Anchorage Methods

Overview

Anchorage methods secure HVAC equipment and components to building structures, providing resistance to seismic forces, wind loads, and operational vibrations. Proper anchor selection and installation ensure structural integrity, code compliance, and system reliability throughout the equipment lifecycle.

Concrete Anchor Types

Expansion Anchors

Expansion anchors develop holding capacity through radial forces against concrete walls:

Sleeve-Type Expansion Anchors:

• Conical sleeve expands against drilled hole sides
• Torque-controlled installation: 50-100 ft-lb typical
• Allowable tension: 1,500-5,000 lbf depending on size and concrete strength
• Minimum embedment: 3 to 4 inches for 3/8 to 5/8 inch diameter
• Not suitable for shallow embedments or cracked concrete
• Requires 3,000 psi minimum concrete strength

Drop-In Anchors:

• Flush-mount design for through-bolt applications
• Setting tool expands internal cone against anchor body
• Allowable tension: 1,200-3,500 lbf for 1/4 to 3/4 inch sizes
• Embedment depth: 1.5 to 2.5 times anchor diameter
• Cannot be removed after installation
• Pre-drilled holes required before concrete pour completion

Wedge Anchors

Wedge anchors provide superior holding strength through mechanical wedging action:

Design Characteristics:

• Cone-shaped wedge expands into anchor sleeve during tightening
• Single-expansion mechanism ensures consistent performance
• Allowable tension: 2,000-12,000 lbf for 3/8 to 1 inch diameter
• Allowable shear: 1,500-10,000 lbf depending on size
• Minimum embedment: 4 times anchor diameter
• Torque specification: 75-200 ft-lb based on diameter

Installation Requirements:

• Hole diameter: Anchor diameter + 1/16 inch tolerance
• Drill depth: Embedment depth + 1/2 inch minimum
• Concrete strength: 2,500 psi minimum at installation
• Clean hole thoroughly with wire brush and compressed air
• Thread engagement: Minimum 3 threads beyond nut face

Adhesive Anchors (Epoxy)

Adhesive anchors transfer loads through chemical bonding:

Two-Component Epoxy Systems:

• Highest load capacity of all post-installed anchors
• Ultimate tension: 8,000-20,000 lbf for 5/8 to 1 inch threaded rod
• Effective in cracked concrete applications
• Cure time: 1-24 hours depending on temperature
• Service temperature range: -40°F to 250°F

Installation Process:

1. Drill hole 1/8 to 1/4 inch oversize of rod diameter
2. Clean hole with wire brush (3 complete cycles)
3. Blow out debris with oil-free compressed air
4. Insert epoxy cartridge in dispensing tool
5. Inject adhesive from bottom to top, filling 2/3 of hole depth
6. Insert threaded rod with twisting motion
7. Allow full cure before loading

Critical Parameters:

• Embedment depth: 10 to 15 times rod diameter for full capacity
• Edge distance: 6 inches minimum for 3/4 inch rods
• Spacing: 12 inches minimum between anchors
• Substrate temperature: 40°F to 110°F during installation

Cast-In-Place Anchors

Cast-in anchors provide maximum load capacity through integral concrete embedment:

Types:

• Headed studs: Welded to embed plates, 5,000-25,000 lbf capacity
• L-bolts: Bent threaded rod, economical for light loads
• J-bolts: Hooked end prevents pullout, 3,000-15,000 lbf
• Slotted channel systems: Adjustable positioning, 2,000-10,000 lbf per connection

Design Advantages:

• Highest load capacity per anchor
• No concrete damage during installation
• Inspectable before concrete placement
• Suitable for high seismic and dynamic loads
• Embedment depth: 12 to 18 times anchor diameter

Installation Coordination:

• Precise template fabrication required
• Anchor locations surveyed and verified
• Protection during concrete pour critical
• Thread protection caps prevent damage and contamination

Undercut Anchors

Undercut anchors create mechanical interlock through enlarged embedment cavity:

Performance Characteristics:

• Load transfer through concrete bearing at undercut
• Superior performance in cracked concrete
• Allowable tension: 3,000-15,000 lbf for 1/2 to 1 inch diameter
• Reduced edge distance requirements: 4 to 5 times diameter
• Approved for seismic applications in critical facilities

Installation:

• Carbide bit drills standard cylindrical hole
• Undercutting tool enlarges bottom of hole
• Anchor body expands into undercut during installation
• Torque: 100-250 ft-lb depending on size

Through Bolts

Through bolts penetrate full structural element thickness:

Applications:

• Thin concrete slabs (4 to 8 inches)
• Steel structures with drilled holes
• Sandwich wall panels
• Locations requiring removable connections

Capacity:

• Limited by concrete bearing or steel plate bearing
• Typical allowable tension: 2,000-8,000 lbf
• Shear capacity: 3,000-12,000 lbf with proper washers
• Requires access to both sides of substrate

Anchor Embedment Requirements

Embedment Depth Calculations

Minimum embedment depth ensures adequate load transfer:

Tension Loading:

• Embedment depth ≥ 12 × anchor diameter (concrete breakout mode)
• Deeper embedment increases tension capacity
• Reduced embedment requires capacity reduction factors

Shear Loading:

• Edge distance governs shear capacity
• Minimum edge distance: 7 × anchor diameter for full shear capacity
• Embedment depth ≥ 6 × diameter prevents pryout failure

Combined Tension and Shear:

• Unity equation: (T/T_allow)^1.5 + (V/V_allow)^1.5 ≤ 1.0
• Both load components must satisfy interaction criteria

Edge Distance and Spacing

Minimum Edge Distance:

Spacing Requirements:

• Minimum spacing prevents group effect reduction
• Closer spacing reduces individual anchor capacity
• Group capacity ≠ sum of individual capacities when spacing < 12 diameters

Anchor Load Capacity

Tension Capacity

Tension failure modes govern design:

Concrete Breakout:

• Cone-shaped failure surface at 35° from vertical
• Projected area: A_c = π × (1.5 × embedment depth)²
• Breakout strength proportional to √f’_c
• Edge effects reduce capacity when distance < 1.5 × embedment

Steel Failure:

• Anchor steel yields before concrete failure
• Capacity: T_steel = 0.75 × A_s × f_u (ACI 318)
• Ductile failure mode preferred for seismic design
• Requires minimum 5 diameters embedment

Pullout Failure:

• Anchor pulls through concrete without breakout
• Critical for shallow embedments
• Reduced capacity for expansion anchors in cracked concrete

Shear Capacity

Shear load resistance depends on failure mode:

Steel Shear:

• Anchor shears at base material interface
• Capacity: V_steel = 0.60 × A_s × f_u
• Highest capacity when edge distance > 20 × diameter

Concrete Breakout:

• Occurs when edge distance < 10 × embedment depth
• Breakout area = (1 + edge distance / (1.5 × embedment))²
• Pryout failure possible when embedment < 2.5 × diameter

Concrete Pryout:

• Anchor rotates about fulcrum point
• Capacity: V_cp = k_cp × N_cb (where N_cb = concrete breakout in tension)
• Pryout factor k_cp = 1.0 to 2.0 depending on anchor type

Seismic Anchor Requirements

Seismic Design Loads

Seismic anchors resist equipment accelerations:

Force Calculation:

• F_p = 0.4 × a_p × S_DS × W_p × (1 + 2z/h) / (R_p/I_p)
• Where: a_p = component amplification factor (1.0-2.5)
• S_DS = design spectral acceleration
• R_p = component response modification factor
• I_p = component importance factor

Load Distribution:

• Distribute to multiple anchors based on tributary area
• Consider torsional effects for eccentric loading
• Account for vertical seismic component (0.2 × S_DS)

Anchor Qualification

ICC-ES ESR Reports:

• Pre-qualified anchors for seismic applications
• Testing per ACI 355.2 (concrete) or AISI S100 (steel)
• Category 1: Highest seismic performance (cracked concrete)
• Category 2: Moderate seismic (uncracked concrete only)

Installation Requirements:

• Special inspection required for seismic anchors
• Torque verification mandatory
• Pull testing for 10% of anchors or minimum 4 anchors
• Proof load: 1.5 × design load held for 2 minutes

Powder-Actuated Fasteners Limitations

Powder-actuated fasteners (PAFs) have restricted HVAC applications:

Code Restrictions:

• Not permitted for seismic restraints per ASCE 7
• Prohibited for vibrating equipment connections
• Not allowed for tension applications in concrete
• Limited to light shear loads only

Acceptable Uses:

• Ductwork track attachment to steel (shear loading)
• Electrical conduit supports
• Sheet metal accessories
• Maximum 200 lbf per fastener in shear

Installation Safety:

• Qualified operator certification required
• Hearing and eye protection mandatory
• Verify substrate thickness > 3 times penetration depth

Steel Attachment Methods

Welded Connections

Welding provides rigid, high-capacity connections:

Weld Types:

• Fillet welds: Most common, 3/16 to 1/4 inch typical
• Allowable shear: 2,400 lbf/inch for E70 electrodes
• All-around welds increase capacity and fatigue life

Requirements:

• AWS D1.1 qualified welders
• Base metal: A36 steel minimum
• Preheat required for plates > 3/4 inch thick
• Visual inspection: 100% of structural welds

Bolted Connections

Bolted connections allow disassembly and adjustment:

High-Strength Bolts:

• ASTM A325 or A490 bolts for structural connections
• Allowable shear: 10,000-17,000 lbf per 3/4 inch bolt
• Allowable tension: 19,000-28,000 lbf per 3/4 inch bolt
• Pretension required: 28,000-39,000 lbf for 3/4 inch A325

Installation:

• Turn-of-nut method: 1/3 to 1/2 turn after snug tight
• Torque method: Calibrated wrench to specified value
• Direct tension indicator washers for verification

Anchor Testing and Verification

Field Testing Requirements

Proof Load Testing:

• Test load: 1.5 × design load for tension
• 2.0 × design load for shear
• Hold for 2 minutes without displacement > 1/16 inch
• Test frequency: 10% of anchors or minimum 4

Testing Equipment:

• Hydraulic ram with calibrated pressure gauge
• Load cell accuracy: ±2% of applied load
• Dial indicator: 0.001 inch resolution
• Reaction frame or adjacent anchor resistance

Failure Criteria:

• Displacement > 1/16 inch during 2-minute hold period
• Continued displacement after load removal
• Visible concrete cracking around anchor
• Pull-through of expansion mechanism

Vibration Isolation Integration

Isolation Hanger Anchorage

Vibration isolators require special anchorage consideration:

Load Components:

• Static load: Equipment weight + attachments
• Dynamic load: Unbalanced forces from operation
• Seismic load: Earthquake-induced accelerations
• Combination typically governs anchor design

Anchor Selection:

• Cast-in or epoxy anchors preferred for reliable long-term performance
• Minimum 4 anchors per isolation point for stability
• Anchor capacity includes 1.5 safety factor for dynamic loading

Spring Isolator Mounting

Top Connection (Equipment Side):

• Through-bolted to equipment base rails
• Minimum 3/8 inch diameter bolts
• Lock washers or thread-locking compound required

Bottom Connection (Structure Side):

• Anchor to withstand seismic overturning moments
• Moment = Equipment weight × CG height × seismic coefficient / number of isolators
• Typical anchor size: 1/2 to 3/4 inch diameter for equipment 500-5000 lbf

Neoprene Pad Anchorage

Neoprene pads reduce high-frequency vibration transmission:

Anchorage Requirements:

• Positive attachment not always required for static loads
• Seismic restraints independent of isolation pads
• Snubbers limit displacement to 1/4 inch during seismic events
• Anchor snubbers to structure, not to equipment

Installation Details:

• Level pad supports ensure uniform compression
• Deflection: 0.20 to 0.30 inches at full equipment load
• Anchor housekeeping pads supporting equipment base

Quality Assurance

Special Inspection

Special inspection ensures code-compliant anchor installation:

Inspection Points:

• Hole diameter and depth verification
• Cleaning procedure observation
• Anchor installation torque verification
• Edge distance and spacing confirmation
• Load testing witnessing and documentation

Documentation Requirements:

• Anchor manufacturer and model
• Installation date and conditions
• Torque values applied
• Test results with load and displacement data
• Inspector certification and signature

This comprehensive approach to anchorage methods ensures reliable, code-compliant HVAC equipment installations capable of resisting seismic, wind, and operational loads throughout the building service life.