21.3 Anchors, Post-Installed Anchors, and Wood/Masonry Connections
Key Takeaways
- Concrete-anchor design envelopes distinct steel, breakout, pullout, side-face, pryout, shear, and interaction modes; the smallest compatible resistance governs.
- Edge distance, spacing, member thickness, cracked-concrete condition, group geometry, eccentricity, and supplementary reinforcement change anchor resistance and force distribution.
- Post-installed anchor capacity depends on verified product scope and installation, including hole geometry, cleaning, moisture and temperature limits, adhesive cure, setting or torque, and inspection.
- Dowels, bolts, and screws in wood require NDS 2018 ASD checks for fastener and wood yield modes, withdrawal where applicable, grain direction, splitting, spacing, group action, and adjustments.
- Masonry anchorage follows TMS 402/602-16 and IBC 2018, with unit, mortar, grout, cell condition, edge, embedment, installation, and load-path details established rather than assumed.
- An anchor or fastener group must transfer force through the attached plate and substrate into the surrounding member; catalog connector strength alone is incomplete.
Anchors connect an attached part to a substrate whose failure can be brittle and highly sensitive to geometry. For July 2026, use the April 2024 PE Civil: Structural specification, ACI 318-14, TMS 402/602-16, IBC 2018, and NDS 2018 using ASD only for wood. Do not import April 2027 editions.
Cast-In Concrete Anchor Modes
Cast-in anchors include headed bolts, headed studs, and other permitted embedded shapes installed before concrete placement. Resolve applied load into tension, shear, and moment at the attachment. Then identify every applicable resistance.
For tension, common ACI 318-14 modes include anchor steel strength, concrete breakout, pullout, and side-face blowout for applicable headed anchors. For shear, check anchor steel, concrete edge breakout, and pryout. Combined tension and shear can require interaction. These modes are not alternative names for one capacity: each represents a different failure surface or material mechanism.
Concrete breakout depends on projected failure area. Nearby free edges, closely spaced anchors, group overlap, member thickness, and eccentric group demand reduce or reshape that area. Cracked versus uncracked concrete matters under the controlling provisions. Pullout is local around the head; breakout forms a larger concrete cone-type surface; pryout is associated with shear and embedment behavior. Reinforcement crossing a potential failure surface helps only when the ACI model permits it and the bars are positioned and developed to carry the anchor force.
The attached plate also matters. A flexible plate can concentrate force in one anchor, and prying can amplify tension. Do not divide moment or tension equally merely because anchors are symmetrically drawn. Use the force-distribution model specified by the problem and compatible with plate stiffness and contact bearing.
Worked Anchor-Group Envelope
A four-anchor group carries concentric LRFD tension N_u = 42 kips. The problem supplies compatible group design strengths: anchor-steel tension φN_sa = 72 kips, concrete breakout φN_cb = 48 kips, and pullout φN_p = 60 kips. No other mode is stated for this comparison.
The governing design strength is the minimum, not the sum:
φN_group = min(72, 48, 60) = 48 kips
Utilization:
N_u/(φN_group) = 42/48 = 0.875
The stated tension envelope passes, with concrete breakout governing. Dividing 42 kips by four and comparing 10.5 kips only with individual steel strength would miss the group breakout surface. A complete design still evaluates other applicable modes, eccentric distribution, plate behavior, edges, spacing, reinforcement, and combined shear.
Post-Installed Anchors
Post-installed anchors are placed in hardened concrete and can be mechanical expansion, undercut, screw-type, adhesive, or another evaluated system. Their resistance is inseparable from installation. Verify substrate, cracked-concrete scope, diameter, embedment, hole formation, cleaning method, moisture condition, temperature limits, edge and spacing, installation orientation, setting procedure or torque, adhesive dispensing, cure time, and inspection requirements.
Dust left in an adhesive hole or an uncured cartridge can reduce bond even when rod steel is strong. A mechanical anchor installed in a wrong-size hole may not develop its evaluated mechanism. Sustained tension and elevated temperature require the post-installed adhesive provisions and product scope applicable to the problem. Do not substitute a generic catalog value from another product, substrate, or installation condition.
Field-drilled anchors can strike reinforcement or utilities. Relocating a hole changes spacing, edges, group distribution, and plate fit; it requires review rather than improvisation. Abandoned holes and damaged concrete may also affect the new location.
Dowels and Force Transfer
A dowel crossing a joint can transfer shear through dowel action and bearing while also experiencing bending. Reinforcement crossing a concrete interface can provide shear-friction clamping when developed on both sides and detailed under ACI. Do not model every dowel as a pure-shear bolt. Trace load through the joint, reinforcement development, surrounding concrete, and connected member.
Wood Bolts, Screws, and Dowels
For this exam, wood design uses NDS 2018 ASD only. Dowel-type connection resistance depends on wood bearing and fastener bending yield modes, member thickness, fastener diameter and strength, side and main member properties, and number of shear planes. Withdrawal for screws or nails is a different action from lateral yield. Loads at an angle to grain, moisture, duration, temperature, end and edge distances, spacing, group action, and toe-nailing or installation details can alter capacity.
A row of bolts can split wood before every bolt reaches a tabulated single-fastener value. Washers and bearing plates distribute compression, but holes, checks, and shrinkage influence performance. Lead-hole and penetration requirements matter for screws and lag screws. Apply the NDS adjustment and group provisions identified by the question; do not multiply one connector value by count without checking distribution and spacing.
Masonry Anchorage
Masonry anchorage is governed by TMS 402/602-16 with applicable IBC 2018 requirements. Establish whether the anchor bears in a grouted cell, solid unit, hollow face shell, mortar joint, or another approved condition. Unit type and strength, mortar and grout, reinforcement, embedment, edge distance, spacing, wall thickness, and proximity to openings affect the load path. An anchor drawn through a face shell cannot be assumed to develop a strength based on fully grouted masonry.
Cast-in masonry anchors, embedded bolts, and post-installed systems have different installation and approval requirements. Drilling can damage a face shell or miss a grouted cell. Adhesive installation needs the evaluated cleaning, cure, and substrate conditions. Check steel, pullout or bond, masonry breakout, pryout or edge behavior as required by the controlling provision, then transfer force through wall reinforcement and diaphragms or supports.
Final Material Check
Before choosing a capacity, label the substrate: concrete, wood, or masonry. Mark cast-in or post-installed, force direction, edges, spacing, group geometry, installation condition, and design format. Then take the minimum applicable resistance and confirm the attached plate and surrounding member can receive it. That sequence prevents one material's anchor equation from being applied to another.
Anchor Mode Inventory
- Tension: steel, breakout, pullout, and side-face blowout where applicable.
- Shear: steel, edge breakout, and pryout where applicable.
- Group and interface: combined tension-shear, plate prying, welds, and force distribution.
- Substrate and detail: cracking, edges, spacing, thickness, reinforcement, installation, and development.
A concrete anchor group has compatible LRFD tension strengths of 72 kips for steel, 48 kips for breakout, and 60 kips for pullout. What design strength governs these modes?
Which condition is most essential when using a post-installed adhesive-anchor resistance?
Which statement correctly compares wood and masonry connection design for this exam?