1. Overview
Block foundations are the most common foundation type for compressor installations. They consist of a solid concrete mass that supports the compressor, driver, and auxiliary equipment. The foundation serves three purposes: resisting dynamic forces, providing sufficient mass to limit vibration amplitudes, and transferring loads to the soil.
Function 1
Mass & Inertia
Reduces vibration amplitude by increasing system mass
Function 2
Load Distribution
Spreads concentrated loads to allowable soil bearing
Function 3
Rigidity
Maintains equipment alignment under dynamic loads
Function 4
Isolation
Attenuates vibration transmission to adjacent structures
Foundation Types Comparison
| Type | Description | Best For | Mass Efficiency |
|---|---|---|---|
| Block (solid) | Solid rectangular concrete mass | Reciprocating compressors | Highest (all mass contributes) |
| Table-top | Elevated deck on columns | Large centrifugal, turbines | Lower (columns add height, not mass) |
| Spring-mounted | Block on isolation springs | Sensitive nearby equipment | Variable (tuned for isolation) |
| Pile-supported | Block on driven/drilled piles | Poor soil conditions | Similar to block |
When to use block foundations: Block foundations are the default choice for reciprocating compressors up to approximately 10,000 HP. They provide the most mass per unit cost, are straightforward to design and construct, and provide excellent rigidity. Table-top foundations are used when process piping must pass beneath the compressor or when equipment elevation is required.
2. Sizing Rules
Block dimensions are governed by mass ratio requirements, minimum overhang distances, and soil bearing capacity. ACI 351.3R and manufacturer specifications provide the design criteria.
Mass Ratio Requirements
Mass Ratio Definition:
R_m = W_foundation / W_machine
Where:
W_foundation = Weight of concrete block + soil on block (lbs)
W_machine = Total machinery weight including compressor, driver,
skid, piping, and all mounted auxiliaries (lbs)
Minimum mass ratios (ACI 351.3R):
Reciprocating compressors: R_m ≥ 3.0 (minimum), 5.0 (recommended)
Centrifugal compressors: R_m ≥ 2.0 (minimum), 3.0 (recommended)
Motors/generators: R_m ≥ 1.5 (minimum), 2.5 (recommended)
Turbines: R_m ≥ 2.0 (minimum), 3.0 (recommended)
Concrete weight:
Normal weight concrete: 150 lb/ft^3
W_block = L x W x D x 150
Minimum Dimensions
| Parameter | Rule | Purpose |
|---|---|---|
| Length | Machine length + 12" min overhang each end | Anchor bolt edge distance |
| Width | Machine width + 12" min overhang each side | Soil bearing area; bolt clearance |
| Depth | Max of: (a) mass ratio, (b) 1/5 of smallest plan dim, (c) manufacturer min | Mass requirement; rigidity |
| Overhang (sides) | ≥ 6" from bolt CL to edge (12" preferred) | Concrete breakout capacity |
| Overhang (ends) | ≥ 12" from skid edge | Crankshaft access; maintenance |
| Top of block | +6" to +12" above grade | Drainage, flooding protection |
Depth sizing procedure:
Step 1: Calculate required depth from mass ratio
D_mass = (R_m x W_machine) / (L x W x 150)
Step 2: Check minimum depth rule
D_min = max(L, W) / 5
Step 3: Check manufacturer minimum
D_mfr = per vendor drawing (typically 3' to 6')
Step 4: Governing depth
D = max(D_mass, D_min, D_mfr)
Round up to nearest 6" increment for formwork simplicity.
Rule of thumb: For quick sizing estimates before detailed analysis, use these guidelines: Foundation length = machine length + 2 feet. Foundation width = machine width + 2 feet. Foundation depth = solve for mass ratio of 4:1. This typically yields a block 4 to 6 feet deep for reciprocating compressors in the 500 to 3,000 HP range.
3. Center of Gravity Analysis
The combined center of gravity of the machine and foundation must be within acceptable eccentricity limits to prevent uneven soil loading and rocking vibration.
Combined CG calculation (each axis):
X_cg = (W_machine x X_m + W_foundation x X_f) / (W_machine + W_foundation)
Y_cg = (W_machine x Y_m + W_foundation x Y_f) / (W_machine + W_foundation)
Z_cg = (W_machine x Z_m + W_foundation x Z_f) / (W_machine + W_foundation)
Where:
X_m, Y_m, Z_m = Machine CG coordinates (from vendor)
X_f, Y_f, Z_f = Foundation CG coordinates (geometric center)
Eccentricity:
e_x = |X_cg - X_block_center|
e_y = |Y_cg - Y_block_center|
Eccentricity limits (ACI 351.3R):
e_x ≤ 0.05 x L (5% of foundation length)
e_y ≤ 0.05 x W (5% of foundation width)
If exceeded: adjust block dimensions or add counterweight pads.
CG Height Considerations
| Parameter | Criterion | Effect if Violated |
|---|---|---|
| Height of combined CG | Below top of foundation preferred | Increased rocking; higher soil edge pressure |
| Rocking stability ratio | W_total x B/2 ≥ 1.5 x M_overturning | Potential for rocking instability |
| Vertical CG limit | Z_cg ≤ D/2 (within lower half) | Higher CG amplifies dynamic response |
Soil Pressure Distribution
With eccentricity, soil pressure varies across the base:
q_max = (W_total / A) x (1 + 6*e_x/L + 6*e_y/W)
q_min = (W_total / A) x (1 - 6*e_x/L - 6*e_y/W)
Where:
A = L x W = foundation base area
q_max must be ≤ q_allowable (soil bearing capacity)
q_min must be ≥ 0 (no tension / uplift)
For dynamic loading:
q_dynamic = q_static + delta_q_vibration
delta_q = M_dynamic / S_base
S_base = (W x L^2) / 6 (section modulus of base)
q_total_max ≤ 0.75 x q_ultimate (per ACI 351.3R)
4. Reinforcement Detailing
Block foundations require reinforcement to resist shrinkage cracking, temperature stresses, and localized loading from anchor bolts and equipment pads.
Minimum Reinforcement
| Location | Minimum Steel | Typical Practice |
|---|---|---|
| Top mat | As = 0.0018 x b x h (ACI 318) | #5 @ 12" each way |
| Bottom mat | As = 0.0018 x b x h | #5 @ 12" each way |
| Vertical (sides) | As = 0.0012 x b x h | #5 @ 12" each way |
| Around blockouts | 2 x #5 each side of opening | Frame openings with hairpins |
| Anchor bolt cages | 4 vertical + ties @ 12" | #4 ties with #5 verticals |
Reinforcement for dynamic loading (ACI 351.3R):
Top and bottom mats: As ≥ 0.5% of cross-section area
Vertical reinforcement: As ≥ 0.5% of vertical cross-section
Cover requirements:
Cast against soil: 3" minimum clear cover
Exposed sides: 2" minimum clear cover
Top surface: 2" minimum clear cover (3" if exposed to weather)
Bar development length (ACI 318-19):
ld = (fy x psi_t x psi_e x psi_s) / (25 x lambda x sqrt(f'c)) x d_b
For #5 Grade 60 in 4,000 psi concrete:
ld = approx. 19" (top bars, psi_t = 1.3)
ld = approx. 15" (bottom bars, psi_t = 1.0)
Practical note: Many compressor vendors require specific rebar configurations around anchor bolt pockets and equipment mounting pads. Always obtain the vendor foundation loading drawing before detailing reinforcement. The vendor drawing specifies bolt loads, pipe penetration locations, and blockout dimensions that directly affect rebar layout.
5. Concrete Requirements
Compressor foundations require concrete with specific properties for durability, strength, and low shrinkage to maintain equipment alignment over the foundation's service life.
| Property | Minimum | Recommended | Notes |
|---|---|---|---|
| Compressive strength f'c | 3,000 psi | 4,000-5,000 psi | 28-day strength; ACI 351.3R |
| Max w/c ratio | -- | 0.45 | Durability; low permeability |
| Cement content | 564 lb/cy | 600-658 lb/cy | Type I/II Portland cement |
| Max aggregate size | -- | 3/4" to 1" | Workability around reinforcement |
| Slump | -- | 4" to 6" | Consolidation around blockouts |
| Air entrainment | 4% (freeze-thaw) | 5-7% | Required in freeze-thaw zones |
Pour Considerations for Mass Concrete
Mass concrete threshold:
Foundations exceeding 4 feet in any dimension are classified as mass
concrete per ACI 207.1R. Temperature control measures are required.
Temperature differential limit:
delta_T_max = 35 deg F between core and surface
Thermal control strategies:
1. Low-heat cement (Type II or IV) or fly ash replacement (25-40%)
2. Pre-cooling: chilled water, ice replacement (up to 50% of mix water)
3. Post-cooling: embedded cooling pipes (1" PE at 4-ft spacing)
4. Insulated formwork: maintain surface temperature
5. Staged pours: limit lift heights to 4-5 feet with 72-hr intervals
Curing requirements:
Moist cure for 7 days minimum (14 days for mass concrete)
Maintain surface moisture; no rapid drying
Do not load foundation until reaching 75% of design f'c
6. Worked Examples
Example 1: Block Foundation Sizing for a Reciprocating Compressor
Given:
2,000 HP reciprocating compressor
Machine weight: 85,000 lbs (including skid, driver, piping)
Machine footprint: 20 ft (L) x 8 ft (W) x 6 ft (H above foundation)
Machine CG: X = 10.5 ft, Y = 4.2 ft, Z = 3.5 ft above foundation top
Required mass ratio: 4:1 (per ACI 351.3R for reciprocating)
Soil bearing capacity: 3,000 psf
Step 1: Determine plan dimensions
L_fdn = 20 + 2(1.0) = 22 ft (12" overhang each end)
W_fdn = 8 + 2(1.0) = 10 ft (12" overhang each side)
Step 2: Required mass from mass ratio
W_fdn_req = 4.0 x 85,000 = 340,000 lbs
Step 3: Required depth
D = W_fdn_req / (L x W x 150)
D = 340,000 / (22 x 10 x 150) = 10.3 ft
Round up to D = 10.5 ft (nearest 6")
Step 4: Check depth rule (1/5 of smallest plan dimension)
D_min = 10 / 5 = 2.0 ft < 10.5 ft OK
Step 5: Verify soil bearing
W_total = 85,000 + (22 x 10 x 10.5 x 150) = 85,000 + 346,500 = 431,500 lbs
q = 431,500 / (22 x 10 x 144) = 13.6 psi = 1,963 psf < 3,000 psf OK
Step 6: Eccentricity check
X_cg = (85,000 x 10.5 + 346,500 x 11.0) / 431,500 = 10.9 ft
e_x = |10.9 - 11.0| = 0.1 ft
e_x / L = 0.1 / 22 = 0.0045 = 0.45% < 5% OK
Result: 22 ft x 10 ft x 10 ft-6 in block foundation
Actual mass ratio = 346,500 / 85,000 = 4.08:1 OK
Example 2: Eccentricity Correction
Given:
Machine CG offset: X_m = 12.0 ft (vs block center at 11.0 ft)
W_machine = 85,000 lbs, W_foundation = 346,500 lbs
Combined CG:
X_cg = (85,000 x 12.0 + 346,500 x 11.0) / 431,500 = 11.2 ft
Eccentricity:
e_x = |11.2 - 11.0| = 0.2 ft
e_x / L = 0.2 / 22 = 0.9% < 5% OK
Soil pressure with eccentricity:
q_max = (431,500 / 220 x 144) x (1 + 6 x 0.2/22)
q_max = 13.6 x 1.055 = 14.3 psi = 2,065 psf < 3,000 psf OK
q_min = 13.6 x (1 - 0.055) = 12.9 psi = 1,854 psf > 0 OK (no uplift)
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