Smart-Book • ACI 318
ACI 318 RC Beam Calculation Prompt + Workflow
Copy the ACI prompt, fill your beam data, then run the calculation. This post also shows the exact workflow used for loads → wu → Mu, Vu → flexure → shear → detailing.
ACI Prompt for RC Beam Calculation (copy/paste)
Act as a structural engineer and design an RC beam per ACI 318 strength design (LRFD).
Use φ factors, ACI load combos, and show steps clearly.
Inputs:
- Beam system: [simply supported / continuous / cantilever]
- Span L = [m]
- Section: b = [mm], h = [mm], clear cover = [mm]
- Stirrups size = [R8/R10] and main bar sizes available = [DB12/DB16/DB20/DB25]
- Concrete f’c = [MPa] (or psi)
- Steel fy = [MPa] (or psi)
- Slab tributary width = [m], slab thickness = [mm]
- Finishes = [kN/m²], live load = [kN/m²]
- Wall on beam? [yes/no], wall thickness = [mm], wall height = [m], unit weight = [kN/m³]
Required outputs:
1) Convert all loads → line load (kN/m), including beam self-weight
2) Factored load w_u using ACI combos
3) Max M_u and V_u
4) Flexural design: compute A_s required, select bars, check tension-controlled (εt), φ
5) Shear design: compute V_c, V_s, stirrup spacing s limits
6) Provide final detailing notes: bottom/top bars, stirrup spacing zones, anchorage/lap guidanceACI 318 Workflow — 1) Effective depth (d)
Quick estimate used before detailed bar arrangement.
d ≈ h − cover − stirrup_dia − 0.5 × bar_dia2) Loads → line load (kN/m)
Convert everything to line load on the beam using tributary width.
Beam self-weight:
w_beam = b · h · γ_c (use γ_c ≈ 24 kN/m³)
Slab dead load on beam:
w_slab = (t_slab · γ_c + finishes) · (tributary width)
Wall load (if any):
w_wall = t_wall · h_wall · γ_wall
Live load on beam:
w_L = (LL) · (tributary width)
Total dead load:
w_D = w_beam + w_slab + w_wall
Common ACI factored combo:
w_u = 1.2 w_D + 1.6 w_L3) Actions (Mᵤ, Vᵤ) — quick formulas
For simply supported beam with UDL. For continuous beams, use moment coefficients or your analysis.
If simply supported with UDL:
M_u = w_u L² / 8
V_u = w_u L / 2
For continuous beams:
- Use moment coefficients (if applicable), OR
- Use analysis results you provide (preferred for accuracy).4) Flexural design (ACI, rectangular, singly reinforced)
Typical: φ = 0.90 for tension-controlled flexure (most beams). Check εt to confirm.
Strength factor (typical):
φ = 0.90 (tension-controlled)
Compression block depth:
a = (A_s f_y) / (0.85 f’_c b)
Nominal moment:
M_n = A_s f_y ( d − a/2 )
Design requirement:
φ M_n ≥ M_u
After solving A_s required:
- Select practical bars (example): Bottom: 2DB20 + 1DB16
- For continuous beams: add top steel over supports for negative moment
Also check:
- ACI minimum flexural steel (A_s,min)
- Bar spacing practicality / constructability5) Shear design (stirrups)
Shear strength uses φ = 0.75 (typical).
Strength factor (typical):
φ = 0.75 (shear)
Design requirement:
φ (V_c + V_s) ≥ V_u
Stirrup contribution:
V_s = (A_v f_y d) / s
Solve for spacing:
s = (A_v f_y d) / V_s
Apply ACI limits:
- tighter spacing near supports
- overall maximum spacing limits (and minimum shear reinforcement rules)6) Final output format (detailing notes)
This is the site-friendly format you’ll receive after calculations.
Deliverable will include:
- Loads summary: w_D, w_L, w_u (kN/m)
- Actions: M_u (kN·m), V_u (kN)
- Flexure: A_s required, provided bars, φ check, notes
- Shear: V_c, V_s, stirrup size and spacing zones
- Detailing note:
* Bottom bars: ___ (continuous through span unless noted)
* Top bars: ___ (over supports for negative moment where applicable)
* Stirrups: ___ @ ___ near supports; ___ @ ___ midspan
* Cover: ___ mm
* Anchorage/lap: practical guidance + where to place lapsFill & Copy (calculate your beam fast)
Fill these 10 items, then copy the filled ACI prompt.
Preview (filled ACI prompt)
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Tags:
RC