1. Overview
Anchor bolts transfer dynamic and static loads from the compressor frame to the concrete foundation. In reciprocating compressor installations, bolts experience cyclic tension and shear from unbalanced forces, gas loads, and frame vibration. Proper anchor bolt design is critical because bolt failure can cause catastrophic frame misalignment, crankshaft damage, and piping stress exceedances.
Static Loads
Dead Weight
Compressor, driver, skid, piping
Dynamic Loads
Unbalanced Forces
Primary, secondary shaking forces
Operating Loads
Gas & Torque
Cylinder gas loads, drive torque
Environmental
Wind & Seismic
ASCE 7 wind/seismic per site class
Applicable Codes and Standards
| Standard | Scope | Key Provisions |
|---|---|---|
| ACI 318 Appendix D | Anchoring to concrete | Breakout, pullout, side-face blowout capacity |
| ACI 351.3R | Machinery foundations | Dynamic bolt preload, pretension criteria |
| API 686 | Machinery installation | Bolt tightening, alignment tolerances |
| ASCE 7 | Minimum design loads | Seismic, wind load combinations |
| ASTM A193 / A307 | Bolt materials | Grade B7 (105 ksi), Grade A (60 ksi) |
Design philosophy: Anchor bolts should be designed so that ductile steel failure governs over brittle concrete failure modes (breakout, blowout). ACI 318 App. D requires checking multiple concrete failure modes and applying appropriate strength reduction factors.
2. Anchor Bolt Types
The choice of anchor bolt type depends on whether the bolt is cast in place during the concrete pour or installed after the concrete has cured.
Cast-in-Place Anchors
Cast-in-place anchors are the preferred choice for compressor foundations because they develop full concrete capacity and provide the most reliable anchorage for dynamic loads.
| Type | Description | Advantages | Typical Use |
|---|---|---|---|
| J-Bolt | L-shaped hook at embedded end | Simple fabrication, low cost | Light equipment, static loads only |
| L-Bolt | 90-degree bend at embedded end | Good pullout resistance | Moderate loads, pumps |
| Headed Bolt | Heavy hex head or plate washer | Best pullout; full ACI 318 App. D | Compressors, dynamic equipment |
| Sleeve Anchor | Bolt within PVC sleeve for adjustment | Lateral adjustment after pour | Precision alignment required |
Compressor best practice: Use cast-in-place headed bolts with sleeves for compressor foundations. The headed bolt provides maximum concrete breakout capacity, and the sleeve allows 1/4" to 3/8" lateral adjustment during alignment. J-bolts and L-bolts are not recommended for dynamic machinery.
Post-Installed Anchors
Post-installed anchors are used for retrofits or when bolt locations cannot be precisely set during the pour. They require qualification testing per ACI 355.2 (mechanical) or ACI 355.4 (adhesive).
| Type | Mechanism | Dynamic Suitability | Notes |
|---|---|---|---|
| Expansion (wedge) | Mechanical friction/wedge | Poor; loosens under vibration | Not recommended for compressors |
| Undercut | Mechanical bearing in undercut hole | Moderate; preload-sensitive | Must maintain torque |
| Adhesive (epoxy) | Chemical bond to concrete | Good if properly installed | Temperature-limited; requires clean holes |
| Grouted-in | Non-shrink grout fills oversized hole | Good for large-diameter bolts | Common retrofit solution |
3. Load Analysis
Anchor bolts for compressor foundations must resist combined tension, shear, and moment from multiple load sources. ACI 318 load combinations govern the design.
Load Sources
Static Loads:
W_dead = Weight of compressor + driver + skid + piping
W_live = Maintenance loads (typically 50-100 psf on access areas)
Dynamic Loads (Reciprocating):
F_primary = m_recip x r x omega^2 (primary unbalanced force)
F_secondary = m_recip x r x (r/L) x omega^2 x cos(2*theta)
F_gas = (P_max - P_min) x A_piston (cylinder gas load)
T_drive = (5252 x HP) / RPM (drive torque, ft-lbs)
Environmental Loads:
F_wind = q_z x G x C_f x A_f (ASCE 7 wind)
F_seismic = S_DS x W / R (ASCE 7 seismic)
ACI 318 Load Combinations
| Combination | Expression | Governs When |
|---|---|---|
| LC1 | 1.4D | Dead load dominant |
| LC2 | 1.2D + 1.6L | Live load dominant |
| LC5 | 1.2D + 1.0E + 0.5L | Seismic zones |
| LC6 | 0.9D + 1.0E | Minimum dead + seismic (max tension) |
| Dynamic | 1.0D + 1.0F_dynamic | Operating vibration (per ACI 351.3R) |
Bolt Tension and Shear Distribution
Tension per bolt from overturning moment:
T_bolt = M / (n x d_cg)
Where:
M = Overturning moment about base (ft-lbs)
n = Number of bolts in the tension group
d_cg = Distance from bolt group centroid to neutral axis (ft)
Shear per bolt:
V_bolt = V_total / n_total
Where:
V_total = Total horizontal shear force (lbs)
n_total = Total number of anchor bolts
Combined loading check (ACI 318 Eq. D-31):
(N_ua / phi_N_n)^(5/3) + (V_ua / phi_V_n)^(5/3) ≤ 1.0
Where:
N_ua = Factored tension demand
V_ua = Factored shear demand
phi_N_n = Design tensile capacity
phi_V_n = Design shear capacity
Pretension requirement: ACI 351.3R recommends pretensioning anchor bolts to 50-70% of bolt yield strength. Pretension prevents joint separation under dynamic loads and provides clamping force for friction-based shear transfer. Always use direct-tension-indicating (DTI) washers or calibrated torque wrenches.
4. Capacity Calculations
ACI 318 Appendix D requires checking multiple failure modes for each anchor bolt. The governing (lowest) capacity determines the design strength.
Tensile Failure Modes
| Failure Mode | Formula | Variables |
|---|---|---|
| Steel failure | N_sa = A_se x f_uta | A_se = effective area; f_uta = ultimate tensile |
| Concrete breakout | N_cb = (A_Nc/A_Nco) x psi_ed,N x psi_c,N x psi_cp,N x N_b | 35-degree cone model |
| Pullout | N_p = psi_c,P x 8 x A_brg x f'c | A_brg = bearing area of head |
| Side-face blowout | N_sb = 160 x c x sqrt(A_brg) x sqrt(f'c) | c = edge distance |
Basic concrete breakout strength (ACI 318 Eq. D-7):
N_b = k_c x sqrt(f'c) x h_ef^(1.5)
Where:
k_c = 24 (cast-in-place) or 17 (post-installed)
f'c = Concrete compressive strength (psi)
h_ef = Effective embedment depth (in.)
Projected failure area:
A_Nco = 9 x h_ef^2 (single anchor, no edge effects)
A_Nc = Actual projected failure area (reduced for edges/spacing)
Edge distance modification factor:
psi_ed,N = 1.0 if c_min ≥ 1.5 x h_ef
psi_ed,N = 0.7 + 0.3 x c_min / (1.5 x h_ef) if c_min < 1.5 x h_ef
Strength reduction factors:
phi = 0.75 (ductile steel failure with supplementary reinforcement)
phi = 0.70 (concrete breakout, Condition B)
phi = 0.65 (concrete breakout, Condition A with no supplementary reinf.)
Shear Failure Modes
| Failure Mode | Formula | Notes |
|---|---|---|
| Steel failure | V_sa = 0.6 x A_se x f_uta | Ductile; preferred governing mode |
| Concrete breakout | V_cb = (A_Vc/A_Vco) x psi_ed,V x V_b | Half-cone breakout toward edge |
| Concrete pryout | V_cp = k_cp x N_cb | k_cp = 1.0 (h_ef < 2.5") or 2.0 |
Embedment Depth Guidelines
| Bolt Diameter | Minimum h_ef | Recommended h_ef | Edge Distance (c_min) |
|---|---|---|---|
| 3/4" | 9" (12d) | 15" (20d) | 6" min |
| 1" | 12" (12d) | 20" (20d) | 6" min |
| 1-1/4" | 15" (12d) | 25" (20d) | 8" min |
| 1-1/2" | 18" (12d) | 30" (20d) | 8" min |
| 2" | 24" (12d) | 40" (20d) | 10" min |
5. Installation & Detailing
Proper installation is critical for achieving design capacity. Common field issues include misalignment, inadequate grouting of sleeves, and insufficient torque.
Bolt Pattern Layout
Minimum bolt spacing (ACI 318 D.8.1):
s_min = 3 x d_bolt (absolute minimum)
s_recommended = 6 x d_bolt to 10 x d_bolt
Minimum edge distance (ACI 318 D.8.2):
For cast-in headed bolts:
c_min = 6 x d_bolt (to prevent side-face blowout)
c_min ≥ 4" (absolute minimum per ACI 351.3R)
Sleeve dimensions (typical):
Sleeve ID = d_bolt + 1/2" to 3/4" (for adjustment)
Sleeve length = h_ef - 4" (leave bottom 4" for head bearing)
Bolt Pretension Requirements
| Bolt Diameter | ASTM A193 B7 | 50% Yield Pretension | Target Torque (ft-lbs) |
|---|---|---|---|
| 3/4" | 105 ksi yield | 17,200 lbs | 90 |
| 1" | 105 ksi yield | 31,500 lbs | 220 |
| 1-1/4" | 105 ksi yield | 50,200 lbs | 440 |
| 1-1/2" | 105 ksi yield | 73,500 lbs | 770 |
| 2" | 105 ksi yield | 133,000 lbs | 1,860 |
Installation Checklist
Step 1
Template Setup
Verify bolt locations within 1/8" of design using steel template; plumb bolts within 1:40 slope
Step 2
Concrete Pour
Vibrate around sleeves carefully; maintain bolt position during pour; protect threads
Step 3
Sleeve Grouting
Fill sleeves with non-shrink grout after equipment alignment; cure 72 hours minimum
Step 4
Torque Sequence
Tighten in star pattern: 50%, 75%, 100% of target torque; re-torque after 24 hours
Common failure cause: Bolt loosening under vibration is the most frequent anchor bolt issue in reciprocating compressor foundations. Use hardened washers, Nordlock locking washers, or double-nut arrangements. Re-torque all bolts after the first 500 hours of operation and annually thereafter per API 686.
6. Worked Examples
Example 1: Tension Capacity of Cast-in-Place Headed Bolt
Given:
1-1/4" diameter ASTM A193 Grade B7 headed bolt
f'c = 4,000 psi concrete
h_ef = 20" embedment depth
c_min = 10" edge distance (nearest edge)
No edge effects on other sides
Step 1: Steel tensile capacity
A_se = 0.969 in^2 (1-1/4" bolt, 8 TPI)
f_uta = 125 ksi
N_sa = 0.969 x 125,000 = 121,125 lbs
phi x N_sa = 0.75 x 121,125 = 90,844 lbs
Step 2: Concrete breakout capacity
N_b = 24 x sqrt(4,000) x 20^1.5
N_b = 24 x 63.25 x 89.44 = 135,793 lbs
A_Nco = 9 x 20^2 = 3,600 in^2
A_Nc = 3,600 in^2 (no edge reduction; c_min = 10" < 1.5 x 20 = 30")
Actually: A_Nc reduced by edge proximity
psi_ed,N = 0.7 + 0.3 x 10 / 30 = 0.80
N_cb = (A_Nc/A_Nco) x 0.80 x 1.0 x 1.0 x 135,793
phi x N_cb = 0.70 x 108,634 = 76,044 lbs
Step 3: Pullout capacity
A_brg = 2.25 in^2 (1-1/4" heavy hex head)
N_p = 1.0 x 8 x 2.25 x 4,000 = 72,000 lbs
phi x N_p = 0.70 x 72,000 = 50,400 lbs
Governing capacity: Pullout = 50,400 lbs
(Use larger head plate or deeper embedment to increase)
Example 2: Combined Tension and Shear Check
Given:
Factored demands: N_ua = 25,000 lbs, V_ua = 12,000 lbs
Design capacities: phi_N_n = 50,400 lbs, phi_V_n = 42,000 lbs
ACI 318 Interaction check (Eq. D-31):
(N_ua / phi_N_n)^(5/3) + (V_ua / phi_V_n)^(5/3) ≤ 1.0
(25,000 / 50,400)^(5/3) + (12,000 / 42,000)^(5/3)
= (0.496)^(5/3) + (0.286)^(5/3)
= 0.312 + 0.131
= 0.443 ≤ 1.0 OK
Utilization ratio = 44.3% (adequate margin for dynamic loads)
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