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LecturesModule 16

Base Plates and Anchor Rods

Column base plate design and anchor rod sizing (AISC Design Guide 1).

AISC Reference Box
  • AISC 360-22 §J8 — Column Bases and Concrete Bearing
  • AISC Design Guide 1 — Base Plate and Anchor Rod Design

Lecture Notes

This module introduces base plates and anchor rods. Lecture content here covers the governing physics, LRFD philosophy, and how the relevant AISC 360-22 chapter organizes the limit states.

Instructors can replace this text in Admin Mode. Each section is structured around: (1) behavior, (2) failure modes, (3) AISC limit-state equations, (4) design workflow, (5) detailing requirements.

A short comparison to ASD is included only where the resistance factor / safety factor relationship clarifies the LRFD design check.

Project case study — Cardinal Square — 4-story braced-frame office

Every chapter's worked example is one step in the design of the same building: Plan: 4 bays N–S × 3 bays E–W, each 30 ft × 30 ft. Stories: 4 @ 13 ft (52 ft roof). Composite floor: 4.5 in NW concrete on 3 VLI20 deck. Roof: 1.5 in B-deck + insulation + membrane. Materials: Wide-flange members A992 (Fy = 50 ksi, Fu = 65 ksi). Plates A572 Gr. 50. HSS bracing A500 Gr. C. Bolts A325-N 7/8 in dia. Welds E70XX. Concrete f'c = 4 ksi. Anchor rods F1554 Gr. 36.

Chapter 16 — Column base plate
Base plate under the same interior column from Chapter 5
Demand carried forward
From Chapter 5: Pu ≈ 612 k bearing on f'c = 4 ksi.
This chapter contributes
Sizes plate N × B for concrete bearing per §J8 and the Design Guide 1 cantilever model. Picks plate thickness tp from the m, n, λn' equation.
Feeds into next chapter
Anchor rod sizing handles the brace-bay uplift from Chapter 4.
PuN×B plate0.85f'c bearing on A1; anchor rods for uplift / shear
Column base plate on pedestal: concrete bearing (φc·Pp) and anchor rods for uplift/shear (AISC DG-1).

Formula Sheet

NameEquationAISC Ref
Concrete bearingφc Pp = 0.65 · 0.85 · fc' · A1 · √(A2/A1) ≤ 0.65 · 1.7 · fc' · A1AISC §J8

Worked Example

Base Plates and Anchor Rods

Given
Replace with project-specific given data (loads, geometry, material).
Load combination
Controlling LRFD load combination from ASCE 7.
Required strength
Compute required strength Ru from the controlling combination.
Limit states
  • Limit state 1
  • Limit state 2
AISC reference
AISC 360-22 — applicable chapter
Solution steps
  1. 1. Required strength
    Compute Ru.
  2. 2. Trial section
    Pick a trial from AISC shape tables Instructor should verify with official AISC Manual.
  3. 3. Check each limit state
    Apply φ Rn ≥ Ru for every governing limit state.
  4. 4. Iterate
    Resize until the most economical section satisfies all checks.
Final design decision
Select the lightest section that satisfies all LRFD limit states.
Common mistakes in this example
  • Skipping a limit state
  • Using the wrong φ factor
  • Forgetting serviceability checks

FE-Style Worked Examples (6)

Each example mirrors the NCEES FE Civil Reference Handbook style: brief givens, a labeled figure, AISC section reference, step-by-step numeric solution, and a single boxed answer.

Given
BP 16×16 (A1=256 in²); pedestal 24×24 (A2=576 in²), fc'=4 ksi.
AISC Reference
AISC §J8
Step-by-step solution
  1. √(A2/A1)
    √(576/256) = 1.5 ≤ 2 ✓
  2. φPp
    0.65 × 0.85 × 4 × 256 × 1.5 = 849 k (≤ 0.65×1.7×4×256 = 1131 k) → use 849 k
Answer φPp = 849 k.
PuN×BColumn base plate on pedestal

Textbook — Aghayere & Vigil (2009)(3 worked examples with figures + numerical answers)

Worked examples scanned directly from the CEGR 436 course textbook. Each card shows the original page (figure + full step-by-step solution) and adds an FE-style numerical multiple-choice prompt with answer key.

Chapter summary

Chapter 8 §8.13–§8.18 of the textbook covers base plate and anchor rod design (AISC Design Guide 1). The plate transfers column axial load and moment into the pier via bearing; anchor rods carry uplift and shear from wind/seismic.

  • Concrete bearing: φcPp = 0.65·0.85·fc'·A1·√(A2/A1) ≤ 0.65·1.7·fc'·A1 (AISC §J8).
  • Plate thickness for axial load: tmin = ℓ·√(2·Pu/(0.9·Fy·BN)) per DG1.
  • Anchor rods: shear via friction + bearing on rod, or use shear lugs (DG1 §3.5).
  • Uplift: rod tension capacity = 0.75·Fnt·Ab per AISC §J3.6.
  • Always check the leveling-plate / grout pad thickness (≥ 1 in typical).
Setup
Base plate B×N = 18×18 in on a 30×30 in concrete pier, fc' = 4 ksi.
AISC Reference
AISC §J8 / DG1
Numerical practice
φcPp (kips)?
Textbook page 414 — Concrete bearing capacity
Aghayere & Vigil (2009), p. 414 — full worked solution & sketch.

Interactive Calculator

Base Plate Concrete Bearing

AISC §J8
A1 = B·N196.0 in²
φc Pp612.6 kipsOK

Practice Problems

  1. [E] State φc = 0.65 for concrete bearing.
  2. [E] State φc·Pp = 0.65·0.85·f'c·A1·√(A2/A1).
  3. [E] Sketch base plate showing N x B dimensions.
  4. [E] List four anchor-rod ASTM specs (F1554 Gr 36, 55, 105; A36).
  5. [E] State cantilever lever-arm distances m and n.
  6. [M] Size base plate for Pu = 400 k, W12x72, f'c = 4 ksi, A2/A1 = 4.
  7. [M] Required plate thickness using cantilever method (above).
  8. [M] Anchor-rod design for 50 k uplift (Combo 6) at a 16 x 16 in. base plate.
  9. [M] Shear-key design for Vu = 20 k at an interior column base.
  10. [M] Anchor-rod pull-out check per ACI 318 Appendix D for two 1 in. F1554 Gr 55 rods.
  11. [H] Full base plate: Pu = 800 k + Mu = 60 k-ft, W14x90, f'c = 4 ksi, A2/A1 = 2.25.
  12. [H] Base plate under axial + uplift + shear: plate thickness, anchor rods, shear key.
  13. [H] 30 x 30 in. concrete pier with 4 #8 verticals: check concrete breakout per ACI 318-19 Ch. 17.
  14. [H] Large-e base plate (e > N/6): bearing area + anchor-rod tension per DG-1 Method 2.
  15. [H] Portal-frame base: design for Mu = 250 k-ft, Pu = 220 k with leveling nuts.

Quiz

1. Which AISC 360-22 chapter primarily governs base plates and anchor rods?
2. In LRFD, the basic design inequality is:

Common Student Mistakes

  • Mixing ASD and LRFD load combinations in the same problem.
  • Using nominal strength Rn instead of design strength φRn.
  • Forgetting to check every limit state listed in the AISC chapter.

"Professor Explains" Script

Today we're talking about base plates and anchor rods. Think of this topic as one step in the LRFD workflow: identify the demand, identify the limit states from the relevant AISC chapter, then check that φ·Rn is at least equal to Ru. We'll walk through the failure modes, the equations, and a worked example. Pay close attention to where the resistance factor changes — that's where students lose points on exams.