AAI AAI Load Calculations & Structural Requirements 1 — Questions and Answers
Question 1: In ACI 318 anchor design, what does the term 'characteristic bond strength' (Ï„k) represent?
- The 5th percentile bond strength from testing used as the basis for design (Correct answer)
- The average bond strength measured across all test samples
- The maximum bond strength recorded in a test series
- The bond strength at 50% probability of failure
Correct answer: The 5th percentile bond strength from testing used as the basis for design
The characteristic value is the 5th percentile of the test population, meaning 95% of tested anchors will meet or exceed this strength, providing a conservative design basis.
Question 2: Which failure mode for adhesive anchors involves pulling out a cone of concrete around the anchor?
- Concrete cone breakout (Correct answer)
- Bond failure at the adhesive-concrete interface
- Steel rod tensile rupture
- Side-face blowout
Correct answer: Concrete cone breakout
Concrete cone breakout occurs when tensile stress radiates out from the anchor head and fractures the concrete in a conical shape, governed by ACI 318 equations based on embedded depth and concrete strength.
Question 3: How does increasing the embedment depth of an adhesive anchor affect its tensile bond capacity?
- It increases capacity because a larger bonded surface area develops greater total bond force (Correct answer)
- It decreases capacity because deeper holes are harder to clean
- It has no effect because bond strength is a material constant
- It increases shear capacity but decreases tensile capacity
Correct answer: It increases capacity because a larger bonded surface area develops greater total bond force
Bond capacity is proportional to the bonded perimeter (π × diameter) times embedment depth; doubling the depth approximately doubles the bonded area and total bond force.
Question 4: When adhesive anchors are installed in a group, what design reduction factor must be applied?
- A group effect factor (ψg) that accounts for overlapping stress cones (Correct answer)
- A factor of 0.5 applied to all anchors regardless of spacing
- No reduction is needed for groups of more than two anchors
- A reduction only for shear loads, not tension
Correct answer: A group effect factor (ψg) that accounts for overlapping stress cones
Overlapping stress cones in anchor groups reduce the effective breakout area, captured by the group factor ψg in ACI 318 Chapter 17 calculations.
Question 5: What is the significance of the sustained load reduction factor for adhesive anchors under long-term tension?
- Bond strength can creep under sustained tension, so a reduction factor lowers allowable sustained loads (Correct answer)
- Sustained loads always increase bond strength through consolidation
- The reduction factor only applies to shear-loaded anchors
- Sustained load reduction applies only in seismic zones
Correct answer: Bond strength can creep under sustained tension, so a reduction factor lowers allowable sustained loads
Polymeric adhesives can exhibit creep (slow deformation) under constant tension, so ACI 318 and ICC-ES reports require a sustained load reduction to prevent progressive failure.
Question 6: Which load combination controls the design of adhesive anchors according to ACI 318 for a typical gravity load case?
- 1.2D + 1.6L (Correct answer)
- 0.9D + 1.0W
- 1.0D + 1.0L
- 1.4D
Correct answer: 1.2D + 1.6L
For gravity loads, the factored load combination 1.2D + 1.6L typically produces the largest design demand and governs the anchor design per ACI 318 and ASCE 7.
AAI AAI Load Calculations & Structural Requirements 1
In ACI 318 anchor design, what does the term 'characteristic bond strength' (Ï„k) represent?