SABP-Q-003-1

Previous Issue: 31 July 2002 Next Planned Update: 1 May 2009 Page 1 of 41 Primary contact: Abu-Adas, Hisham on phone 874-6908

Best Practice SABP-Q-003 30 April 2005 Vertical Vessel Foundation Design Guide

Document Responsibility: Onshore Structures Standards Committee

Vertical Vessel Foundation Design Guide

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

Document Responsibility: Onshore Structures SABP-Q-003 Issue Date: 30 April 2005 Next Planned Update: 1 May 2009 Vertical Vessel Foundation Design Guide

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VERTICAL VESSEL FOUNDATION DESIGN GUIDE

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 Process Industry Practices (PIP) ....................................... 4 2.2 Industry Guides And Standards ......................................... 4 2.3 Saudi Aramco Standards................................................... 4 2.4 Saudi Aramco Best Practices ............................................ 4

3 General......................................................................................... 5 4 Design Procedure......................................................................... 5

4.1 Design Considerations....................................................... 5 4.2 Vertical Loads .................................................................... 6 4.3 Horizontal Loads................................................................ 7 4.4 Load Combinations.......................................................... 10 4.5 Pedestal........................................................................... 12 4.6 Anchor Bolts .................................................................... 14 4.7 Footing Design................................................................. 16

Attachments: Figures, Tables, and Example Figure A - Foundation Pressures for Square Bases ................................... 24 Figure B - Foundation Pressures for Octagon Bases ................................. 25 Table 1 - Octagon Properties...................................................................... 26 Table 2 - Foundation Pressures for Octagon Bases................................... 31 Table 3 - Basic Development Length.......................................................... 32 Example - Vertical Vessel Foundation Design............................................ 33


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1 Introduction

1.1 Purpose

The purpose of this Practice is to establish guidelines and recommended procedures for the analysis and design of vertical vessel foundations for use by engineers working on Saudi Aramco projects and Saudi Aramco engineers. It shall be used where applicable unless otherwise specified.

1.2 Scope

This design guide defines the minimum requirements for the analysis and design of vertical vessel foundations for Saudi Aramco plants. In the ensuing sections, pertinent references are given, and design loadings and general design consideration are presented and discussed. This Practice addresses isolated foundations supported directly on soil. Pile supported footings are not included in this practice. The Process Industry Practice STE03350 forms the basis for the development of this design guide.

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.

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.


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2 References

This Best Practice is based on the latest edition of the references below, unless otherwise noted.

2.1 Process Industry Practices (PIP) PIP STE03350 Vertical Vessel Foundation Design Guide

2.2 Industry Guides and Standards

American Concrete Institute (ACI)

ACI 318-02 Building Code Requirements for Reinforced Concrete

American Society of Civil Engineers (ASCE)

ASCE 7-02 Minimum Design Loads for Buildings and Other Structures

Wind Load and Anchor Bolt Design for Buildings and Other Structures

2.3 Saudi Aramco Standards

Saudi Aramco Engineering Standards (SAES)

SAES-A-112 Meteorological and Seismic Design Data SAES-A-204 Preparation of Structural Calculations SAES-M-001 Structural Design Criteria for Non-Building

Structures SAES-Q-001 Criteria for Design and Construction of Concrete

Structures SAES-Q-005 Concrete Foundations

2.4 Saudi Aramco Best Practices SABP-Q-001 Anchor Bolt Design and Installation SABP-Q-002 Spread Footings Design


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3 General

3.1 The design and specifications for construction of vertical vessel foundation shall be adequate for the structure intended use, in accordance with commonly accepted engineering practice, Saudi Aramco Standard SAES-Q-005 and this guideline.

3.2 A geotechnical investigation is required for all new structures and foundations as described in SAES-A-113. (Ref. SAES-Q-005, para. 4.1.1)

3.3 The allowable soil bearing pressure shall be based on the results of the geotechnical investigation, and a consideration of permissible total and differential settlements. Soil pressures shall be calculated under the action of vertical and lateral loads using load combinations that result in the maximum soil pressures. The maximum soil pressure shall not exceed the applicable allowable value. (Ref. SAES-Q-005, para. 4.1.2)

3.4 Foundations shall be founded on either undisturbed soil or compact fill and at least 600 mm below the existing or finished grade surface, unless a detailed soils investigation indicated otherwise. In the case of foundations supported on compacted fill, the geotechnical investigation and/or SAES-A-114 shall govern the type of fill material and degree of compaction required. (Ref. SAES-Q-005, Para. 4.1.3)

3.5 The design and construction of all concrete foundations shall comply with the requirements of SAES-Q-001, SAES-Q-005 and ACI 318-02. (Ref. SAES-Q-005, para. 4.3.1)

3.6 The design concrete compressive strength of concrete shall be 27.6 MPa (4000 psi) at 28 days. (Ref. SAES-Q-005, para. 4.3.2.b)

3.7 The structural calculations shall be prepared in accordance with the requirements of SAES-A-204.

4 Design Procedure

4.1 Design Considerations

4.1.1 Vertical vessel wind and seismic loads shall be in accordance with Saudi Aramco Enginering Standard SAES-A-112.

4.1.2 Vertical vessel foundation design shall be based on approved certified vendor drawing.

4.1.3 For general foundation requirements and guidelines, refer to Saudi Aramco Best Practice SABP-Q-002 "Spread Footings Design".


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4.1.4 The engineer shall verify anchor bolts design, type and size to ensure compliance with ACI 318-02 Code Appendix D, Saudi Aramco Standard drawing and with the Vendor specific requirements.

4.1.5 For very tall or heavy vessels, sufficient capacity cranes may not be available for erection. The engineer shall determine whether additional loading may be imposed on the foundation during erection.

4.2 Vertical Loads

4.2.1 Dead Loads

4.2.1.1 The following nominal loads shall be considered as dead loads when applying load factors used in strength design.

A. Structure dead load (Ds) Vessels foundation weight which is defined as the combined weight of footing, pedestal dead load (Dp), and the overburden soil.

B. Erection dead load (Df) Fabricated weight of vessel, generally taken from the certified vessel drawing.

C. Empty dead load (De) Empty weight of the vessel, including all attachments, trays, internals, insulation, fireproofing, agitators, piping, ladders, platforms, etc. generally taken from the certified vessel drawing.

D. Operating dead load (Do) Empty dead load of the vessel plus the maximum weight of contents (including packing/catalyst) during normal operation. Operating dead load shall be taken from the certified vessel drawing.

E. Test dead load (Dt) Empty dead load of the vessel plus the weight of test medium contained in the system. The test medium shall be as specified in the contract documents. Unless otherwise specified, a minimum specific gravity of 1.0 shall be used for test medium. Cleaning load shall be used for test dead load if cleaning fluid is heavier than test medium. Whether to test or clean in the field should be determined. Designing for test dead load is generally desirable because unforeseen circumstances may occur. generally taken from the certified vessel drawing.

4.2.1.2 Eccentric vessel loads caused by large pipes or reboilers shall be considered for the applicable load cases.


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4.2.2 Live loads (L)

4.2.2.1 Live loads shall be calculated in accordance with SAES-M-001.

4.2.2.2 Load combinations that include live load as listed in Tables 3 and 4 of Section 4.4 will typically not control any part of the foundation design.

4.3 Horizontal Loads

4.3.1 Wind Loads (W)

4.3.1.1 Wind loads shall be calculated in accordance with the requirements of SAES-A-112 Meteorological and Seismic Design Data, SAES-M-001 "Structural Design Criteria for Non-Bulding Structures", and the guidelines of ASCE "Wind Load and Anchor Bolt Design for Buildings and Other Structures".

4.3.1.2 The engineer is responsible for determing wind loads used for the foundation design.

Comment: Loads from vendor or other engineering disciplines without verification shall not be accepted.

4.3.1.3 Partial wind load (Wp) shall be based on the requirements of ASCE 37-02, Section 6.2.1, for the specified test or erection duration. The design wind speed shall be 75% of the actual wind speed.

4.3.1.4 When calculating or checking wind loads, due consideration shall be given to factors which may significantly affect total wind loads, such as the application of dynamic gust factors or the presence of spoilers, platforms, ladders, piping, etc., on the vessel.

4.3.1.5 If detailed information (number of platforms, platform size, etc.) is unkown at the time of foundation design, the following Simplified Method may be used:

a) For the projected width, add 5-ft (1.52 m) to the diameter of the vessel, or add 3-ft (0.91 m) plus the diameter of the largest pipe to the diameter of the vessel, whichever is greater. This will account for platforms, ladders, nozzles and piping below the top tangent line.


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b) The vessel height should be increased one (1) vessel diameter to account for a large diameter and platform attached above the top tangent, as is the case with most tower arrangements.

c) The increases in vessel height or diameter to account for wind on appurtenances shoul not be used in calculating the h/D ratio for force coefficients or flexibity.

d) The force coefficient (Cf) should be deermined from ASCE 7-02, Figure 6-19 shown below.

e) If most design detail items (platforms, piping, ladders, etc.) of the vessel are known, the Detailed Method of the guidelines of ASCE 'Wind Load and Anchor Bolt Design for Buildings and Other Structures" shall be used.


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Figure 6-19 (Adapted from ASCE 7-02)


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4.3.2 Earthquake Loads (E)

4.3.2.1 Seismic forces shall be calculated in accordance with SAES-A-112 Meteorological and Seismic Design Data and the requirements of SAES-M-001 "Structural Design Criteria for Non-Bulding Structures".

4.3.2.2 Seismic loads calculated by the Vessel Vendor shall be independently verified as appropriate by the Engineer prior to performing foundation design to ensure compliance with the project specifications and the applicable Saudi Aramco Standards.

4.3.2.3 For skirt-supportrd vertical vessel classified as SUG III in accordace with ASCE 7-02, Section 9, the critical earthquake provisions and implied load combinations of ASCE 7-02, Section 9.14.7.3.10.5, shall be followed.

4.3.3 Other Loading

4.3.3.1 Thrust forces caused by thermal expansion of piping shall be included in the operating load combinations, if deemed advisable. Dead load factors shall be applied to the resultants of piping thermal loadings. The pipe stress engineer shall be consulted for any thermal loads that are to be considered.

4.3.3.2 Consideration shall be given to process upset conditions that could occur and could increase loading on the foundation.

4.4 Load Combinations

4.4.1 General

A. Structure, equipment, and foundations shall be designed for the appropriate load combinations from ASCE 7, this guideline, and any other probable realistic combination of loads. This document shall be used for load combiantions for both strength design and allowable stress design. Load combintions for vertical vessels shall be as listed below.

B. The load combinations shown below are the most common load combinations but may not cover all possible conditions. Any credible load combination that could produce the maximum stress or govern for stability should be considered in the calculations. The use of a one-third stress increase for load combinations


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including wind or seismic loads shall not be allowed for design of foundation.

C. The non-comprehensive list of typical load combinations provided below shall be considered and used as applicable. Engineering judgment shall be used in establishing all appropriate load combinations.

Service load combinations shall be used to check soil bearing pressures and foundation stability against overturning and sliding. In computing moments and shears for footing slab design, the service loads are factored. In designing the pedestal, load factors are applied to the service load reactions and the pedestal is designed in accordance with Section 4.5.

Table 3 Allowable Stress Design (Service Loads)

Load Comb.

#

Load Combination

Allowable Stress

Multiplier

Description

1 Ds + Do + L 1.00 Operating Weight + Live Load

2 Ds + Do + (W or 0.7 Eoa) 1.00 Operating Weight + Wind or Earthquake

3 Ds + De + W 1.00 Empty Weight + Wind (Wind uplift case)

4a 0.9 (Ds + Do) + 0.7 Eoa 1.00 Operating Weight + Earthquake (Earthquake uplift case)

4b 0.9 (Ds + De) + 0.7 Eea 1.00 Empty Weight + Earthquake (Earthquake uplift case)

5 Ds + Df + Wp 1.00 Erection Weight + Partial Windb (Wind uplift case) 6 Ds + Dt + Wp 1.20 Test Weight + Partial Wind

a. For skirt supported vertical vessels and skirt-supported elevated tanks classified as SUG III per

SEI/ASCE 7-02 Section 9, the critical earthquake provisions and implied load combination of SEI/ASCE 7 - 02 Section 9.14.7.3.10.5 shall be followed.

b. Erection Weight + Partial Wind is only required when the erection weight of the vessel is significantly less than the empty weight of the vessel.

c. Thrust forces caused by thermal expansion of piping should be included in the calculations for operating load combinations, if deemed advisable. The pipe stress engineer should be consulted for any thermal loads that are to be considered.


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Table 4 Loading Combinations and Load Factors Strength Design

Load Comb. #

Load Combination Description

1 1.4 (Ds + Do) Operating Weight

2 1.2 (Ds + Do) + 1.6 L Operating Weight + Live Load

3 1.2 (Ds + Do) + (1.6 W or 1.0 Eoa) Operating Weight + Wind or Earthquake

4 0.9 (Ds + De) + 1.6 W Empty Weight + Wind (Wind uplift case)

5a 0.9 (Ds + Do) + 1.0 Eoa Operating Weight + Earthquake (Earthquake uplift case)

5b 0.9 (Ds + De) + 1.0 Eea Empty Weight + Earthquake (Earthquake uplift case)

6 0.9 (Ds + Df) + 1.6 Wp Erection Weight + Partial Windb (Wind uplift case)

7 1.4 (Ds + Dt) Test Weight

8 1.2 (Ds + Dt) + 1.6 Wp Test Weight + Partial Wind

a. For skirt supported vertical vessels and skirt-supported elevated tanks classified as SUG III per SEI/ASCE 7 - 02 Section 9, the critical earthquake provisions and implied load combination of SEI/ASCE 7 - 02 Section 9.14.7.3.10.5 shall be followed.

b. Erection Weight + Partial Wind is only required when the erection weight of the vessel is significantly less than the empty weight of the vessel.

c. Thrust forces caused by thermal expansion of piping should be included in the calculations for operating load combinations, if deemed advisable. The pipe stress engineer should be consulted for any thermal loads that are to be considered. The same load factor as used for dead load shall be used.

4.5 Pedestal

4.5.1 Concrete pedestal dimensions shall be sized on the basis of standard available forms for the project. When form information is not available, octagon pedestal dimensions shall be sized with pedestal faces in 2-inch increments to allow use of standard manufactured forms. The folloiwng criteria shall be used to determine the size and shape for the pedestal.

4.5.1.1 Face-to-face pedestal size shall be no less than the largest of the following:

BC + 9 inches (Eq. 1a) BC + 8 (BD) (for Grade 36 or A307 anchor bolts) (Eq. 1b) BC + 12 (BD) (for high-strength anchor bolts) (Eq. 1c) BC + SD + 9 inches (BD) (Eq. 1d)


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BC + SD + 7 (BD) (Grade 36 or A307 anchor bolts) (Eq. 1e) BC + SD + 11 (BD) (for high strength anchor bolts (Eq. 1f) where:

BC = bolt circle, inches

BD = bolt diameter, inches

SD = sleve diameter, inches

4.5.1.2 Pedestals 6 ft and larger shall be octagonal.Dimensions for octagon pedestals are provided in Table 1. Octagons highlighted in gray in Table 1 have faces in 2-inch increments.

4.5.1.3 Pedestals smaller than 6 ft shall be square, or round if forms are available.

4.5.2 Anchorage It is normally desirable to make the pedestal deep enough to contain the anchor bolts and keep them out of the footing. Consideration shall be given to anchor bolt development and foundation depth requirements. Pedestal size may need to be increased to provide adequte AN (projected concrete area) for anchor bolts when additional reinforcement for anchor bolts is not used.

4.5.3 Pedestal reinforcement The pedestal shall be tied to the footing with sufficient dowels around the pedestal perimeter to prevent seperation of the pedestal and footing. Development of reinforcing steel shall be checked.

4.5.4 Dowels Dowels shall be sized by computing the maximum tension existing at the pedestal perimeter due to overturning moments. Conservatively, the following formula may be used. More exact tension loads may be obtained by using ACI 318 strength design methodology.

Tension Fu = 4(Muped)/[(Nd)(DC)] 0.9[(De or Do) + Dp]/Nd (Eq. 2)

(De or Do) = nominal empty or operating vessel weight. Use empty weight for wind loads. Use empty or operating for for earthquake loads depending on which condition is used to calculate Muped.

As required = tension / design stress = Fu / fy (Eq. 3)

where:

Fu = maximum ultimate tension in reinforcing bar


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Muped = maximum factored overturning moment at base of pedestal, calculated by using load factors in load combinations for uplift cases in Table 4 (Loading Combinations and Load Factors Strength Design).

Nd = number of dowels (assumed); shall be a multiple of 8

DC = dowel circle diameter (assume pedestal size minus 6 inches)

De+Dp = nominal empty weight of vessel and pedestal weight

Do+Dp = nominal operating weight of vessel and pedestal weight

= strength reduction factor = 0.90

fy = yield strength of reinforcing steel

4.5.5 Minimum pedestal reinforcing shall be as follows:

Octagons 6'-0" to 8'-6": 16-#4 verticals with #3 ties at 15" max.

Octagons larger than 8'-6" to 12'-0": 24-#5 vert. with #4 ties at 15" max.

Octagons larger than 12'-0": #5 verticals at 18" maximum spacing with #4 ties at 15" max.

4.5.6 Top Reinforcement A mat of reinforcing steel at top of pedestal shall be provided. Minimum steel shall be #4 bars at 12-inch maximum spacing across the flats in two directions only.

4.5.7 Ties - See minimum pedestal reinforcemnt, Section 4.5.5, this Guideline.

4.6 Anchor Bolts

Anchor bolts shall conform to the requirements of Para. 4.8 of SAES-Q-005 "Concrete Foundations" and SABP-Q-001 Anchor Bolt Design and Installation.

4.6.1 Conservatively, the maximum tension on an anchor bolt may be determined using the following formula. More exact tension loads may be obtained by using ACI 318 strength design methodology.

Nu = 4Mu/[(Nb)(BC)] 0.9(De or Do)/Nb (Eq. 4)

where:

Nu = factored maximum tensile load on an anchor bolt


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Mu = factored moment at the base of vessel, calculated using load factors in load combinations for uplift cases in Table 4 (Loading Combinations and Load Factors Strength Design).

Nb = number of anchor bolts

BC = bolt circle diameter

(De or Do) = nominal empty or operating vessel weight. Use empty weight for wind loads. Use empty or operating for for earthquake loads depending on which condition is used to calculate Mu.

4.6.2 For most cases, there is no shear on the anchor bolts because the load is resisted by friction caused primarily by the overturning moment. If friction cannot resist the load, the bolts shall be designed to resist the entire shear load, or other methods may be used to resist the shear load. The friction resistance can be calcualated using the following formulas:

Pu = Mu/LA + 0.9(De or Do) (Eq. 5)

Vf = Pu (Eq. 6)

where:

Pu = factored compression force at top of pedestal

LA = lever arm between centroid of tension loads on bolts and centroid of the compression load on the pedestal. This is a complicated distance to determine exactly. A conservative approximation is to use 2/3 of the bolt circle diameter as the lever arm.

= coefficient of friction. For the normal case of grout at the surface of pedestal, = 0.55.

Vf = frictional resisting force (factored)

To have no shear load on the bolts: Vu Vf (Eq. 7)


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where:

Vu = factored shear load at base of vessel, calculated using load factors in load combinations for uplift cases in Table 4 (Loading Combinations and Load Factors Strength Design).

= strength reduction factor = 0.75

For anchor bolt design procedure and guidelines, refer to Saudi Aramco Best Practice SABP-Q-001 Anchor Bolt Design and Installation.

4.7 Footing Design

4.7.1 Sizing

The size of spread footings may be governed by stability requirements, sliding, soil bearing pressure, or settlement.

Footings for vertical vessels shall be octagonal or squre and sized based on standard available form sizes. When form information is not available, footing dimensions shall be sized with footing faces in 2-inch increments to allow use of standard manufactured forms. (Octagons highlighted in gray in Table 1 are those having faces in 2-inch increemts). If extended to the recommended depth specified in the geotechncial report, the pedestal may be adequate without a fooitng. Footings smaller than 7 ft 0 inch in diameter shall be square.

Where a footing is required, the footing thickness shall be a minimum of 12 inches.The footing thickness shall be adequate to develop pedestal reinforcement and satisfy the shear requirements of ACI 318.

The footing thickness shall also be checked for top tension without top reinforcement in accordance with ACI 318. If the thickness is not adequate, either a thicker footing or top reinforcing steel is required (see Section 4.7.5). Note that increasing the footing thickness is typically more cost effective for construction than adding a top mat of reinforcing steel except where seismic effects create tensile stresses requiring top reinforcement.

For the first trial, the diameter of an octagonal footing may be approximated by the following formula:

Diameter D = 2.6 (Mftg/SB)1/3 (Eq. 8)


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where:

Mftg = nominal overturning moment at base of footing, kip-ft

SB = allowable gross soil bearing, ksf

A common assumption in the design of soil bearing footings is that the footing behaves as a rigid unit. Hence, the soil pressure beneath a footing is assumed to vary linearly when the footing is subjected to axial load and moment. The ensuing footing formulas are based on the linear pressure assumption.

Footings shall be designed so that under sustained loads (operating loads) the total settlement and the differential settlement between footings do not exceed the established limits. The maximum allowable amount of total settlement and differential settlement is typically set by the Project Structural Engineer based on the sensitivity of the equipment or structure being supported.

4.7.2 Soil Bearing Octagon Footing

4.7.2.1 Soil bearing pressure shall be checked for maximum allowable on the diagonal.

4.7.2.2 Soil bearing pressure used for footing design shall be computed on the flat.

4.7.2.3 Where the total octagonal footing area is in compression (e/D 0.122 on the diagonal and e/D 0.132 on the flat), the soil bearing pressure shall be computed using the following formulas:

f = (P/A) Mftg/S (Eq. 9) f(diagonal) = P/A Mftg/SDiagonal (Eq. 10a) f(diagonal) = P/A [1 (8.19e/D)] (Eq. 10a) f (flat) = P/A Mftg/SFlat (Eq. 10b)

f (flat) = P/A [1 (7.57e/D)] (Eq. 10b)

where:

D = distance between parallel sides, ft

f = toe pressure, ksf

P = nominal total vertical load including soil and foundation, kips


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A = bearing area of octagonal footing (0.828D2), ft

Mftg= nominal overturning moment at base of footing, kip-ft

S = section modulus, ft3

SDiagonal = 0.1011 D

SFlat = 0.1095 D

e = eccentricity (Mftg/P), ft

4.7.2.4 Where the total octagonal footing area is not in compression (e/D > 0.122 on the diagonal and e/D > 0.132 on the flat), the soil bearing pressure shall be computed using Figure B and the following formula:

f = L P/A (Eq. 11)

where the value of L is obtained from Figure B

4.7.2.5 The e/D ratios for octagon footings may go above the limits of the chart in Figure B because of the load factors in strength design L and K values for these conditions are tabulated in Table 2 for lateral loads perpendicular to a face. These values shall be used for calculating moments and shears in the footing. They shall not be used to check soil-bearing pressures.

4.7.3 Soil Bearing Square Footing

4.7.3.1 Where the total footing is in compression (e/D 0.167), the soil bearing pressure shall be computed using the following formula:

f = P/A M/S (Eq. 12)

f (Diagonal) = P/A M/SDiagonal (Eq. 12a)

f (Flat) = P/A M/SFlat (Eq. 12b)

where:

D = footing width, ft

A = bearing area of square footing, ft2

S = section modulus, ft3

SDiagonal = 0.1179 D3, ft3


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SFlat = D3 /6 , ft3

e = eccentricity (M/P), ft

When the total square footing is not in compression (e/D > 0.167), the soil bearing pressure shall be computed using the following formula:

fflat = 2P / 3D(D/2 e) (Eq. 13)

4.7.3.2 Maximum soil-bearing pressure (on diagonal) shall be calculated using the design aid for soil pressure for biaxial loaded foortings as shown in Figure A (See Attachments).

4.7.4 Stability/Sliding

4.7.4.1 All foundations subject to buoyant forces shall be designed to resist a uniformly distributed uplift equal to the full hydrostatic pressure. The minimum safety factor against floatation shall be 1.20, considering the highest anticipated water level. (Ref. SAES-Q-005, Para. 4.2.7)

4.7.4.2 The Stability Ratio shall be defined as "The ratio of the resisting moment to overturning moment about the edge of rotation'.

4.7.4.3 The minimum safety factor Stability Ratio against overturning for service laods other than earthquake shall be 1.5 (Ref. SAES-Q-005, Para. 4.2.1)

Compute the Stability Ratio (S.R.) using the following formula:

S.R. = D/2e (Eq. 15)

or S.R. = MR/ MO.T (Eq. 15a) where

D = dimension of footing in the direction of the overturning moment, ft.

e = eccentricity (ft)= overturning moment at the base of the footing divided by the total vertical load, ft. The moment and loads shall be factored in accordance with load combinations in Table 3 Loading Combinations Alllowable Stress Design (Service Loads).


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Eccentricity e = MO.T./P Resisting Moment MR = P x D/2

4.7.4.4 Foundation Sliding

The minimum safety factor against sliding for service loads other than earthquake shall be 1.5. The coefficient of friction used in computing the safety factor against sliding for cast-in-place foundations shall be 0.40, unless specified otherwise in a detailed soil investigation. Passive earth pressure from backfill shall not be considered in computing these safety factors. (Ref. SAES-Q-005, Para. 4.2.6).

The mimimum overturning stability ratio and the minimum factor of safety against sliding for earthquake service loads shall be 1.0. In addition, the minimum overturning stability ratio for the anchorage and foundations of skirt-supported vertical vessels classified as SUG III in accordance with ASCE 7-02, Section 9, shall be 1.2 for the critical earthquake loads specified in ASCE 7-02, Section 9.14.7.3.10.5. For foundations designed using seismic load combinations from Table 3, the reduction in the foundation overturning moment permitted in ASCE 7, Chapter 9, Section 9.5.5.6, Overturning shall not be used (Ref. SAES-Q-005, Para. 4.2.3).

4.7.5 Reinforcement

4.7.5.1 Standard Factored Design

Reinforced concrete design using factored strength design loads shall be in accordance with ACI 318. The critical section for moment shall be taken with respect to the face of a square with an area equivalent to that of the pedestal.

Moment shall be checked at the face of the equivalent square. Moment shall be calculated for a 1 foot-wide strip as a simple cantilever from the edge of the equivalent square. Punching shear may need to be checked in some situations in accordance with with ACI 318. The resulting reinforcing steel shall be placed continuously across the entire footing in a grid pattern, the minimum bottom reinforcement being #5 bars at 12 inches on-center, each way.


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The minimum amount of bottom steel (grade 60 ksi) shall not be less than the mimimum shrinkage reinforcement as required by ACI Code Sect. 10.5.4:

As (min) = 0.0018 b h

Where:

b = width of footing

d = distance from top of footing to center of bottom bars

h = depth of footing

4.7.5.2 Top Reinforcement

Except where seismic effects create tensile stresses, top reinforcement in the footing is not necessary if the factored tensile stress at the upper face of the footing does not exceed the flexural strength of structural plain concrete, as follows:

ft = 5(fc)1/2 (Eq. 16) where:

ft = flexural strength of structural plain concrete, psi

fc = compressive strength of concrete, psi

= strength reduction factor for structural plain concrete = 0.55

The effective thickness of the footing for tensile stress calculations should be 2 inches less than the actual thickness for footings cast against soil (ACI 318-02, Section R22.7.4). For footings cast against a seal slab, the actual thickness of the footing may be used for the effective thickness. If the factored tensile stress exceeds the flexural strength of structural plain concrete, top reinforcement should be used if an increase in the footing thickness is not feasible.

See the following formulas for footing thicknesses that do not require top reinforcing steel:

For footings cast against soil:

treqd = teff + 2 inches (Eq. 17a)


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For footings cast against a seal slab:

treqd = teff (Eq. 17b)

with teff calculated as follows:

teff = (6Mu/ft)1/2 (Eq. 18)

where:

treqd = required footing thickness with no top reinforcing steel, inches

teff = effective footing thickness, inches

Mu = factored moment caused by the weight of soil and concrete acting on a 1-inch strip in the footing at the face of the equivalent square pedestal, inch-pounds per inch, calculated using a load factor of 1.4

ft = flexural strength of structural plain concrete, psi (from Eq. 18)

4.7.6 Shear Consideration

Both wide-beam action and two-way action (punching shear) must be checked to determine the required footing depth. Shear, as a measure of diagonal tension, shall be checked at the critical section specified in ACI 318-02, Section 11.1.3.1 (at a distance d from the face of the equivalent square). The shear shall be calculated for a 1 foot-wide strip as a simple cantilever from the edge of the equivalent square. Beam action assumes that the footing acts as a wide beam with a critical section across its entire width. Punching shear may need to be checked in some situations in accordance with with ACI 318-02. Two-way action (at a distance d/2 from the face of the equivalent square) for the footing checks "punching" shear strength. The critical section for punching shear is a perimeter bo around the supported member with the shear strength computed in accordance with ACI Code Sect. 11.12.2.

For footing design, the depth must be selected so that shear reinforcement is not required. If either permissible shear is exceeded, the thickness of the footing must be increased. The shear strength equations may be summarized as follows:


Page 23 of 41

Shear Strength of Concrete in Footings

Note: For beam shear & punchinG shear sketch, refer to Fig. 3 in SABP-Q-002 Revision Summary 31 July 2002 New Saudi Aramco Best Design Practice SABP-003. 30 April 2005 New revision to comply with revised ACI 318-02 Code, ASCE 7-02 and revised Saudi

Aramco Standards.


Page 24 of 41

Attachments Figure A Foundation Pressures for Square Bases


Page 31 of 41

Table 2 - Foundation Pressures for Octagon Bases (Large Eccentricities - Load Perpendicular to Face)

e/D K L e/D K L e/D K L

0.300 0.4935 4.503 0.370 0.6676 7.844 0.440 0.8369 19.432 0.305 0.5065 4.656 0.375 0.6794 8.233 0.445 0.8496 21.421 0.310 0.5195 4.819 0.380 0.6912 8.654 0.450 0.8625 23.812 0.315 0.5323 4.991 0.385 0.7030 9.113 0.455 0.8754 26.742 0.320 0.5450 5.174 0.390 0.7149 9.615 0.460 0.8885 30.411 0.325 0.5577 5.369 0.395 0.7267 10.167 0.465 0.9017 35.138 0.330 0.5703 5.576 0.400 0.7387 10.775 0.470 0.9151 41.452 0.335 0.5828 5.797 0.405 0.7507 11.450 0.475 0.9287 50.304 0.340 0.5951 6.032 0.410 0.7628 12.203 0.480 0.9424 63.601 0.345 0.6074 6.284 0.415 0.7749 13.046 0.485 0.9564 85.785 0.350 0.6196 6.553 0.420 0.7872 13.998 0.490 0.9707 130.1920.355 0.6317 6.842 0.425 0.7995 15.080 0.495 0.9852 263.4870.360 0.6438 7.152 0.430 0.8119 16.320 0.500 See Note See Note0.365 0.6557 7.485 0.435 0.8244 17.754 > 0.500 See Note See Note

Note: For e/D values greater or equal to 0.500, assume soil bearing as a line load with a length equal to the

face dimension of the octagon footing applied at a distance of e from the centerline of the footing.


Page 35 of 41

PEDESTAL DESIGN

Pedestal Dimensions and Weight

BC + 9 inches = 178.5 inches + 9 inches = 187.5 inches (Eq. 1a) BC + 8(BD) = 178.5 inches + 8(1.5 inches) = 190.5 inches (Eq. 1b) BC + SD + 9 inches - BD = 178.5 inches + 4 inches + 9 inches - 1.5 inches

= 190.0 inches (Eq. 1d) BC + SD + 7(BD) = 178.5 + 4 + 7(1.5 inches) = 193 inches Controls = 16.083 ft (Eq. 1e)

Use 16-ft - 1-1/8-inch octagon. Note: Pedestal diameter had to be increased to 17-ft - 8-1/2-inches to provide a sufficiently large projected concrete failure area to resist the tensile load in the anchor bolts. Alternatively, additional reinforcing steel may be used to transfer anchor bolt forces to concrete.

Pedestal Reinforcement

Pedestal area = 259.7 ft2 (Ref. Table 1) Pedestal weight (Dp) = (259.7 ft2)(4.5 ft)(0.15 kcf) = 175.3 kip Mped = O.T.M. at pedestal base = (1,902 kip-ft) + (4.5 ft)(44.75 kip) = 2,104 kip-ft Muped = 1.6Mped = 1.6(2,104 kip-ft) = 3,366 kip-ft (Load Factors shall be in accordance with Table 4, Load Comb. 4) Nd = number of dowels = assume 40 DC = (17.71-ft pedestal) - (say 0.5 ft) = 17.21 ft De + Dp = empty weight of vessel + pedestal weight = 170.3 kip + 175.3 kip

= 345.6 kip Fu = 4(Muped)/[(Nd)(DC)] - 0.9(De + Dp)/Nd= 4(3,366 kip-ft)/[(40)(17.21 ft)] - 0.9(345.6

kip)/40 = 11.78 kip (Eq. 2)

Asreqd = Fu/fy = (11.78 kip)/(0.9)(60 ksi) = 0.22 inch2 (Eq. 3)

Use 40 #5 bars (As = 0.31 inch2) with #4 ties at 15 inches c/c (minimum reinforcement controls)

Anchor Bolt Check Maximum Tension on Anchor Bolt:

Nu = 4Mu/[(Nb)(BC)] - 0.9(De)/Nb (Eq. 4) Nu = 4[(1.6)(1,902 kip-ft)]/[(24)(14.88 ft)] - 0.9(170.3 kips)/24 = 27.7 kips

(Load factors shall be in accordance with Table 4, Load Comb. 4)


Page 36 of 41

Maximum Shear on Anchor Bolt:

Vu at base top of grout = 1.6(44.75 kip) = 71.6 kips Check whether shear load can be taken by friction between base of vessel and top of grout. Pu = Mu/LA + 0.9(De) (Eq. 5) Conservatively, take LA as 2/3 of BC diameter = (2/3)(14.875 ft) = 9.92 ft Pu = 1.6(1902 kip-ft)/(9.92 ft) + 0.9(170.3 kips) = 307 kips + 153 kips = 460 kips Vf = Pu = (0.55)(460 kips) = 253 kips (Eq. 6) Vf = (0.75)(253 kips) = 190 kips > 71.6 kips (Eq. 7)

Therefore, the anchor bolts are not required to resist shear. Projected Concrete Failure Area:

Note: Several iterations were required to determine that D = 16 ft - 1-1/8 inches would not provide enough projected concrete failure area to resist the maximum tensile load Nu = 27.7 kips, regardless of what embedment depth, hef, was used. To save space, these trial calculations are not shown here. Reinforcing steel either should be added to transfer the tensile load from the anchor bolts to the pedestal or the pedestal diameter should be increased. This second alternative is shown here. Try increasing D to 17 ft - 8-1/2 inches (17.704 ft)

FOOTING DESIGN

Select a Trial Octagon Size:

Mftg = O.T.M. at footing base = (1,902 kip-ft) + (6.0 ft)(44.75 kip) = 2,171 kip-ft

SB = allowable gross soil bearing = (3.25 ksf) + (5 ft)(0.11 kcf) = 3.80 ksf Trial diameter = (2.6)(Mftg/SB)1/3 = (2.6)[(2,171 kip-ft)/(3.80 ksf)]1/3 = 21.57 ft (Eq. 8) Try a 21-ft - 8-3/4-inch octagon. Area = 391.1 ft

2 (Ref. Table 1)

Check Required Thickness for Pedestal Reinforcing Embedment:

For #5 hooked bar,

ldh = [(0.02fy)/ c'f ](db) (ACI 318-02, Section 12.5.2) = [(0.02)(1.0)(1.0)(60,000 psi)/ psi 4,000 ](0.625 inches) = 11.9 inches Asreq'd / Asprov = (0.22 inch

2)/(0.31 inch2) = 0.71


Page 37 of 41

Treq'd = (3 inch clear) + (2 layers)(0.75 inch bar) + (0.71)(0.7)(11.9 inches) = 10.4 inches

Tmin = 12 inches (Ref. Section 4.7.1) Try footing thickness = 18 inches

Footing Weights Weight of pedestal Dp = (259 7 ft

2)[(4.5 ft)(0.15 kcf)] = 175.3 kip

Weight of footing = (391.1 ft2)[(1.5 ft)(0.15 kcf)] = 88.0 kips

Weight of soil = (391.1 ft2 -259.7 ft2)(3.5 ft)(0.11 kcf)] = 50.6 kip

Total (Ds) = 175.3 kip + 88.0 + 50.6 kip = 313.9 kip Pe = De + Ds = 170.3 kip + 313.9 kip = 484.2 kip Po = Do + Ds = 345.2 kip + 313.9 kip = 659.1 kip Pt = Dt + Ds = 624.1 kip + 313.9 kip = 938.0 kip

Check Soil Bearing and Stability Empty + Wind (Ref. Table 3, Load Comb. 3): P = Pe = 484.2 kip Mftg = 2,171 kip-ft e = Mftg/P = (2,171 kip-ft)/(484.2 kip) = 4.48 ft

Stability ratio = D/2e = (21.73 ft)/[2(4.48 ft)] = 2.43 > 1.5 O.K. (Eq. 15) e/D = (4.48 ft)/(21.73 ft) = 0.206 > 0.122 Ldiag = 2.85 (Ref. Figure B) f = LP/A = (2.85)(484.2 kip)/(391.1 ft

2) = 3.53 ksf < 3.80 ksf O.K.

Operating + Wind (Ref. Table 3, Load Comb. 2): P = Po = 659.1 kip Mftg = 2,171 kip-ft e = Mftg/P = (2,171 kip-ft)/(659.1 kip) = 3.29 ft e/D = (3.29 ft)/(21.73 ft) = 0.152 > 0.122 Ldiag = 2.25 (Ref. Figure B) f = LP/A = (2.25)(659.1 kip)/(391.1 ft

2)

= 3.79 ksf < 3.80 ksf O.K. (controlling case) (Eq. 11) Test + Partial Wind (Ref. Table 3, Load Comb. 6): P = Pt = 938.0 kip Partial wind velocity = 68 mph


Page 38 of 41

5.46 3

.82

4.55

0.91

0.73

16.84 ft. 4.89 ft.

soil + concrete

2.81 ft.

Mftg = (68 mph/115 mph)2(2,171 ft-kip) = 759.1 ft-kip e = Mftg/P = (759.1 ft-kip)/(938.0 kip) = 0.81 ft e/D = (0.81 ft)/(21.73 ft) = 0.037 < 0.122 f = P/A [1 + (8.19)(e/D)] (Eq. 10a) = [(938.0 kip)/(391.1 ft

2)][1 + (8.19)(0.037)] = 3.13 ksf < 3.80 ksf O.K.

Use 21-ft - 8-3/4-inch octagon. Bottom Reinforcement

Check Operating + Wind (Ref. Table 4, Load Comb. 3 controls): [1.2(Ds + Do) + 1.6W]


Page 39 of 41

8.50

4.72

5.27

3.23

0.55

7.39 ft. 14.34 ft.

soil + concrete

2.81 ft.

Pu = 1.2(659.1 kip) = 790.9 kip Mu = 1.6(2,171 kip-ft) = 3,474 kip-ft e = Mu /Pu = (3,474 kip-ft)/(790.9 kip) = 4.39 ft e/D = (4.39 ft)/(21.73 ft) = 0.202 > 0.132 (flat) L = 2.70 (flat) K = 0.225 (flat) (Ref. Figure B) KD = (0.225)(21.73 ft) = 4.89 ft SB = LP/A = (2.70)(790.9 kip)/(391.1 ft2) = 5.46 ksf Find equivalent square for pedestal:

side2 = 259.7 ft2 side = 16.12-ft projection = (21.73 ft - 16.12 ft)/2 = 2.81 ft SB at face of equivalent square:

= 5.46 ksf(16.84 ft - 2.81 ft)/(16.84 ft) = 4.55 ksf Soil + concrete = 1.2(238.6 kip)/(391.1 ft2) = 0.73 ksf Mu = (4.55 ksf - 0.73 ksf)(2.81 ft)2/2 + (5.46 ksf - 4.55 ksf )(2.81 ft)2/3 = 17.48 kip-ft Check Empty + Wind (Ref. Table 4, Load Comb. 4, controls): [0.9(De + Ds) + 1.6W] Pu = 0.9(484.2 kip) = 435.8 kip Mu = 1.6(2,171 kip-ft) = 3,474 kip-ft e = Mu/Pu = (3,474 kip-ft)/(435.8 kip) = 7.97 ft e/D = (7.97 ft)/(21.73 ft) = 0.367 > 0.132 (flat)


Page 40 of 41

L = 7.63 (flat) K = 0.660 (flat) (Ref. Table 2) KD = (0.660)(21.73 ft) = 14.34 ft SB = LPu/A = (7.63)(435.8 kip)/(391.1 ft2) = 8.50 ksf Find equivalent square for pedestal:

side2 = 259.7 ft2 side = 16.12-ft projection = (21.73 ft - 16.12 ft)/2 = 2.81 ft

SB at face of equivalent square: = (8.50 ksf)(7.39 ft - 2.81 ft)/(7.39 ft) = 5.27 ksf Soil + concrete = 0.9(238.6 kip)/(391.1 ft2) = 0.55 ksf Mu = (5.27 ksf - 0.55 ksf)(2.81 ft)2/2 + (8.50 ksf - 5.27 ksf)(2.81 ft)2/3 = 27.14 kip-ft Controls d = 18 inches - 3 inches - 1.125 inches = 13.875 inches = 1.16 ft F = bd2/12,000 = (12 inches)(13.875 inches )2/12,000 = 0.193 Ku = Mu/F = (27.14 kip-ft)/(0.193) = 140.6 au = 4.390 As = Mu/(aud) = (27.14 kip-ft)/[(4.390)(13.875 inches)] = 0.45 inch2/ft Controls As min = (0.0018)(12 inches)(18.00 inches) = 0.39 inch2/ft < 0.45 inch2/ft

Use #6 at 9 inches E.W. (bottom); As = 0.59 inch2/ft. > 0.45 O.K.


Page 41 of 41

Shear Check

Beam Shear Empty + Wind Case:

SB (at distance d from face): = (8.50 ksf)(7.39 ft - 2.81 ft + 1.16 ft)/(7.39 ft) = 6.60 ksf

Vu (at distance d from face): = (6.60 ksf - 0.55 ksf)(2.81ft - 1.16 ft) + (8.50 ksf - 6.60 ksf)(2.81 ft - 1.16 ft)/2 = 9.98 kip/ft + 1.57 kip/ft = 11.55 kip/ft

vu = (11.55 kip/ft)(1,000 lb/kip)/[(12 inches/ft)(13.875 inches)] = 69.4 psi < 2 c'f = 94.9 psi O.K.

Punching Shear Test Load Case

Pu/A = 1.4(938.0 kip)/(391.1 ft2) = 3.36 ksf (Ref. Table 4, Load Comb. 7)

Vu (total at d/2 away from equivalent square) = [3.36 ksf - (1.4/1.2)(0.73 ksf)][391.1 ft2 - (16.12 ft + 1.16 ft)2] = (2.51 ksf)(92.5 ft2) = 232 kip

bo = 4(16.12 ft + 1.16 ft) = 69.1 ft vu = Vu/(dbo) = (232 kip)(1,000 lb/kip) /[(13.875 inches)(69.1 ft)(12 inches/ft)] = 20 psi

vc (allowable) = the smaller of ACI 318-02, Eq. 11-34 or 11-35 vc (allowable) = (sd/bo + 2)(fc)1/2

= 0.75[(40)(1.16 ft)/(69.1 ft) + 2](4,000 psi)1/2 = 127 psi > 20 psi O.K. (ACI 318-02, Eq. 11-34)

vc (allowable) = (4)(fc)1/2 = 0.75(4)(4,000 psi)1/2 = 190 psi > 20 psi O.K. (ACI 318-02, Eq. 11-35)

Top Reinforcement

Check to see if concrete can take weight of concrete plus soil above footing without top reinforcement. Use load factor of 1.4.

Mu = (1.4/1.2)(0.73 ksf)(2.81 ft)2/2 = 3.36 kip-ft = 3,360 inch-lb/inch ft = 5(fc)1/2 = 5(0.55)(4,000 psi)1/2 = 173.9 psi (Eq. 16) treqd = teff + 2 inches

= (6Mu/ ft)1/2 + 2 inches = [6(3,360 inch-lb/inch)/(173.9 psi)]1/2 + 2 inches = 12.8 inches < 18 inches (Eq. 17a and 18)

Wind loads (rather than earthquake) govern footing design. Therefore, no top reinforcement is required.

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