1. Pipe Rack Overview
Pipe racks are elevated structural frameworks that support process piping, electrical cable trays, and instrumentation runs between equipment in oil and gas facilities. They serve as the primary transportation corridor for fluids and utilities throughout a plant, and their structural integrity is critical to safe operations.
Structural Load Path
Pipe racks transfer loads through a clear hierarchy: individual pipes and cable trays load the transverse beams (stringer beams), which transfer reactions to the longitudinal beams or directly to columns, which in turn transfer all forces to the foundations. Understanding this load path is essential for proper structural design and ensuring that no member is overlooked in the analysis.
Pipe Rack Configurations
The configuration of a pipe rack depends on the number of piping levels, the width required for the pipe count, the height needed for road or equipment clearance, and whether the rack carries cable trays or instrument trays in addition to piping.
| Configuration | Typical Application | Key Consideration |
|---|---|---|
| Single-level rack | Small facilities, satellite stations | Simplest design, lowest cost, limited capacity |
| Two-level rack | Most common in midstream and process plants | Upper level for piping, lower for cable trays or additional pipes |
| Multi-level rack (3+) | Large refineries, LNG plants | Higher lateral loads, more complex bracing requirements |
| T-type rack | Branch connections from main rack | Requires moment connections or bracing at intersection |
| Cantilever rack | Adjacent to buildings or equipment | Higher column moments, heavier foundations |
Governing Standards
| Standard | Scope | Application to Pipe Racks |
|---|---|---|
| ASCE 7-22 | Minimum Design Loads | Wind velocity pressure, seismic base shear, load combinations |
| PIP STC01015 | Structural Design Criteria | Pipe rack-specific loading criteria, deflection limits, design guidelines |
| AISC 360 | Steel Structures Specification | Member design, connection design, stability requirements |
| AISC 341 | Seismic Provisions for Steel | Seismic detailing requirements for high seismic zones |
| ASCE/SEI 48 | Design of Steel Transmission Poles | Sometimes referenced for tall pipe rack columns |
| PIP STE05121 | Anchor Bolt Design | Foundation anchor bolt sizing and embedment |
Typical Pipe Rack Dimensions
| Parameter | Typical Range | Notes |
|---|---|---|
| Rack width | 6 – 30 ft | Based on number of pipes and cable trays |
| Bay spacing | 20 – 30 ft | 20 ft most common; longer spans need deeper beams |
| Bottom of steel elevation | 14 – 22 ft | Minimum 14 ft for road clearance per PIP |
| Level spacing | 5 – 8 ft | Based on largest pipe diameter plus supports |
| Column spacing (transverse) | Equal to rack width | One column each side, wider racks may need intermediate support |
2. Dead Loads
Dead loads on pipe racks include the weight of all permanently attached components: piping, insulation, pipe contents, cable trays with cables, structural steel self-weight, fireproofing, and any permanently mounted equipment. Accurate dead load estimation is the foundation of the entire structural analysis.
Piping Dead Load
The piping dead load is the sum of the empty pipe weight, the weight of insulation and cladding, and the weight of the heaviest expected fluid contents. For hydrotest conditions, the pipes are filled with water, which is often the heaviest operating case and may govern the design of individual beams.
Pipe Dead Load per Foot:
wpipe = wsteel + winsulation + wcontents
Where each component is in lb/ft for the given NPS, schedule, insulation thickness, and fluid density.
Typical Pipe Weights (Steel + Insulation + Water)
| NPS | Schedule | Steel (lb/ft) | 2" Insulation (lb/ft) | Water (lb/ft) | Total (lb/ft) |
|---|---|---|---|---|---|
| 4" | STD | 10.8 | 5.4 | 5.5 | 21.7 |
| 6" | STD | 19.0 | 7.2 | 12.5 | 38.7 |
| 8" | STD | 28.6 | 9.0 | 21.7 | 59.3 |
| 10" | STD | 40.5 | 10.8 | 34.2 | 85.5 |
| 12" | STD | 49.6 | 12.6 | 49.0 | 111.2 |
| 16" | STD | 62.6 | 15.5 | 79.2 | 157.3 |
| 20" | STD | 78.6 | 19.1 | 126.2 | 223.9 |
| 24" | STD | 94.6 | 22.7 | 184.0 | 301.3 |
PIP STC01015 Preliminary Design Load
When individual pipe sizes and quantities are not yet defined during early project phases, PIP STC01015 recommends using 40 PSF (pounds per square foot) as a uniform distributed piping load per level for preliminary pipe rack design. This value represents a moderate piping density typical of midstream gas processing facilities. For heavy piping (large NPS lines with water), the actual load may be 60-80 PSF or higher. Always verify with actual pipe loads when available.
Cable Tray Dead Load
Cable trays carrying power, control, and instrumentation cables are commonly supported on pipe racks. The dead load includes the tray structure itself plus the weight of all cables routed through it.
| Component | Typical Load | Notes |
|---|---|---|
| Ladder tray (empty) | 3 – 6 PSF | Varies with tray width (12" to 36") |
| Power cables (filled) | 8 – 15 PSF | Heavy copper conductors in large trays |
| Control/instrument cables | 5 – 8 PSF | Lighter cables, smaller conductors |
| PIP recommendation | 10 – 15 PSF | Combined tray + cables, per level |
Structure Self-Weight
The self-weight of the pipe rack structural steel (beams, columns, bracing, stringer beams, grating, handrails) must be included in the dead load. This is typically estimated as a percentage of the total load or as a PSF value during preliminary design, then refined once member sizes are selected.
| Bay Span | Estimated Self-Weight (PSF) | Notes |
|---|---|---|
| 20 ft or less | 15 – 18 | Lighter beams, standard columns |
| 20 – 30 ft | 18 – 22 | Deeper beams required for span |
| 30 – 40 ft | 22 – 28 | Heavy beams, may need intermediate support |
| >40 ft | 25 – 35 | Truss beams or plate girders may be required |
Hydrotest Loading
During hydrostatic testing, pipes are filled with water at test pressure (typically 1.5 times the design pressure). Water-filled large-diameter pipes can be significantly heavier than their normal operating weight. For gas service pipes that normally carry negligible fluid weight, the hydrotest condition can increase the beam loading by 50-100%. PIP STC01015 treats hydrotest as an occasional load and applies different load factors than normal operating dead load.
3. Wind Loads (ASCE 7)
Wind loads on pipe racks are calculated using the velocity pressure method from ASCE 7. The wind applies horizontal forces to the projected areas of pipes, cable trays, structural members, and any equipment mounted on the rack. Pipe racks are classified as open structures, and the analysis must account for shielding effects between multiple rows of pipes.
Velocity Pressure
The fundamental wind pressure at any height z above ground is determined from the basic wind speed and site-specific factors.
Velocity Pressure:
qz = 0.00256 × Kz × Kzt × Kd × V²
Where qz is in psf, V is the basic wind speed in mph (3-second gust), Kz is the velocity pressure exposure coefficient at height z, Kzt is the topographic factor (1.0 for flat terrain), and Kd is the wind directionality factor (0.85 for open structures).
Exposure Categories
| Category | Terrain Description | Kz at 20 ft | Kz at 40 ft | Typical Use |
|---|---|---|---|---|
| B | Urban, suburban, wooded areas | 0.62 | 0.76 | Within cities, forested areas |
| C | Open terrain with scattered obstructions | 0.90 | 1.04 | Most industrial and midstream sites |
| D | Flat, unobstructed coastal areas | 1.08 | 1.22 | Coastal facilities, offshore platforms |
Exposure C Is Standard for Industrial Facilities
Most midstream and process facilities are located in open terrain and should use Exposure C. Even facilities located near towns may qualify as Exposure C if the open fetch distance in the prevailing wind direction exceeds the transition distance specified in ASCE 7. Using Exposure B when Exposure C is appropriate results in unconservative wind loads.
Wind Force on Pipes
The wind force on pipes is calculated using the projected area of the pipes exposed to wind, adjusted for the drag coefficient of round cylinders and shielding from upstream rows.
Wind Force on Pipes:
Fw = qz × G × Cf × Af
Where G = 0.85 (gust-effect factor for rigid structures), Cf = 0.7 (force coefficient for round pipes per ASCE 7), and Af is the total projected area including shielding adjustments.
Shielding Effects
When multiple rows of pipes are arranged on the rack, the downstream (leeward) rows experience reduced wind force due to shielding from the upstream (windward) row. PIP STC01015 and common practice provide shielding factors for this reduction.
| Pipe Row Position | Load Factor | Notes |
|---|---|---|
| 1st row (windward) | 1.0 | Full wind load |
| 2nd row | 0.5 – 0.7 | Significant shielding |
| 3rd and subsequent rows | 0.3 – 0.5 | Maximum shielding effect |
Conservative practice (and the approach used in most pipe rack designs) is to apply 50% reduction to all shielded rows. Some engineers prefer to use no shielding reduction for the conservative case, particularly when the pipe arrangement is not yet finalized.
Wind on Structural Members
In addition to pipes, the wind applies forces to the structural steel members themselves. Columns and beams are treated as flat-sided members with a higher force coefficient than round pipes.
| Element | Cf | Projected Area |
|---|---|---|
| Round pipes | 0.7 | Diameter × length |
| W-shape beams/columns | 2.0 | Depth × length (flat side) |
| Angle bracing | 1.8 | Width × length |
| Cable trays (with covers) | 2.0 | Height × width of tray face |
Wind Direction
Wind loads must be checked for both the transverse direction (perpendicular to the rack run) and the longitudinal direction (parallel to the rack run). The transverse direction typically governs because the projected area of pipes is maximized. However, longitudinal wind on the column faces plus any equipment mounted on the rack can produce significant forces in the bracing system. ASCE 7 requires checking the worst-case direction.
4. Seismic Loads
Seismic loads on pipe racks are calculated using the Equivalent Lateral Force (ELF) procedure from ASCE 7 Chapter 12. The seismic force is a horizontal load proportional to the weight of the structure and its contents, adjusted for the site seismicity, the structural system type, and the importance of the facility.
Seismic Base Shear
Base Shear:
V = Cs × W
Seismic Response Coefficient:
Cs = SDS / (R / Ie)
Where SDS is the design spectral response acceleration at short periods (from ASCE 7 seismic maps or USGS), R is the response modification coefficient, Ie is the importance factor, and W is the effective seismic weight (dead load).
Seismic Design Parameters
| Parameter | Value for Pipe Racks | Reference |
|---|---|---|
| Response Modification (R) | 3.0 | ASCE 7 Table 12.2-1, ordinary steel moment frame |
| Importance Factor (Ie) | 1.0 – 1.5 | 1.0 standard, 1.25 essential piping, 1.5 hazardous materials |
| Minimum Cs | 0.01 | ASCE 7 Eq. 12.8-5 |
| Seismic Weight (W) | Total dead load | Includes pipe, insulation, contents, structure |
R = 3.0 for Pipe Racks
Pipe racks are typically designed as ordinary steel moment frames (R = 3.0) or ordinary steel braced frames (R = 3.25). The low R value means that pipe racks must be designed for a larger proportion of the elastic seismic force compared to buildings with more ductile systems (R = 8). This is because pipe rack connections are generally simpler and provide less energy dissipation than fully ductile building connections. In high seismic zones, using special moment frame connections (R = 8) may be economical for tall or heavily loaded racks.
Vertical Distribution of Seismic Force
The total base shear is distributed vertically to each level based on the weight and height of that level per ASCE 7 Equation 12.8-11:
Force at Level x:
Fx = Cvx × V
Cvx = (wx × hxk) / Σ(wi × hik)
Where wx is the weight at level x, hx is the height of level x, and k = 1 for short-period structures (T ≤ 0.5 s, typical for pipe racks).
Seismic vs. Wind Comparison
In many locations, one lateral load type governs over the other. The governing lateral load determines the design of columns, bracing, and foundations for lateral resistance.
| Region | Typical SDS | Wind Speed (mph) | Governing Lateral |
|---|---|---|---|
| Gulf Coast (TX, LA) | 0.10 – 0.30 | 120 – 160 | Usually wind |
| Oklahoma / Midwest | 0.20 – 0.60 | 105 – 120 | Can be either |
| California | 0.80 – 2.0 | 95 – 115 | Usually seismic |
| Appalachian (Marcellus) | 0.10 – 0.20 | 100 – 115 | Usually wind |
| Rocky Mountain | 0.15 – 0.50 | 100 – 130 | Varies by site |
Both Loads Must Be Checked
Even when one lateral load type clearly governs, both wind and seismic loads must be calculated and checked against the applicable ASCE 7 load combinations. The governing combination depends not only on the magnitude of the lateral load but also on the load factors applied. Seismic combinations include the vertical seismic effect (0.2*SDS*D), which increases the effective gravity load.
5. Load Combinations (LRFD)
ASCE 7 Section 2.3 specifies the Load and Resistance Factor Design (LRFD) combinations that must be checked for structural adequacy. Each combination applies different factors to the various load types to account for uncertainty and probability of simultaneous occurrence.
ASCE 7 LRFD Combinations
| Combination | Expression | Typical Governing Case |
|---|---|---|
| LC1 | 1.4D | Dead load only; governs lightly loaded racks |
| LC2 | 1.2D + 1.6L + 0.5S | Maximum gravity; governs beam design |
| LC3 | 1.2D + 1.0W + L + 0.5S | Wind + gravity; governs columns in wind zones |
| LC4 | 1.2D + 1.0E + L | Seismic + gravity; governs in seismic zones |
| LC5 | 0.9D + 1.0W | Minimum gravity + wind; checks uplift at foundations |
| LC6 | 0.9D + 1.0E | Minimum gravity + seismic; checks overturning |
Understanding Load Factors
The 1.2 and 0.9 factors on dead load represent the upper and lower bounds of dead load uncertainty. The 1.2D combinations maximize vertical load on members and foundations, while 0.9D combinations minimize the stabilizing effect of gravity to check against overturning and uplift. Wind and seismic loads use 1.0 factors because the design loads already include appropriate return period adjustments.
Pipe Rack-Specific Considerations
Several load conditions specific to pipe racks may not appear in standard building combinations but must be addressed in design:
| Special Load Condition | Treatment | Load Factor |
|---|---|---|
| Hydrotest (water-filled) | Occasional load, usually with no wind/seismic | 1.0 – 1.2 (as dead load) |
| Thermal anchor/guide loads | Self-limiting; treated as live load or thermal load | 1.0 – 1.2 |
| Pipe support friction | Longitudinal force from sliding supports | Included with thermal loads |
| Future pipe load allowance | 10 – 25% additional capacity per PIP | Applied to dead load |
| Bundle pull load (maintenance) | Temporary lateral load during exchanger bundle pull | Occasional load, reduced factors |
6. Member Sizing
Pipe rack structural members are sized per AISC 360 using LRFD methodology. The primary members are transverse beams (spanning between columns), longitudinal struts or beams, columns, and bracing members. Each member type has distinct design requirements governed by the loads it carries.
Transverse Beams
Transverse beams are the primary load-carrying members, spanning between columns and directly supporting the pipes. They are designed for bending from distributed gravity loads and must satisfy both strength and serviceability (deflection) criteria.
Simply Supported Beam:
Mmax = wL² / 8
Required Section Modulus:
Sx,req = Mu / (φb × Fy)
Where φb = 0.90 for flexure per AISC, Fy is yield strength (50 ksi for A992).
Deflection Criteria
Beam deflection limits ensure that piping slopes are maintained, pipe supports remain effective, and the structure appears and feels rigid to operations personnel.
| Load Condition | Deflection Limit | Reference |
|---|---|---|
| Total load (D + L) | L/240 | PIP STC01015, AISC |
| Live load only | L/360 | AISC, common practice |
| Sensitive equipment supported | L/360 total | Project-specific requirement |
Simply Supported Beam Deflection:
δmax = 5wL4 / (384EI)
Required Moment of Inertia:
Ix,req = 5wL4 / (384Eδallow)
Where E = 29,000 ksi for structural steel and δallow = L/240.
Deflection Often Governs Beam Size
For typical pipe rack bay spacings of 20-30 feet, the deflection criterion (L/240) frequently requires a deeper beam than the strength criterion alone. This is because deflection depends on the moment of inertia (I), which increases with the fourth power of beam depth, while bending strength depends on the section modulus (S), which increases with the cube of depth. Engineers often select beams primarily for stiffness and then verify strength adequacy.
Column Design
Pipe rack columns are designed as beam-columns subjected to combined axial compression (gravity loads) and bending (lateral loads from wind or seismic). The column effective length factor K depends on the end conditions and bracing configuration.
| Column End Condition | K Factor | Application |
|---|---|---|
| Fixed base, pinned top (cantilever) | 2.0 | Unbraced pipe rack in transverse direction |
| Fixed base, fixed top (portal frame) | 1.0 | Moment-connected beam-column joint |
| Pinned base, braced frame | 1.0 | Braced pipe rack (X-bracing in longitudinal direction) |
Steel Grade Selection
| Grade | Fy (ksi) | Fu (ksi) | Application |
|---|---|---|---|
| ASTM A992 | 50 | 65 | W-shapes (beams and columns) -- most common |
| ASTM A36 | 36 | 58 | Plates, angles, channels, miscellaneous steel |
| ASTM A500 Gr B | 46 | 58 | HSS (hollow structural sections) for bracing |
| ASTM A572 Gr 50 | 50 | 65 | Plates and shapes, alternative to A992 |
Bracing Systems
Pipe racks require bracing to resist lateral loads (wind and seismic) in both the transverse and longitudinal directions. The bracing configuration affects the lateral stiffness, the column effective length, and the foundation design.
| Direction | Common System | Key Consideration |
|---|---|---|
| Transverse | Moment frame (beam-column) | Allows piping to pass through bays unobstructed |
| Longitudinal | X-bracing or V-bracing | Located every 3-5 bays; coordinate with thermal expansion |
7. Thermal Expansion Provisions
Pipes operating at elevated or depressed temperatures expand or contract relative to the pipe rack structure. This thermal movement must be accommodated through proper pipe support design and, in some cases, special provisions in the pipe rack structure itself.
Pipe Support Types
| Support Type | Function | Load on Rack |
|---|---|---|
| Shoes (sliding) | Support vertical load, allow axial movement | Vertical + friction (longitudinal) |
| Guides | Restrain lateral movement, allow axial | Vertical + lateral force |
| Anchors | Fixed point, restrain all movement | Vertical + lateral + longitudinal (thermal force) |
| Spring hangers | Support vertical load with constant or variable force | Vertical (reduced variation due to thermal displacement) |
| Trunnions | Direct pipe-to-beam welded support | Vertical + horizontal as designed |
Friction Loads from Pipe Shoes
When pipes move thermally on sliding shoes, friction between the shoe and the beam generates a horizontal force on the pipe rack. The friction coefficient for steel-on-steel is approximately 0.3, and for PTFE (Teflon) slide plates it is approximately 0.05-0.10. For large-diameter, heavy pipes, these friction forces can be significant and must be included in the longitudinal design of the pipe rack. PIP STC01015 recommends using a friction coefficient of 0.3 for bare steel and 0.1 for PTFE in the absence of specific manufacturer data.
Anchor and Guide Spacing
Pipe anchors are fixed points where thermal expansion is controlled. Guides are placed between anchors to prevent lateral buckling of the pipe while allowing axial expansion. The spacing of anchors and guides is determined by the pipe stress engineer based on the allowable stresses in the piping system.
| Pipe Size (NPS) | Typical Guide Spacing | Typical Anchor Spacing |
|---|---|---|
| 4" – 6" | 15 – 25 ft | 100 – 200 ft |
| 8" – 12" | 20 – 30 ft | 150 – 300 ft |
| 14" – 20" | 25 – 35 ft | 200 – 400 ft |
| 24" – 36" | 30 – 40 ft | 250 – 500 ft |
Thermal Expansion Coefficients
| Material | Coefficient (in/in/°F) | Expansion per 100 ft per 100°F Rise |
|---|---|---|
| Carbon steel (A106) | 6.5 × 10-6 | 0.78" |
| Stainless steel (304) | 9.6 × 10-6 | 1.15" |
| Chrome-moly (P11) | 6.7 × 10-6 | 0.80" |
Longitudinal Bracing and Thermal Growth
Longitudinal bracing locations must be coordinated with pipe anchor locations to avoid creating excessive thermal stresses in the piping or excessive forces in the bracing. Braced bays should ideally be located at or near pipe anchor points. If a braced bay is located between two anchors, the thermal movement of pipes sliding on supports between those anchors will induce friction forces into the bracing system. These forces must be evaluated and accommodated in the bracing design.
8. Foundation Design
Pipe rack foundations must resist the combined effects of vertical gravity loads, horizontal lateral loads (wind or seismic), and overturning moments. The foundation type and size depend on the soil conditions, the load magnitudes, and the project requirements.
Foundation Types
| Type | Application | Key Consideration |
|---|---|---|
| Spread footing | Most common for pipe racks | Soil bearing capacity must exceed applied pressure |
| Combined footing | Adjacent columns with high lateral loads | Combines two column loads into one large footing |
| Drilled piers | Poor soil conditions or high lateral loads | Resists lateral and uplift through skin friction and bearing |
| Driven piles | Coastal or soft soil sites | Groups of piles with pile cap |
Foundation Loading
Each column foundation receives vertical load, horizontal shear, and moment. The vertical load comes from gravity (dead + live loads). The horizontal shear comes from wind or seismic base shear distributed to each column. The moment comes from the overturning effect of lateral loads and from column base fixity.
Vertical Reaction per Column:
Rv = (Total Gravity Load) / (Number of Columns per Bay)
Horizontal Reaction per Column:
Rh = (Governing Lateral Base Shear) / (Number of Columns per Bay)
Overturning Moment:
MOT = Σ(Fi × hi)
Where Fi is the lateral force at level i and hi is the height of level i above the foundation.
Anchor Bolt Design
Anchor bolts connect the steel column base plate to the concrete foundation. They must resist shear forces (from lateral loads) and tension forces (from overturning moments or uplift).
| Bolt Diameter | Tensile Capacity (kips)* | Shear Capacity (kips)* | Typical Application |
|---|---|---|---|
| 3/4" | 10.8 | 6.5 | Light pipe racks, single level |
| 1" | 19.6 | 11.8 | Standard pipe racks |
| 1-1/4" | 31.1 | 18.7 | Heavy pipe racks, tall columns |
| 1-1/2" | 45.0 | 27.0 | High wind or seismic zones |
| 1-3/4" | 61.7 | 37.0 | Critical pipe racks, high loads |
*F1554 Grade 55 bolts, ASD capacity. Actual values depend on embedment, edge distance, and concrete strength per ACI 318.
Overturning Check
For pipe racks with significant lateral loads, the overturning moment at the foundation can create tension (uplift) in the anchor bolts on the windward or seismic-leeward side. The factor of safety against overturning should be at least 1.5 for service loads. The 0.9D load combinations (LC5 and LC6) specifically check this condition by minimizing the stabilizing dead load. If uplift exceeds the column dead load, anchor bolts must be designed for the net tension.
Soil Bearing Pressure
The foundation must be sized so that the maximum soil bearing pressure under the combined effects of vertical load and moment does not exceed the allowable soil bearing capacity. For eccentrically loaded footings:
Maximum Bearing Pressure:
qmax = P/A + M/S
Where P is the vertical load, A is the footing area, M is the applied moment, and S is the section modulus of the footing plan shape.
| Soil Type | Allowable Bearing (psf) | Notes |
|---|---|---|
| Soft clay | 1,000 – 2,000 | May require piles; settlement concern |
| Stiff clay | 2,000 – 4,000 | Adequate for most pipe racks |
| Loose sand | 1,500 – 3,000 | Densification may improve capacity |
| Dense sand/gravel | 3,000 – 6,000 | Good foundation material |
| Weathered rock | 6,000 – 10,000 | Excellent bearing capacity |
| Sound rock | 10,000+ | Limited by structural capacity |
Geotechnical Investigation Required
Allowable bearing pressures vary significantly with soil type, water table depth, and loading conditions. A site-specific geotechnical investigation with borings and laboratory testing is required for any pipe rack foundation design. The values shown above are general ranges for preliminary sizing only and must not be used for final design without geotechnical confirmation.