DESIGN OF PIPERACK STRUCTURE – ASCE 7-10

The length of Pipe rack 42m is considered to avoid forces due to thermal expansion of pipe rack under ambient temperature and free to expand at ends. This is general engineering practice for Pipe rack design with single vertical bracings and mentioned in “Design Specification for Steel Structures”

The geometry of the pipe rack structure as below

• Width of the structure 9.0m
• Length 42.0m
• Frame spacing 6.0m
• Height 20.0m.

Pipe rack consists of 3 levels of pipe support and 4 levels of cable tray at top. The transverse beam elevations as below:

• EL (+) 7.5m – Pipe supporting Level
• EL (+) 11.5m – Pipe supporting Level
• EL (+) 15.5m – Pipe supporting Level
• EL (+) 20.0m – Cable Tray & Grating Level

Column bases of the structure are fixed in transverse direction and pinned in longitudinal direction. The lateral stability of the structure is provided by concentric bracing system in longitudinal bracing and moment resisting frames in transverse direction. Pipe rack is supported on pedestal of height 0.3m above the ground level.

The structure is designed according to AISC 360-10.

Figure 4‑1 3D Isometric View of Pipe Rack

1. The analysis and design is performed using STAAD Pro Connect Edition design software.

LOADS AND LOAD COMBINATIONS OF PIPERACKS

DEAD LOAD

Structural Dead load (Ds)

Structural self-weight of modelled steel elements are automatically generated in STAAD Pro

• Structural self-weight of all steel members are computed considering a unit weight of 78.5 kN/m³ multiplied by a contingency factor of 1.2 taking into account for connection details.
• Self-weight of secondary beam and grating is considered as 1.0 kN/m2 and applied as dead load on the structure.

Figure 5‑1 Grating load at EL (+) 20.000 m

EMPTY DEAD LOAD (De)

• Empty load of the pipe is considered as 60% of the pipe operating load.

• Empty Cable tray load is assumed as 10% of cable tray operating load. Considered 600mm wide cable tray for loadings.

Figure 5‑2 Pipe Empty Dead load at EL (+) 7.5m, 11.5m, 15.5m & 20.0 m

PIPE OPERATING LOAD (D_O)

1. The pipe operating load is considered as per SLD from piping layout.

1. The Cable tray load is considered as 1.5 kN/m2 per level of cable tray in the absence of electrical data. Considered 600mm wide cable tray for loadings.

Figure 5‑3 Pipe Operating load at EL (+) 7.5m, 11.5m, 15.5m & 20.0 m

PIPE TEST LOAD (D_T)

1. The pipe test load is considered as per SLD from piping layout.

1. The Cable tray load is considered as 1.5 kN/m2 per level of cable tray in the absence of electrical data. Assumed 600 wide cable tray for loadings.

Figure 5‑4 Pipe Test load at EL (+) 7.5m, 11.5m, 15.5m & 20.0 m

LIVE LOAD (LL)

Live load is considered as 2.87 kN/m² and applied as UDL.

Figure 5‑5 Live load at EL (+) 20.0 m

THERMAL LOAD (Ts1 & Ts2)

1. Temperature load are taken from the Basic Engineering Design Data. The thermal load is considered for the structure with maximum temperature variation of +42°C and minimum temperature variation of 6°C from the ambient temperature.
   • 1. Maximum Temperature : 42°C
     2. Minimum Temperature : 6°C
     3. Mean Ambient Temperature : 26°C
     4. TL1 max = 42 – 26 = +16 °C
     5. TL2 min = 6 – 26 = – 20 °C

2. 

Figure 5‑6 Thermal Load showing maximum and minimum temperature

PIPE ANCHOR/GUIDE LOAD (Ttsgu)

• 1. No pipe Anchor/Guide load is considered.

PIPE FRICTION LOAD (Tt_F)

The 10% of the pipe operating load has been considered as pipe friction load for pipes numbers > 6. The loads are applied in longitudinal direction along the pipe direction.

Figure 5‑7 Pipe Empty load at EL (+) 4.45 m & EL (+) 6.35 m

WIND LOAD (WL_X & WL_Z)

Wind Parameters:

Basic wind speed (3sec-gust) = 85.70 m/s

Category = III

F, applied wind force = qz G Cf Af (Eq. 29.5-1), ASCE 7-10

qz – Velocity Pressure at height z above Ground.

G – Gust effect factor

Cf – Net force coefficient

Af – Projected area normal to wind

Velocity Pressure qz:

qz, (N/m2) = 0.613 Kz Kzt Kd V2 (Eq. 30.3-1), ASCE 7 -10

Kz, Velocity pressure exposure coefficient

Kzt, Topographic factor

Kd, Wind directionality factor

V, Basic wind speed (3 sec – Gust) m/s

Exposure Factor = D

H, Height of hill or escarpment = 27 m (Grading level)

Lh, Distance upwind of crest at H/2 = 50 m (shore to escarpment = 100m)

x, Distance from crest to building site = 100 m (escarpment to bldg = 100m)

z, Ht above ground surface at bldg site = 47 m 27+ ht of pipe rack

Considering 2-D escarpments

μ, Horizontal attenuation factor = 4 Figure 26.8-1 (ASCE 7-10) – down wind

γ, Height attenuation factor = 2.5 Figure 26.8-1 (ASCE 7-10)

k1, (k1/(H/Lh) = 0.95 = 0.513

k2, = 0.5

k3, = 0.095

Kzt, = 1.05

Kd, Wind directionality factor = 0.85 Table 26.6-1, ASCE 7-10

G – Gust effect factor = 0.85 26.9.1, ASCE 7-10

Table 5‑2 : wind pressure for height z

Cf – Net force coefficient

Ref: Sec. 4.1, ASCE-Report – Wind Loads and Anchor Bolt Design for Petrochemical Facilities

All structural members = 1.8 uniformly at all tier levels

For Cable tray = 2.0

For Pipes = 0.7 Fig 29.5-1, ASCE 7-10 (smooth pipe)

Wind load Calculated for member with fire proof insulation – X Direction

Wind load Calculated for member with fire proof insulation – Z Direction

Transverse Wind Force due to Pipe:

Ref: Sec. 4.1, ASCE-Report – Wind Loads and Anchor Bolt Design for Petrochemical Facilities

Tributary Area: Af = (D + 10%W) x Bent Spacing (m)

where, D is maximum pipe diameter at respective level

W is useable width of pipe rack

Force on Pipe: F = qzxGxCfxAf

Transverse Wind Force due to Cable Tray:

Ref: Sec. 4.1, ASCE-Report – Wind Loads and Anchor Bolt Design for Petrochemical Facilities

Tributary Area: Ae = (D + 10%W) x Bent Spacing (m)

where, D is maximum pipe diameter at respective level sss

W is useable width of pipe rack

Force on Pipe: F = qzxGxCfxAf

Figure 5‑8 Wind load in X – Direction

Figure 5‑9 Wind load in Z – Direction

SEISMIC LOAD (SL_X/SL_Z/SL_Y)

1. Risk Factor = III Table 1.5-1/ASCE-7-10
2. Importance Factor, I = 1.25 Table 1.5-2/ASCE-7-10
3. Seismic Design Category = C Table 11.6-1/ASCE-7-10
4.  
5. Response modification coefficient, R
6. Steel ordinary braced frame X-Dir = 3.25 Table C15.5-1/ASCE-7-10
7. Steel ordinary moment frames Z-Dir = 3.5 Table C15.5-1/ASCE-7-10
8.  
9. For initial Staad factor in X Dir I/R 0.38
10. For initial Staad factor in Z Dir I/R 0.36
11. The Site Specific Design Response Spectrum shall be considered from Section 6.7 of project document “Design Specification for Loads and Load Combinations”
    
12. 
    Figure 5‑10 Seismic load in X/Z/Y – Direction

SERVICIABILITY LOAD COMBINATION

STRENGTH LOAD COMBINATION (LRFD)

RESULTS

BEAM DEFLECTION CHECK

1. Local deflections of the beams are checked using STAAD software under serviceability load combinations and ensured design is adequate for the limit specified in project document “Design Specification for Steel Structures”
2. Pipe rack main beams : L/400
3. Pipe rack secondary beams : L/250
4. Critical Beam Deflection results:

COLUMN DEFLECTION CHECK

WIND DRIFT

1. Horizontal displacement at top of columns due to wind is limited to H/100 where H is the total column height.
2. Governing Load Case: LC- 1018 (DS+DO+TS2+TTSGU+0.60 WLZ)
3. 
4. 
   Figure 6‑1 Column Drift due to wind along Z Direction
5. The following table shows the maximum deflection at top of column and relevant ratio:

1. 
   Table 6‑1 : Horizontal Displacement check due to wind

SEISMIC DRIFT

1. Allowable seismic drift limits shall be in accordance with ASCE/SEI 7-10 (Sect.12.8.6). Being the structure assigned to Seismic Category C and Occupancy Category IV, the design story drift shall not exceed h_sx/200 (where h_sx is the story height below level x).
2. 
3. 
   Figure 6‑2 Column Drift due to seismic along Z Direction
4. Actual Drift
5. C_d*δ_xe / I < h_sx/200
6. Where:
7. C_d = deflection amplification factor = 3.0 (as per Table 15.4-1) – steel ordinary moment frame;
8. δ_xe = calculated deflection;
9. I = importance factor = 1.25 (as per Table 1.5-2)
10. Governing Load Case: LC- 1022 (DS+DO+TS2+TTSGU+0.21 SLX+0.70 SLZ+0.21 SLY)

1. 
   Table 6‑2 : Seismic Storey drift check

UNITY RATIO

1. Design of the structure is performed using STAAD software and ensured member design adequacy. The utility ratios of the members are presented below.

1. 
   Table 6‑3 : Maximum utility ratio of the structure – Strength Case

STAAD UNITY STRESS RATIO

1. 
   Figure 6‑3 Staad Unity Stress picture