (referenced in dialogue/attendance)
• Michaelo

Standards / sources explicitly referenced

• SNI 2847 (called “holy book” for concrete course; latest 2019 referenced)
• ACI (American Concrete Institute; referenced alongside SNI design equations)

Indicated equation/figure numbering

• Figure 5.1 through 5.9 (as referenced in the subtitles)