PIP Code_Wind Load on Piperack and Structure

Previous Issue: New Next Planned Update: 1 September, 2007 Page 1 of 39 Primary contact: Abu-Adas, Hisham on phone 874-6908

Best Practice

SABP-M-006 31 August, 2002 Wind Loads on Piperacks and Open Frame Structures Document Responsibility: Onshore Structures

Wind Loads on Piperacks and Open Frame Structures

Developed by: Hisham Abu-Adas Developed: July, 2002 Civil Engineering Unit/M&CED Consulting Services Department

Document Responsibility: Onshore Structures SABP-M-006 Issue Date: 31 August, 2002 Wind Loads on Piperacks Next Update: 1 September, 2007 and Open Frame Structures

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Table of Contents Page

1 Introduction .............................................................................................. 3 1.1 Purpose ............................................................................................ 3 1.2 Scope ............................................................................................... 3 1.3 Disclaimer......................................................................................... 3 1.4 Conflicts with Mandatory Standards ................................................. 3

2 References............................................................................................... 4 2.1 Saudi Aramco References................................................................ 4 2.2 Industry Codes and Standards ......................................................... 4

3 General .................................................................................................... 4 3.2 Wind Speed V................................................................................... 4 3.3 Importance Factor I .......................................................................... 4 3.4 Exposure Category........................................................................... 4 3.5 Basic ASCE 7 Formulas ................................................................... 5 3.6 Velocity Pressure qz.......................................................................... 5 3.7 Gust Effect Factors G ....................................................................... 6 3.8 Force Coefficient Cf .......................................................................... 6 3.9 Projected Area Ae ............................................................................. 6

4 Wind Loads on Pipe Racks & Open Frame Structures ........................... 6 4.1 Piperacks.......................................................................................... 6 4.2 Open Frame Structures .................................................................... 8

ATTACHMENTS:

Attachment 1 – List of Tables 1. Basic Wind Speed V for Saudi Aramco Sites 2. Velocity Pressure qz (Customary Units) 3. Velocity Pressure qz (Metric Units)

Attachment 2: .................................................................................................22 Example 1 – Wind Load on Piperack Attachment 3: .................................................................................................26 Example 2 – Wind Load on Piperack Attachment 4 ..................................................................................................31 Example 3 – Wind Load on Open Frame Structure


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1 Introduction

Most of the design practices in the petrochemical industry were based on the wind loading provisions of ASCE 7, “Minimum Design Loads for Buildings and Other Structures” or its predecessor (ANSI A58.1). ASCE 7 does not adequately address Piperacks and Open Frame Structures. It is the intent of this practice to provide uniform application of practices to all Piperacks and Open Frame Structures designed for Saudi Aramco projects.

1.1 Purpose

The purpose of this practice is to provide the engineer and designer with guidelines for wind load on Piperacks and Open Frame Structures for use by engineers working on Saudi Aramco projects and Saudi Aramco engineers.

1.2 Scope

This design guideline covers the minimum requirements and provides guidance for calculating wind load on onshore Piperacks and Open Frame Structures typically located in petrochemical facilities. Section 2.0 of this instruction includes reference codes, Saudi Aramco standards, and specifications. In cases where this guideline conflicts with these references, the conflict shall be immediately brought to the attention of the project engineer.

1.3 Disclaimer

The material in this Best Practices document provides the most correct and accurate design guidelines available to Saudi Aramco which comply with international industry practices. This material is being provided for the general guidance and benefit of the Designer. Use of the Best Practices in designing projects for Saudi Aramco, however, does not relieve the Designer from his responsibility to verify the accuracy of any information presented or from his contractual liability to provide safe and sound designs that conform to Mandatory Saudi Aramco Engineering Requirements. Use of the information or material contained herein is no guarantee that the resulting product will satisfy the applicable requirements of any project. Saudi Aramco assumes no responsibility or liability whatsoever for any reliance on the information presented herein or for designs prepared by Designers in accordance with the Best Practices. Use of the Best Practices by Designers is intended solely for, and shall be strictly limited to, Saudi Aramco projects. Saudi Aramco® is a registered trademark of the Saudi Arabian Oil Company. Copyright, Saudi Aramco, 2002.


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1.4 Conflicts with Mandatory Standards

In the event of a conflict between this Best Practice and other Mandatory Saudi Aramco Engineering Requirement, the Mandatory Saudi Aramco Engineering Requirement shall govern.

2 References

This Best Practice is based on the latest edition of the references below, unless otherwise noted. Short titles will be used herein when appropriate. Short titles will be used herein when appropriate.

2.1 Saudi Aramco References

Saudi Aramco Engineering Standards (SAES)

SAES-A-112 Meteorological and Seismic Design Data SAES-M-001 Structural Design Criteria for non-Building

Structures

2.2 Industry Codes and Standards

American Society of Civil Engineers (ASCE)

ASCE 7 – 95 Minimum Design Loads for Buildings and Other Structures

Wind Load and Anchor Bolt Design for Petrochemical Facilities

3 General

3.1 Wind loads shall be computed and applied in accordance with SAES-M-001, ASCE 7-, and the recommended guidelines for Piperacks, and Open Frame Structure in ASCE’s “Wind loads and Anchor Bolt Design for Petrochemical Facilities”.

3.2 Wind load calculations shall be based on basic wind speed V of 3-second gust speed at 33 ft (10 m) above the ground in Exposure C and associated with an annual probability 0.02 of being equaled or exceeded (50-year mean recurrence interval). The basic wind speed V for each site is defined in SAES-A-112, “Meteorological and Seismic Design Data” and Table 1 (Attachment 1).

3.3 The Importance Factor I shall be category IV.

3.4 Exposure Category C shall be used, except for structures close to the shoreline, as defined in ASCE, where Exposure Category D shall be used. Exposure D is defined as ‘Flat, unobstructed areas exposed to wind flowing over open water


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for a distance of at least 1 mi 91.61 km). The Exposure category for each site is defined in SAES-A-112, “Meteorological and Seismic Design Data”.

3.5 Basic ASCE 7 Formulas

The design wind forces for the main wind force resisting system shall be per Table 6-1 (ASCE 7) for “Open Buildings and Other Structures”. The applied wind force F shall be determined by the basic equation:

F = qz G Cf Ae ASCE 7 Table 6-1

where

qz = Velocity pressure at height z above ground

G = Gust response factor

Cf = Force coefficient

Ae = Projected area normal to wind

3.6 Velocity Pressure qz

The velocity pressure qz is determined in accordance with the provisions of Section 6.5 of ASCE 7.

qz = 0.00256 Kz Kzt V2 I (lb/sq ft)ASCE 7 (Eq. 6-1)

qz = 0.613 Kz Kzt V2 I (N/m2) [SI Units]

where

Kz is given in Table 6-3 of ASCE 7.

Kzt as per provisions of 6.5.5 of ASCE 7. Kzt is equal to 1.0 for Piperacks and Open Frame Structures located in Saudi Aramco facilities.

V is basic wind speed of 3-second gust speed at 33 ft above the ground.

I is the Importance Factor set forth in Table 6-2 of ASCE 7. I equal 1.15 for Category IV structures. All Piperacks and Open Frame Structures at Saudi Aramco facilities are considered Category IV structures.

Velocity pressures qz are determined using ASCE 7, Eq. 6-1 shown above. Attachment 1 - Tables 2 (Customary Units) and 3 (Metric Units) provides values for qz at several heights for most Saudi Aramco sites. These values are to be used for Piperacks and Open Frame Structures.


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3.7 Gust Effect Factors G

The gust effect factor G is determined in accordance with the provisions of Table 6-1 and Section 6.6 of ASCE 7. For rigid structures, the simplified method of section 6.6.1 specifies G=0.85 for structures in terrain exposure C and D. This value should be used for all Piperacks and Open Frame Structures.

For flexible or dynamically sensitive structures Gf is used in place of G. Flexible structures defined by ASCE 7 as those structures with a fundamental frequency f < 1 Hz or if the height divided by least horizontal dimension is greater than 4. Gf shall be calculated by a rational method as given in ASCE 7 Commentary Section 6.6.

3.8 Force Coefficient Cf

The force coefficient Cf for the various structures shall be as listed in the following sections of this guideline.

3.9 Projected Area Ae

The projected area Ae normal to wind direction for the various structures in question shall be as defined in this guideline.

4 Wind Loads on Pipe Racks & Open Frame Structures

4.1 Piperacks

Wind on the piperack structure itself should be calculated based on no shielding. For all structural members Cf = 1.8 shall be used, except Cf = 2.0 shall be used for columns.

4.1.1 Tributary Area for Piping

The tributary area for piping should be based on the diameter of the largest pipe plus 10% of the width of the pipe rack. This sum is multiplied by the length of the pipes (bent spacing) to determine the tributary area.

4.1.2 Tributary Area for Cable Trays

The tributary area for cable trays should be based on the height of the largest tray plus 10% of the width of the pipe rack. This sum is multiplied by the length of the pipes (bent spacing) to determine the tributary area.


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4.1.3 Force Coefficients for Pipes

The force coefficient Cf = 0.7 should be used as a minimum. The force coefficient Cf is taken from ASCE 7, Table 6-7 (see below) for a round shape, with h/D=25, D√qz > 2.5 and a moderately smooth surface; that is Cf = 0.7. If the largest pipe is insulated, then consider using a Cf for a rough pipe dependent on the roughness coefficient of the insulation (D′/d).

Table 6-7 – (Adapted from ASCE 7)


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4.1.4 Force Coefficients for Cable Trays

For cable trays the force coefficient Cf = 2.0 shall be used. The force coefficient Cf, for cable trays is taken from ASCE 7, Table 6-7 for a square shape with the face normal to the wind and with h/D = 25; that is Cf = 2.0.

4.2 Open Frame Structures

4.2.1 General

Wind loads should be calculated in accordance with the general procedures and provisions of ASCE 7 for wind loads on “Other Structures”.

4.2.1.1 Main Wind Force Resisting System

Wind forces acting on structural frame and appurtenances (ladders, handrails, stairs, etc.) should be computed in accordance with 4.2.1.2 and 4.2.2.

Wind forces on piping and cable trays located on or attached to the structure should be calculated according to the provisions of 4.1 and added to the wind forces acting on the frames in accordance with 4.2.6.

4.2.1.2 Force Coefficients for Components

Wind loads for the design of individual components, cladding, and appurtenances (excluding vessels, piping and cable trays) should be calculated according to the provisions of ASCE 7. Force coefficients for several items are given in Table 4.1 below.

TABLE 4.1 Force Coefficients for Wind Loads on Components

(Adapted from ASCE – “Wind Loads and Anchor Bolt Design for Petrochemical Facilities”)

Item Cf Projected Area Handrail 2.0 0.80 sq. ft./ft. Ladder without cage 2.0 0.50 sq. ft./ft. Ladder with cage 2.0 0.75 sq. ft./ft. Solid Rectangles & Flat Plates 2.0 Stair w/handrail Side Elevation End elevation

2.0 2.0

Handrail area plus channel depth 50% gross area

Round or Square Shapes See ASCE 7 Table 6-7


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4.2.2 Frame Load

4.2.2.1 Design wind forces for the main wind force resisting system for open frame structure should be determined by the equation:

Fs = qz G Cf Ae ASCE 7 Table 6-1 (Eq. 4.1a)

where

Fs is the wind force on structural frame and appurtenance

qz as defined in Section 3.6

G as defined in Section 3.7

Cf the force coefficient is determined from the provisions of 4.2.3

Ae is the area of application of force as determined per 4.2.5

The design load cases are computed per 4.2.6

The structure is idealized as two sets of orthogonal frames. The maximum wind force on each set of frames is calculated independently.

Note: Cf accounts for the entire structure in the direction of the wind.

4.2.2.2 Limitation of Analytical Procedure

Design wind forces are calculated for the structure as a whole.

The method is described for structure, which are rectangular in plan and elevation.

4.2.3 Force Coefficients Cf

The force coefficient for a set of frames shall be calculated by

Cf = CDg / ε (Eq. 4-2)

where

CDg is the force coefficient for the set of frames given in Figure 4.1, and ε is the solidity ratio calculated in accordance with 4.2.4.

Force coefficients are defined for wind forces acting normal to the wind frames irrespective of the actual wind direction.


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(Adapted from ASCE “Wind Loads and Anchor Bolt Design for Petrochemical Facilities”)


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4.2.4 Solidity Ratio ε

ε = As / Ag (Eq. 4-3)

where

Ag is the gross area of the windward frame and

As is the effective solid area of the windward frame defined by the following:

a) The solid area of a frame is defined as the solid area of each element in the plane of the frame projected normal to the nominal wind direction. Elements considered as part of the solid area of a frame include beams, columns, bracing, cladding, stairs, ladders, handrails, etc. Items such as vessels, tanks, piping and cable trays are not included in calculations of solid area of frame; wind load on these items are calculated separately.

b) The presence of flooring or decking does not cause an increase of the solid area beyond the inclusion of the thickness of the deck.

c) For structures with frames of equal solidity, the effective solid area As should be taken as solid area of the windward frame.

d) For structures where the solid area of the windward frame exceeds the solid area of the other frames, the effective solid area As should be taken as the solid area of the windward frame.

e) For structures where the solid area of the windward frame is less than the solid area of the other frames, the effective solid area As should be taken as the average of all the frames.

4.2.5 Area of Application of Force

Ae shall be calculated in the same manner as the effective solid area in 4.2.4 except that it is for the portion of the structure height consistent with the velocity pressure qz.

4.2.6 Design Load Cases

• The total wind force acting on the structure in a given direction, FT, is equal to the sum of the wind load acting on the structure and appurtenances (Fs), plus the wind load on the equipment and vessels,


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plus the wind load on piping. See Figure 4.2 for complete definitions of FT and FS.

• If piping arrangements are not known, the engineer may assume the piping area to be 10% of the gross area of the face of the structure for each principal axis. A force coefficient of 0.7 should be used for this piping area.

The following two load cases must be considered as a minimum:

• Frame load + equipment load + piping load (FT) for one axis, acting simultaneously with 50% of the frame load (FS) along other axis, for each direction. These two combinations are indicated in Figure 4.2.


Revision Summary 31 August, 2002 New Saudi Aramco Best Design Practice.


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TABLE 1: Basic Wind Speed - 3-sec. Gust per SAES-A-112

Site Miles per hour Kilometer per hour Location mph km/hr

Abha 93 150 Abqaiq 93 150 Al-Jauf 103 165 Ar'ar 112 181 Berri 93 150 Dhahran 93 150 Duba 96 155 Hawiyah 93 150 Haradh 93 150 Hawtah 96 154 Hofuf 93 150 Jeddah 93 150 Jizan 96 155 Ju'aymah 93 150 Khamis Mushayt 93 150 Khurais 101 163 Medina 96 155 Najran 93 150 Qasim 119 191 Qaisumah 114 183 Qatif 93 150 Rabigh 93 150 Ras Tanura 93 150 Riyadh 103 165 Safaniya 96 155 Shaybah 96 155 Shedgum 96 155 Tanajib 96 155 Tabuk 106 171 Turaif 103 165 Udhailiyah 96 155 Uthmaniyah 96 155 Yanbu 93 150


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Attachment 2

EXAMPLE 1 - WIND LOAD ON PIPERACK

Design a piperack in Uthmaniyah Gas Plant. The pipe rack case shall be as shown in Figure 1 (Bent spacing = 20 ft), with a 3-second Gust wind speed of 96 mph per SAES-A-112.

Design wind forces are determined by the equation per Section 3.5 (repeated below) where F is the force per unit length of the piping or cable tray:


Design wind pressure, for 30 ft elevation

qz = 26.59 psf (Ref. Table 2 – Attach. 1)

Gust effect factor, G = 0.85 (ASCE 7, Section 6.6.1)

Force Coefficients For structural members Cf = 1.8 (Ref. Section 4.1) For columns Cf = 2.0 (Ref. Section 4.1) For pipes Cf = 0.7 (Ref. Section 4.1.3) For cable trays Cf = 2.0 (Ref. Section 4.1.4)

Projected Area

Projected Area per foot of pipe rack, Ae = Largest pipe diameter or cable tray height + 10% of pipe rack width. (Ref. Sections 4.1.1 and 4.1.2)

PART I – PIPING AND CABLE TRAY The guidelines require the consideration of the piping or cable trays separately from the structural members. The following calculations are only for piping and cable trays without the structural support members: Force Calculation Force (Pounds) F1 Cable Tray 6” Deep (@ level 30’-0”) Cf = 2.0 Ae = 0.5 + (10% *25 ft) = 3.0 sq ft F1 = qz G Cf Ae

F1 = [(26.59 psf)*(0.85) * (2.0) * (3.0)]*20.0 ft bent spacing F1=2712.2 lb.


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F2 Pipe Level 25 ft – 24” Max. O.D. Cf = 0.7 Ae = 2.0 + (10% *25 ft) = 4.5 sq ft F2 = qz G Cf Ae




PART II – STRUCTURAL MEMBERS For structural members, assume 25 ft wide rack with bent spacing of 20 ft centers, all stringers not shielded. Stringers at elevations 30.0, 22.5 and 17.5 (Refer to Figure 1) Assume qz = 26.59 psf for all 3 levels of stringers (conservative) Cf = 1.8 Ae = 9.73/12 ft (beam depth) * 20 ft (beam length) = 16.22 ft2 F = qz G Cf Ae

F4=F5=F6= (26.59 psf) * 0.85 *1.8 * 16.22 ft2 = 660 lb.

Columns qz = 26.59 psf at elev. 30 ft qz = 25.50 psf at elev. 25 ft qz = 24.42 psf at elev. 20 ft

Use qz = 26.59 psf for the whole column (conservative)

Cf = 2.0 Ae = 8/12 ft (column width) * 1 ft = 0.67 ft2/linear foot F = qz G Cf Ae Force per column F = (26.59 psf) * 0.85 * 2.0 * 0.67 = 30.3 lb / ft For piperack wind load values on the bent, refer to Figures 1 & 1A below:


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Typical Bent Figure 1

25’-0”

W 12 x 45 2.5’

2.5’

W 12 x 40

W 10 x 33

5’-0”

5’-0”

20’-0”

W 14 x 53

F1

F2

F3

F6

WIND LOAD ON PIPES & STEEL MEMBERS

30’-0”

6” Conduit Racks

24” O.D. Max.

18” O.D. Max.

3 ft F5

F6 5 ft

W 10 x 33 TYP.

d = 9.73”

F4

bf = 8”

EL 0.00

F4

F5

W 14 x 53

30.3 #/ft 30.3 #/ft

Example 1


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Typical Bent Example 1

Figure 1A

25’-0”

W 12 x 45 2’-6”

2’-6”

W 12 x 40

W 10 x 33

5’-0”

5’-0”

20’-0”

W 14 x 53 W 14 x 53

2712# 660#

1424#

660#

1266#

660# 660#

660#

660#

WIND LOAD ON PIPES & STRUCTURE

30’-0”

30.3 #/ft 30.3 #/ft

Attach. 2


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Attachment 3

EXAMPLE 2 - WIND LOAD ON PIPERACK

The piperack case shall be as shown in Figure 2 (bent spacing = 20 ft), with a 3-second Gust wind speed of 103 mph.

Design wind forces are determined by the equation per Section 3.5 (repeated below) where F is the force per unit length of the piping or cable tray:


Design wind pressure, for 35 ft elevation

qz = 31.55 psf (Ref. Table 2 – Attach. 1)

Gust effect factor, G = 0.85 (ASCE 7, Section 6.6.1)

Force Coefficients For structural members Cf = 1.8 (Ref. Section 4.1) For columns Cf = 2.0 (Ref. Section 4.1) For pipes Cf = 0.7 (Ref. Section 4.1.3) For cable trays Cf = 2.0 (Ref. Section 4.1.4)

Projected Area

Projected Area per foot of pipe rack, Ae = Largest pipe diameter or cable tray height + 10% of pipe rack width. (Ref. Sections 4.1.1 and 4.1.2)

PART I – PIPING AND CABLE TRAY

The guidelines require the consideration of the piping or cable trays separately from the structural members. The following calculations are only for piping and cable trays without the structural support members:

Force Calculation Force (Pounds) F1 Cable Tray 6” Deep (@ elev. 35’-0”) Cf = 2.0 Ae = 0.5 + (10% *20 ft) = 2.5 sq ft F1 = qz G Cf Ae

F1= [(31.55 psf) * (0.85) * (2.0) * (2.5)] * 20.0 ft bent spa. F1 = 2681.8 lb


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F2 = [(30.80 psf) * (0.85) * (0.7) * (3.0)] * 20.0 ft bent spa. F2 = 1100 lb







PART II – STRUCTURAL MEMBERS

For structural members, assume 20 ft wide rack with bent spacing of 20 ft centers, all stringers not shielded.

Stringers at elevations 35.0, 24.0, 18.0 and 12.5 Cf = 1.8 Ae = 9.73/12 ft (beam depth) * 20 ft (beam length) = 16.22 ft2 F = qz G Cf Ae = qz x 0.85 x 16.22 = qz x 24.82

F6 = 31.55 x 24.82 = 783 lb F7 = 29.11 x 24.82 = 723 lb F8 = 27.49 x 24.82 = 682 lb F9 = 26.55 x 24.82 = 659 lb


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Columns qz = 31.55 psf at elev. 35 ft qz = 28.11 psf at elev. 20 ft

Use qz = 31.55 psf for 20-35’ column (conservative)

Cf = 2.0 Ae = 10/12 ft (column width) * 1 ft = 0.83 ft2/linear foot F = qz G Cf Ae

Force per column F = (31.55 psf) * 0.85 * 2.0 * 0.83 = 44.5 pounds / foot

Force for 0-20’ = F = 28.11 x 0.85 x 2.0 x 0.83 = 39.7 pounds / foot


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For piperack wind load values on bent, refer to Figures 2 & 2A below:

3’

6” Conduit Racks

3’-0”

F6F6

F1

F2

F3

F4

F5

F7F7

F8F8

W 10 x 33 TYP.

W 14 x 61 W 14 x 61

F9 F9

3’-0”

2.5’

W 10

W 12

W 12

W 12

W 12

12”

20”

30” O.D. Max

24” O.D. Max

15’-0”

6’-0”

6’-0”

4’-0”

4’-0”

35’-0”

0.0

20 ft

Typical Bent Figure 2


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3’-0”

3’-0”

783#2682#

1100

1304

1519

1264 W 10 x 33

TYP.

W 14 x 61 W 14 x 61

3’-0”

2’-6”

W 10 x 33

W 12 x 40

W 12 x 40

W 12 x 40

W 12 x 45

bf = 10” 15’-0”

6’-0”

6’-0”

4’-0”

4’-0”

35’-0”

20’-0”

Typical Bent Example 2

Figure 2A

783#

722

682

6593

722

682

39.7 #/ft

659

Attach. 3

El. 35’-0”

El. 20’-0’

44.5 # / ft

El. 0’-0”

39.7 # / ft


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Attachment 4

EXAMPLE 3

3.0 WIND LOAD ON OPEN FRAME STRUCTURE

The plan and elevation views of the structure are shown in Figures 3.1, 3.2 and 3.3. The structure considered is 40 ft (12.19 m) x 40 ft (12.19 m) x 82 ft (24.99 m) high, with three open frames in the direction of wind. The basic wind speed V = 119 mph. This is a 3-second gust wind speed with an annual probability of exceeding of 0.02.

Member sizes are assumed as follows:

Columns - 12 in x 12 in (0.31 m x 0.31 m) Beams at El. 20’-0’’ (6.10 m) - W36 Beams at El. 48’-0” (14.63 m) - W18 Beams at El. 82’-0” (24.99 m) - W18 Braces - W8 Intermediate Beams - W12

Design wind forces are determined by the Equation 4.1a (Sect. 4.2.2)

Fs = qz G Cf Ae ASCE 7 Table 6-1 (Eq. 4.1a)

where Fs is the wind force on structural frame and appurtenance

It is convenient to determine the velocity pressures at the mid-floor heights and at the top of the structure. Table 3.1 below summarizes qz values at various levels:

Table 3.1

Velocity Pressure qz

Height above Ground z (ft) *qz (psf) 10 36.0 34 42.4 65 48.8

h = 83 51.7

Note: To convert psf to N/m2 multiply values in this table by 47.878

*qz is obtained from Table 1 – Attachment 1

Although the top of the third floor level is at 82 ft, the structure height “h” is increased slightly to account for the handrail on top of the structure (see Figures 3.2 and 3.3).


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The gust effect factor is determined next. The ratio of height / least horizontal dimension = 83 ft / 41 ft = 2.02 < 4, therefore the structure is not considered a flexible structure.

Use Gust effect factor, G = 0.85 (ASCE 7, Section 6.6.1)

3.1 ALONG WIND FORCE CALCULATIONS

In order to calculate the force coefficient, the solidity ratio ε must first be computed from Equation 4.3, which stated ε = As / Ag. The gross area (or envelope area) is the area within the outmost projections of the front face normal to the nominal wind direction. Note that the width used below is measured from outside column face to outside column face. For the wind direction shown in Figure 3.1,

Ag = 83 ft (height) x 41 ft (width) = 3,403 ft2 (316 m2)

To determine the effective solid area, the solid area of the windward frame must first be calculated per 4.2.4.a. In order to facilitate the computation of forces later in the problem, it is convenient to calculate the solid areas from mid-floor to mid-floor, and sum these to obtain the total solid area of the frame. Calculation of solid area of the windward frame (column line 3) is summarized in Table 3.2. The stairs are considered as part of the windward frame (see Figure 3.1). The stairs column in Table 3.2 includes area of stair stringer, struts, handrails, and bracing.

TABLE 3.2

Solid Area of Windward Frame - As

Solid Areas (ft2) Floor Level

Tributary Height (ft)

Cols. Beams Interm. Bracing Handrails Stairs Total Beams

0 0-10 30 0 0 19 0 76 125 1 10-34 72 120 40 40 32 150 454 2 34-65 93 60 80 38 40 91 402 3 65-83 51 60 40 38 40 17 236 Total Solid Area of Windward Frame (ft2) = 1217 ft2

Note: To convert ft2 to m2 multiply values in this table by 0.0929

Since the middle and leeward frames (column lines 2 and 1, respectively) are similar to the windward frame with the exception of not having stairs, the solid areas and hence the solidity ratios for these two frames will be less than the windward frame, so As is equal to the solid area of windward frame per 4.2.4.d, which leads to

ε = As / Ag = 1,217 ft2 / 3,403 ft2 = 0.358


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Next, the coefficient CDg is obtained from curves given in Figure 4.1 as a function of the solidity ratio ε, the number of frames N, and the frame spacing ratio SF/B. As defined in Figure 4.1, N=3 and SF/B= 20 ft / 41 ft = 0.488. From Figures 4.1c & 4.1d, for N = 3 and extrapolating slightly for ε = 0.358

CDg = 1.09, for SF/B= 0.5 CDg = 1.03, for SF/B= 0.33 Interpolating for SF/B= 0.488, CDg =1.03 + (1.09 –1.03)[(0.488-0.33) / (0.50-0.33)] = 1.086

Next, the gross area force coefficient CDg is converted into a force coefficient compatible with ASCE 7 by means of Equation 4.2.

Cf = CDg / ε = 1.086 / 0.358 = 3.03

The area of application of force Ae has already been determined per floor level during calculation of solidity ratio. The wind force transmitted to each floor level may now be found by Equation 4-1a, F = qz G Cf Ae as shown below. The total force on the structural frame and appurtenances Fs is 143.1 kips (639.9 kN), found by summing the forces at all levels in Table 3.3.

TABLE 3.3

Total Force – Structural Frame and Appurtenances - Fs

Floor Level

qz (psf)

G Cf Ae (ft2)

F (lbs)

0 36.0 0.85 3.03 125 11,590 1 42.4 0.85 3.03 454 49,577 2 48.8 0.85 3.03 402 50,525 3 51.7 0.85 3.03 236 31,424 Fs = ΣF = 143,116

Note: To convert pounds force (lbs) to newtons (N) multiply F values in this table by 4.448

These forces are due to wind acting on the frames only. Wind forces acting on the vessels, equipment and piping are computed separately. Wind loads on vessels, equipment and piping are not in the scope of this guideline (Refer to Sections 4.1 and 4.3 of the ASCE reference “Wind Loads and Anchor Bolt Design for Petrochemical Facilities” for requirements).

3.2 CROSSWIND FORCE CALCULATIONS

The next step is to repeat the analysis for the nominal wind direction normal to column line A (see Figures 3.1 and 3.3) – “non-windward” frame. The member sizes are the same on this elevation except that the intermediate beams are W10’s and


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Beams at El. 20’-0” - W14



The gross area of the windward face includes the stair tower on the right hand side of the structure.

Ag = (83 x 41) + (9 x 49) = 3,844 ft2

The solid areas for the windward frame are given below. The stairs column in the table includes areas of the stair column, struts, and handrails (see Table 3.4).

TABLE 3.4

Solid Area - As

Solid Areas (ft2) Floor Level

Tributary Height (ft)

Cols. Beams Interm. Bracing Handrails Stairs Total Beams

0 0-10 30 0 0 19 0 24 73 1 10-34 72 46 8 35 40 44 245 2 34-65 93 53 41 36 40 36 299 3 65-83 51 40 0 16 40 0 147 Total Solid Area of Windward Frame (ft2) = 764 ft2

Note: To convert ft2 to m2 multiply As values in this table by 0.0929

Since the solidity of neither the middle and leeward frames (column lines B and C, respectively) exceeds that of the windward frame, As is equal to the solid area of windward frame, yielding

ε = As / Ag = 764 ft2 / 3,844 ft2 = 0.199

The frame spacing ratio in this direction is SF/B = 20 ft / 46 ft = 0.435. Since the width is not uniform (the stair tower stops at the second floor level), an average value of B is used. From Figures 4.1c & 4.1d for N=3 and ε = 0.199

CDg = 0.72, for SF/B= 0.5

CDg = 0.71, for SF/B= 0.33

Therefore use CDg = 0.716

Cf = CDg / ε = 0.716 / 0.199 = 3.60


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The wind forces per floor level are shown in Table 3.5

TABLE 3.5

Total Force – Structural Frame and Appurtenance - Fs

Floor Level

qz (psf)

G Cf Ae (ft2)

F (lbs)

0 36.0 0.85 3.60 73 8,042 1 42.4 0.85 3.60 245 31,787 2 48.8 0.85 3.60 299 44,649 3 51.7 0.85 3.60 147 23,256 Fs = ΣF = 107,734

Note: To convert pounds force (lbs) to newtons (N) multiply F values in this table by 4.448

3.3 Open Frame Example – Summary and Conclusion

The results thus far are summarized in the Table 3.6. The load combinations for design are application of FT in one direction simultaneously with 0.5 Fs in the other, per 4.2.6.1. These combinations are shown in Figure 3.4.

TABLE 3.6

Summary

Wind – Direction 1 Wind – Direction 2 Wind Load on FS Structural Frame

143 kips (636 kN) 108 kips (480 kN)

Wind Load on FE Equipment And Piping

*Assume 27 kips (120 kN)

*Assume 19 kips (84.5 kN)

Total Wind Load on FT Structure

170 kips (756 kN) 127 kips (565 kN)

*No equipment or piping were included in this example, but for illustration purpose, assume that horizontal vessels and piping were included and the wind loads are as shown in the Table.


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Adapted from ASCE “ Wind Loads and Anchor Bolt Design for Petrochemical Facilities”


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PIP Code_Wind Load on Piperack and Structure