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Dongxiao Wu P. Eng. (Alberta, Canada)

 

Home  >> Tutorial >>  Seismic Design for Petrochemical Facilities As Per NBCC 2005

 

Seismic Design for Petrochemical Facilities As Per NBCC 2005        ? Download PDF        ? Buy This Document

 

 

TABLE OF CONTENTS

 

1.0 SCOPE AND APPLICATION.. 2

2.0 GENERAL. 2

2.1 Spectral Acceleration Sa(T) and S(T) 2

2.2 Methods to Determine Site Class. 2

2.3 Determine If Seismic Design Is Required for Project 2

3.0 METHOD OF ANALYSIS.. 3

4.0 DUCTILTY AND OVERSTRENGTH FACTOR.. 5

5.0 STRUCTURE CLASSIFICATION.. 6

Case 01 Building Structures. 8

Case 02 Nonbuilding Structures Similar to Building. 9

Case 03 Self Supported Vertical Vessel 10

Case 04 Braced Leg-Supported Ver Vessel 11

Case 05? Self Supported Horizontal Vessel 12

Case 07 Nonbuilding Structure (Less Than 25% Comb Wt) Supported by Other Structure. 14

Case 08 & 09 Nonbuilding Structure (More Than 25% Comb Wt) Supported by Other Structure. 15

6.0 DESIGN EXAMPLES.. 17

Design Example 01: Nonbuilding Structure Similar to Building - Exchanger Structure. 17

Design Example 02: Skirt-Supported Vertical Vessel 35

Design Example 03: Braced Leg -Supported Vertical Vessel 41

Design Example 04: Self-Supported Horizontal Vessel 50

Design Example 05: Building Structure. 58

Design Example 06: Nonbuilding Structure (> 25% Comb Wt) Supported by Other Structure. 69


 

1.0 SCOPE AND APPLICATION

 

This guideline is intended to be used as supplementary document to NBCC2005 for the seismic design of petrochemical facilities in Canada, with particular focus on Northern Alberta Fort McMurray area.

This document only covers Equivalent Static Force Procedure (ESFP), which is the easiest and most applicable way to implement seismic design in low seismic zone like Fort McMurray area.

There is no provision on seismic design of Nonbuilding Structure in NBCC2005. ASCE 7-05 Chapter 15 Seismic Design Requirements for Nonbuilding Structures is referenced for Nonbuilding Structure seismic design in Canadian location. When ASCE 7-05 is referenced, NBCC2005 version of ground motion parameters is used to interpret the ASCE 7-05 formula. This is what NBCC2005 recommends in Commentary J page J-61, Para. 226.

 

2.0 GENERAL

 

2.1 Spectral Acceleration Sa(T) and S(T)

 

Sa(T)

?          5% Damped Spectral Response Acceleration?

?          Based on Site Class C as per NBCC Table 4.1.8.4.A

?          For most cities in Canada, Sa(T) value can be found in NBCC Appendix C Table C-2

 

S(T)

?          Design Spectral Acceleration

?          Modified from Sa(T) by applying Fa and Fv factors relating to Site Class?????????? NBCC 4.1.8.4 (6)

?          S(T) = Sa(T)? when specific project site class is Class C

 

2.2 Methods to Determine Site Class

 

Two methods are available to determine Site Class if it?s not provided by Geotechnical consultant

1.       Average shear wave velocity Vs NBCC Table 4.1.8.4A

?          Preferable way to classify Site Class????????????? NBCC 4.1.8.4 (2)

?          Shear wave velocity Vs ?is normally available in soil report under dynamic machine foundation section

?          Use? Vs = SQRT(G/ ρ) = SQRT(Gg / γ ) to get shear wave velocity if only shear modulus is provided

 

2.       SPT N60, for sand site. Undrained shear strength, su, for clay site??? NBCC Table 4.1.8.4A

 

2.3 Determine If Seismic Design Is Required for Project

 

From NBCC 4.1.8.1? ? requirements in this Subsection need not be considered in design if S(0.2), as defined in Sentence 4.1.8.4.(6), is less than or equal to 0.12

 

 

Please note it?s S(0.2)<=0.12 , not Sa(0.2) <=0.12

For Fort McMurray, Sa(0.2)=0.12

For Site Class C or better, S(0.2) <= Sa(0.2)=0.12 ? seismic design is not required

For Site Class D or worst, S(0.2) > Sa(0.2)=0.12 ? seismic design is required

For most projects in Fort McMurray, average shear wave velocity is 200~300 m/s, and the Site Class is Class D.

 

3.0 METHOD OF ANALYSIS

 

1.       Equivalent Static Force Procedure (ESFP) ????????? NBCC 4.1.8.11

ESFP may be used for structures that meet any of the following criteria

a)       in cases where IE Fa Sa(0.2) is less than 0.35,

b)       regular structures that are less than 60 m in height and have a fundamental period Ta < 2s

c)       irregular structures, other than those that are torsionally sensitive, that are less than 20 m

in height and have Ta < 0.5s

In Fort McMurray, for the highest importance category Post disaster structure, Site Class D, IE Fa Sa(0.2) = 1.5x1.3x0.12 = 0.234 < 0.35?

? For Site Class D or better, ESFP can be used as the seismic analysis method for all structures in Fort McMurray area.

 

2.       Modal Response Spectrum Method????? NBCC 4.1.8.12

Not covered in this guideline.

 

3.       Time History Method ??????????????? NBCC 4.1.8.12

Not covered in this guideline.

 

Notes on Equivalent Static Force Procedure (ESFP)

 

1.       NBCC2005 4.1.8.11 (3) allow the use of estimated period for seismic calculation.

Computed structure period via computer model is not absolutely required.

2.       Most of the time, the computed period is much longer than estimated one. This is due to the fact that formula for estimation given by code always leans to the conservative side.

Using computed period instead of estimated one gives us the advantage to reduce the seismic base shear.

Below is a comparison of S(T) value based on estimated Ta and computed Ta, from Example 01.

 

?

 

From Example01, Moment Frame direction, estimated period = 0.91 s, STAAD computed period = 2.43 s

 

3.       NBCC2005 4.1.8.11 (3)(d) sets the upper limit? on using longer computed period, considering that the actual structure may be stiffer than the model in STAAD. For example, mechanical equipments, pipings, cable trays etc are conventionally not modeled in STAAD while they may actually contribute to the stiffness of SFRS system.

 

NBCC2005 focuses mainly on residential/commercial buildings, for industrial facilities there are mostly open structures and less partition wall cases. In high seismic zone, should there be a demand for reducing seismic force to achieve a an economical design for industrial structures, engineering judgment is required to identify if this upper limit is applicable, when the engineer is confident that the computer model can reflect the actual SFRS stiffness and give an accurate period.

4.       Seismic serviceability check? NBCC 4.1.8.13

?          Storey drift weighs more important than lateral deflection at top of structure???????? NBCC Commentary J Para 195

?          NBCC 4.1.8.13 (3) specifies storey drift limit 0.025h for normal buildings. 0.025h is an allowable limit for inelastic storey drift, which is applicable when seismic force is not reduced by dividing RdxRo factor.

Use RdxRo / IE to scale up the drift? for comparison with 0.025h when the drift value is obtained from a model with

seismic load scaled down by IE/( RdxRo).


 

4.0 DUCTILTY AND OVERSTRENGTH FACTOR

 

NBCC Table 4.1.8.9

Ductility-Related Seismic Force Reduction Factor?????????????????????? Rd

Overstrength-Related Seismic Force Reduction Factor????????????? Ro

 

In high seismic zone, the total seismic load can be more than 20 times of total wind load.

Refer to attached example 01, exchanger structure, site location: Vancouver

Base shear by seismic =8270 kN, base shear by wind =341 kN? ?????????? 8270/341 = 24.3

It?s almost impractical to design a structure deforming elastically with seismic lateral load 24 times of wind load.

 

RdxRo factor is used to reduce the seismic forces in recognition of the fact that a ductile structure designed based on the reduced forces is able to dissipate the earthquake energy through inelastic deformation without collapsing.

 

Higher Ductility of SFRS for High Seismic Zone

In high seismic zone, higher ductility of SFRS is more desirable.

Refer to attached example 01, exchanger structure, site location: Vancouver

If Ductile SFRS is used, RdxRo =5.0x1.5 for moment frame and RdxRo =4.0x1.5 for eccentrically braced frame, the seismic force for design can be reduced to? 8270 / (4.0x1.5) = 1378 kN , which is more comparable to wind load, 341 kN.

?

Higher Ductility Causes Rigorous Design Requirements for Connection Detailing

The tradeoff of higher ductility for SFRS, is the steel member and connection design requirements.

CSA S16-09 Clause 27 specifies the requirements for design of members and connections for all steel SFRS with Rd >1.5, with the exception of Conventional Construction, Rd=1.5 Ro=1.3 in S16-09 27.11

 

Some direct impacts to structural design, if the SFRS is under Clause 27 coverage

 

1.       Limitation on beam and column size, mainly only Class 1 & 2? section are allowed

2.       For energy dissipating elements, not the min yield strength Fy , but the probable yield strength RyFy = 1.1Fy shall be used, and RyFy shall not be less than 460MPa for HSS or 385MPa for others sections????????? S16-09 27.1.7

3.       S16-09 requires that all bracing connections in SFRS be detailed such that they are significantly stronger than the probable tensile capacity of bracing members.? S16-09 27.5.4.2

Brace connection design to meet such high capacity is very difficult, considering probable capacity using RyFy = 1.1Fy, and for HSS RyFy shall not be less than 460MPa.????? S16-09 27.1.7

4.       The amplification factor U2, to account the P-delta effects for structural element in SFRS, is calculated differently compared to conventional design ? S16-09 27.1.8.2

5.       Ductile moment resisting connections for seismic application must satisfy more rigorous design and detail requirements. Moment Connection shall be pre-qualified connections and designed as per CISC publication Moment Connections for Seismic Applications-2008, which contains design procedure of three types of pre-qualified moment resisting connections.

 

Conventional Construction for Low and Moderate Seismic Zone

From above we can see that, once SFRS is covered by S16-09 Clause 27, the increased complexity of SFRS frame member sizing, frame analysis, connection design and detailing, steel facbrication and erection is tremendous.

In low and moderate seismic zone, Conventional Construction is an advantageous design option to waive all provisions in S16-09 Clause 27, except clause 27.11.

In low seismic zone like Fort McMurray, the low ductility of Conventional Construction SFRS will not cause significant increase to member size, as the seismic load is normally lower or comparable to wind load, even using the lower reduction factor RdxRo value of Conventional Construction.

 

Refer to attached example 01, exchanger structure, location: For McMurray

The seismic base shear before applying / (RdxRo) is 823 kN, wind load base shear is 341 kN

With Conventional Construction, design seismic load reduced to 823 / (RdxRo) = 823 /(1.5x1.3) = 422 kN, which is already close to wind load 341 kN? ? use of higher ductility SFRS is not necessary.

 

In Fort McMurray, always use Conventional Construction, RdxRo = 1.5x1.3, for all SFRS systems.

 

5.0 STRUCTURE CLASSIFICATION

 

Most of petrochemical facilities can be classified as the following categories:

 

1.       Building Structure

2.       Nonbuilding Structure Similar to Building

3.       Nonbuilding Structure Not Similar to Building

4.       Nonbuilding Structure (Less Than 25% Comb Wt)? Supported by Other Structure

5.       Nonbuilding Structure (More Than 25% Comb Wt)? Supported by Other Structure


 

Classification of Petrochemical Facilities and Applicable Code Provisions

 

 

Seismic provision in NBCC2005 is written predominantly to address residential and commercial building structures. ?It covers the seismic requirements for Building Structure (clause 4.1.8.11 and table 4.1.8.9) and Nonstructural Component (clause 4.1.8.17 and table 4.1.8.17), but there is no provision for Nonbuilding Structure.

Nonbuilding Structure includes many popular petrochemical facilities, such as all free-standing vertical vessels, flare stacks, all horizontal vessels, piperacks, exchanger structures, process/equipment modules etc.

?In this guideline, ASCE 7-05 Chapter 15 is referenced for seismic design of Nonbuilding Structure. When ASCE 7-05 is referenced for seismic design in Canadian location, Canadian version of ground motion parameters in NBCC2005 are used to interpret formulas in ASCE 7-05. This is exactly what NBCC2005 suggests in its Commentary J page J-61 Para. 226.

 

Some of the equipments, such as hor vessel, can be treated as either Nonstructural Component or Nonbuilding Structure. When a hor vessel is supported on a steel structure and it?s weight is less than 25% of the combined weight, it?s a Nonstructural Component and NBCC2005 4.1.8.17 is used to calculate the base shear, for equipment local support design only. For the overall structure, NBCC2005 4.1.8.11 is used to calculate the base shear. The hor vessel weight is considered as part of effective seismic weight in the base shear calculation and seismic force distribution.

 

  

Case 01 Building Structures

Building structure seismic force shall be designed as per NBCC 4.1.8.11, with the weight of nonstructural components (Process, HVAC equipment and Bridge Crane etc) considered as effective seismic weight for base shear calculation and base shear distribution along vertical direction.

?       25% of roof snow load shall be counted as effective seismic weight for base shear calculation as per NBCC Commentary J page J-46 note 168

?       All process equipments (piping, tank, vessel, exchanger, pump, crusher etc) content weight under normal operating condition shall be counted as effective seismic weight for base shear calculation as per NBCC Commentary J page J-46 note 168

?       For building with crane, only crane empty weight (bridge+trolley/hoist), excluding lifting weight, shall be counted as effective seismic weight for base shear calculation as per AISC Design Guide 7: Industrial Buildings--Roofs to Anchor Rods 2nd Edition 13.6 on page 50

 

 

Case 01 Building Structure

Case 02 Nonbuilding Structures Similar to Building

 

Nonbuilding Structures Similar to Building seismic force shall be designed as per NBCC 4.1.8.11, with the weight of nonstructural components (Process, Mechanical equipments etc) considered as effective seismic weight for base shear calculation and base shear distribution along vertical direction.

?       25% of snow load, if there is any, shall be counted as effective seismic weight for base shear calculation as per NBCC Commentary J page J-46 note 168

?       All process equipments (piping, tank, vessel, exchanger, pump, crusher etc) content weight under normal operating condition shall be counted as effective seismic weight for base shear calculation as per NBCC Commentary J page J-46 note 168

?       All Process, Mechanical equipments supported on a steel/conc structure with its weight less than 25% of the combined weight, shall be designed as Nonstructural Component and NBCC2005 4.1.8.17, for equipment local support design only. For the overall structure, NBCC2005 4.1.8.11 is used to calculate the base shear. The equipment weight is considered as part of effective seismic weight in the base shear calculation and seismic force distribution.

 

 

 

Case 02 Nonbuilding Structures Similar to Building


 

Case 03 Self Supported Vertical Vessel

 

 

Case 03 Self Supported Vertical Vessel

 

 

 

 

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Case 04 Braced Leg-Supported Ver Vessel

 

Braced Leg Supported vertical vessel seismic force shall be designed as per NBCC 4.1.8.11, with the seismic force distributed as per NBCC 4.1.8.11 (6).

?          The leg brace is normally Tension Only brace which is designed by vendor.

?          The vendor will provide foundation load, including seismic case, for foundation and anchor bolt design. The engineer shall always carry on the seismic load calculation by his own and verify the vendor provided data.

?          The Braced Leg Supported vertical vessel is more flexible compared to Skirt Supported vertical vessel. Engineer can use STAAD to get the fundamental period. Please note that the Tension Only brace makes the support system more flexible and will generate longer period.

?          Depends on the shape of the vertical vessel, it can be classified as Sphere type or Cylinder type. The PSC vessel below is Sphere type vessel, the seismic load can be applied at mass center. The Deaerator vessel below is the Cylinder type, the seismic force can be applied as reverse triangle.

?          The seismic overturn moment to vessel base shall be corrected by multiplying reduction factor J obtained from NBCC Table 4.1.8.11.? This is due to the fact that the higher mode forces, Ft , make a much smaller contribution to the storey and base overturn moment.

 

Case 04 Braced Leg-Supported Ver Vessel

 

Case 05 ?Self Supported Horizontal Vessel

 

 

???????????????????? ???????????????????????????????                                                                                  Case 05 Self-Supported Hor Vessel

Case 06 Concrete Base Mounted Pump and Compressor

 

 

 

 


 

Period Estimation

From ASCE Seismic Design Guideline Page 4.A-6 Case G

Period of Vibration ? Generalized One-Mass Structure

 

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Case 06 ?Concrete Base Mounted Pump and Compressor

 

Nonbuilding structure is not covered in NBCC2005, ASCE 7-05 15.4.2 Rigid Nonbuilding Structures is referenced.

For Concrete Base Mounted Pump and Compressor, the pump and compressor is more like a rigid block and their own fundamental period is normally less than 0.06s. As defined by ASCE 7-05 15.4.2, nonbuilding structures that have a fundamental period Ta < 0.06s is classified as Rigid Nonbuilding Structure. The base shear shall be

 

 

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Case 07 Nonbuilding Structure (Less Than 25% Comb Wt) Supported by Other Structure

 

Nonbuilding structures weight is less than 25% of combined (nonbuilding structures+supporting structure) weight

ASCE 15.3.1

?       For Local Structural Support Design

Nonbuilding structures seismic force shall be designed as Nonstructural Component as per NBCC 4.1.8.17

?       For Supporting Structure Design

Supporting structure seismic force shall be designed as Building Structures or Nonbuilding Structures Similar to Building as per NBCC 4.1.8.11, with the weight of nonbuilding structure considered as effective seismic weight for base shear calculation and base shear distribution along vertical direction.

 

 

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Case 07 Nonbuilding Structure (Less Than 25% Comb Wt) Supported by Other Structure


 

Case 08 & 09 Nonbuilding Structure (More Than 25% Comb Wt) Supported by Other Structure

Nonbuilding structures weight is greater than 25% of combined (nonbuilding structures+ supporting structure) weight

ASCE 15.3.2

 

Case 08 Nonbuilding Structure (More Than 25% Comb Wt)? Supported by Other Structure

??????????? ???Rigid Nonbuilding Sructures????????????

Nonbuilding structures that have a foundamental period less than 0.06s are considered as Rigid Nonbuilding Structures?? ASCE 15.3.2-1

?       Nonbuilding structure shall be considered as a rigid element with appropriate distribution of its effective seismic weight

?       RdxRo value of combined system is permitted to be taken as the supporting structure?s RdxRo value

?       Supporting structure seismic force shall be designed as Building Structures or Nonbuilding Structures Similar to Building as per NBCC 4.1.8.11

 

 

Case 08 Nonbuilding Structure (More Than 25% Comb Wt)? Supported by Other Structure

?????????????????????????????????????????                                                                                   Rigid Nonbuilding Structures

 

 

Case 09 Nonbuilding Structure (More Than 25% Comb Wt)? Supported by Other Structure

?????????????? Nonrigid Nonbuilding Sructures?????

 

Nonbuilding structures that have a fundamental period greater than 0.06s are considered as Nonigid Nonbuilding Structures?? ASCE 15.3.2-2

?       Nonbuilding structure and supporting structure shall be modeled together in a combined model with appropriate stiffness and effective seismic weight distribution

?       RdxRo value of combined system shall be taken as the lesser? RdxRo value of the nonbuilding structure or the supporting structure

?       The combined structure seismic force shall be designed as Nonbuilding Structures Similar to Building as per NBCC 4.1.8.11

 

 

Case 09 Nonbuilding Structure (More Than 25% Comb Wt)? Supported by Other Structure

????????????????????????????????????????                                                                                    Nonrigid Nonbuilding Sructures


 

6.0 DESIGN EXAMPLES

 

Design Example 01: Nonbuilding Structure Similar to Building - Exchanger Structure

Structure Classification: Case 02 & Case 07

 

Calculate the seismic force for an exchanger structure supporting stacked heat exchangers as shown on next page.

Frames along GL1,2,3 are moment frame. Frames along GLA, C are braced frame. Frame along GLB is unbraced.

Single exchanger shell operating weight 500 kN, each floor equipment effective seismic weight = 4 x 500 = 2000 kN.

Assume each floor has 20m long 20? dia pipes to be counted for effective seismic weight.

Structure importance category = High as the exchanger contains flamable hydrocarbon content.

Calculate seismic force for the following scenarios:

 

1.     Site in Fort McMurray, Site D, ?Use SFRS RdxRo of Conventional Construction (CC)

Use Equivalent Static Force Procedure

?          Seismic force calc for overall structure steel design

?          Seismic force calc for local structure steel support design (exchanger support)

?          Compare wind and seismic force, with the RdxRo value of Conventional Construction and Moderately Ductility

 

2.     Site in Vancouver, Site D, Use SFRS RdxRo of Ductile (D) and Moderately Ductility (MD)

Use Equivalent Static Force Procedure

 

From STAAD output, braced frame in N-S direction Ta=0.66s, moment frame in E-W direction Ta=2.43s

 


 

 

 

Example 01 Exchanger Structure

 

 

 


 

Wind Load Calc for Overall Structure

To simplify the calc and for comparison purpose only, use the wind load on enclosed structure for a quick check

Wind load pressure 1/50 yr q=0.35 kPa, Cf=1.3, Ce=1.10, Cg=2.0, Iw=1.15

Wind load base shear = Iw x Cf x q x Cg x Ce x A =1.15 x 1.3 x 0.35 x 2.0 x 1.1 x 12.6 x 23.5 = 341 kN

 

Seismic Base Shear for Overall Structure Design

 

Location: Fort McMurray

 

????? Location: Fort McMurray?? ??????????????? Site Class: Site D

SFRS

Base Shear Ve

before Ve / (RdxRo)

Base Shear SFRS CC

Ve / (1.5x1.3)

Base Shear SFRS MD

Ve / (3.0x1.3)

kN

kN

kN

Moment Frame

328

168

84

Braced Frame

823

422

211

 

From above seismic base shear calc, we can find that, in low seismic zone such as Fort McMurray area, using Conventional Construction (CC) is good enough to bring the lateral seismic force down to a magnitude comparable to wind load, 341 kN.

From CSA S16-09 clause 27.11.1 Conventional construction Rd=1.5 , Ro=1.3

? the requirement of clauses 27.1 to 27.10 and 27.12 shall not apply to these systems.

In low or moderate seismic zone, using a higher RdxRo modification factor is not necessary as it will trade the convenience of non-seismic connection design for nothing. With the use of response reduction factor RdxRo under Conventional Construction, the seismic load is already comparable to wind load, and in many cases, seismic load is actually lower than wind load.

 

NOTES

 

It?s incorrect to conceive that in Fort McMurray area the wind load will govern structural design and the seismic load is negligible compared to wind load. In this case the seismic load for braced frame, 422 kN, is bigger than the wind load,

341 kN. One may argue that the wind load still govern when it goes to the load combination considering wind load factor of 1.4, and seismic load factor of 1.0, but actually in many cases the seismic load will govern in the design of petrochemical structures in Fort McMurray area.

 


 

Location: Vancouver

 

????? Location: Vancouver??????? ??????????????? Site Class: Site D

SFRS

Base Shear Ve

before Ve / (RdxRo)

Base Shear SFRS CC

Ve / (1.5x1.3)

Base Shear SFRS MD

Ve / (3.0x1.3)

Base Shear SFRS D

Ve / (4.0x1.5)

kN

kN

kN

kN

Moment Frame

4185

2146

1073

698

Braced Frame

8270

4241

2121

1378

 

From above seismic base shear calc, we can find that, in high seismic zone such as Vancouver, using higher modification factor of RdxRo is absolutely necessary, otherwise the huge seismic lateral load, 8270 / 341 = 24 times of wind load in this case, will create an impractical structural design.

 

 

 

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Seismic Base Shear for Exchanger Support Design

In this part the equipment is taken as a Nonstructural Component and its seismic force is calculated as per NBCC 4.1.8.17.

This seismic force is used for the design of local equipment support only (steel support for exchangers).

The exchangers sitting on top of structure (EL23.500) get the biggest seismic response as the acceleration increases with the height of structure. This effect is caputured by the height factor, newly introduced in NBCC2005, Ax = 1 + 2hx / hn

For equipments at foundation level Ax = 1.0, and Ax = 3.0 for equipments sitting at roof level.

 

????? Location: Fort McMurray?? ??????????????? Site Class: Site D

Lateral Load Type

Transverse Direction

Longitudinal Direction

kN

kN

Wind

18

4

Seismic

122

243

 

From above we find that, for local equipment support design, the seismic load is much bigger than the wind load if the equipment is located on a higher elevation above grade. This is mainly due to the dynamic amplifying effect (Ar =2.5) for big mass sitting on a flexible supporting structure.

 

 

NOTES

 

It?s incorrect to conceive that in Fort McMurray area the wind load will govern structural design and the seismic load is negligible compared to wind load. In this case the seismic load can be 243/4 = 61 times bigger than the wind load.



 

 

 

 

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Design Example 02: Skirt-Supported Vertical Vessel

Structure Classification: Case 03

 

Calculate the seismic force for a skirt-supported vertical vessel

Vessel diameter = 7.189 m??????????????

Vessel height = 12.400 m

Vessel shell thickness = 0.25 in

 

Vessel empty weight = 221 kN

Vessel operating weight = 3793 kN

Vessel hydrotest weight = 5055 kN

Site location : Fort McMurray

Site class : Class D

Structure importance category : Normal

 

NOTES

 

It?s incorrect to conceive that in Fort McMurray area the wind load will govern structural design and the seismic load is negligible compared to wind load. In this case

seismic base shear is 131.5 kN? vs wind base shear 71.9 kN

seismic overturn moment is 1087.0 kNm? vs wind overturn moment 499.9 kNm

 

In this case, the overturn moment caused by seismic is 2 times of the overturn moment caused by wind. This is mainly due to the reverse triangle distribution of seismic load.

 



 

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Design Example 03: Braced Leg -Supported Vertical Vessel

Structure Classification: Case 04

 

Calculate the seismic force for a braced-leg supported PSC vessel. The PSC vessel is supported by 10 x OD=1450mm wall thk =27mm steel column equally spaced at 22.5m diameter circle. ?The 3D support frame is braced by 25 dia steel tension only rod. Vessel empty weight = 13810 kN , operating weight = 183710 kN

Site location : Fort McMurray ???????????? Site class : Class D??????????? Structure importance category : Normal

 

The PSC vessel is a cone shape, diameter varies from 0m to 32m along the 30m vessel height. To simplify the wind load calculation, assume it?s a dia=16m H=30m cylinder vessel, which gives the same projection area for wind load calc.

 

 

Braced-Leg Supported PSC Vessel

Use Master/Slave to define the top support plane as a rigid diaphragm. Use the central node as a master node, the central node needs not to be physically connecting to the surrounding nodes.

 

 

 

STAAD Model : Rigid Diaphragm and Tension Only Brace

 

 

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NOTES

 

It?s incorrect to conceive that in Fort McMurray area the wind load will govern structural design and the seismic load is negligible compared to wind load. In this case

seismic base shear is 771.6 kN? vs wind base shear 319.0 kN

seismic overturn moment is 10416.4 kNm? vs wind overturn moment 5335.9 kNm

 

In this case, the overturn moment caused by seismic is 2.0 times of the overturn moment caused by wind. This is mainly due to

?          Ta >0.7s causing Ft >0

?          Vessel mass center is located at a higher elevation

 

 

 


 

 

 

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Design Example 04: Self-Supported Horizontal Vessel

Structure Classification: Case 05

 

Calculate the seismic force for a self-supported horizontal vessel

Vessel diameter OD= 3.683 m???????? Insulation thk = 50mm??????

Vessel length = 20.700 m???????????????? Vessel saddle distance = 16.535 m

Vessel empty weight = 533 kN

Vessel operating weight = 2317 kN

 

Site location : Fort McMurray

Site class : Class D

Structure importance category : Normal

 

NOTES

 

It?s incorrect to conceive that in Fort McMurray area the wind load will govern structural design and the seismic load is negligible compared to wind load. In this case

For lateral load on vessel longitudinal direction

seismic base shear is 80.3 kN? vs wind base shear 13.1 kN

seismic overturn moment is 157.4 kNm? vs wind overturn moment 20.1 kNm

 



 

 

 

 

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Design Example 05: Building Structure

Structure Classification: Case 01

 

Calculate the seismic force for a pump house building

 

Building span = 11.1 m????????????????????? Building total length = 33.37m????????? Roof slope = 1:12

Building eave height = 7.94m?????????? Crane runway height = 5.32m

Building has a 18 tonne overhead crane

Crane bridge wt = 8600kg???????????????? Trolley + hoist? wt = 1365kg??????????????

Site location : Fort McMurray

Site class : Class D

Structure importance category : Normal

 

 


 

 

 

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Design Example 06: Nonbuilding Structure (> 25% Comb Wt) Supported by Other Structure

Structure Classification: Case 09

 

Calculate the seismic force for a vertical surge drum supported by a steel frame table top.

 

 

 

Vessel diameter D= 7.550 m = 24.770 ft???????

Vessel height H= 33.150 m = 108.760 ft

Vessel shell thickness t = 25.4mm = 1 in

 

Vessel empty weight = 2243 kN = 504675 lb

Vessel operating weight=20081kN = 4518225 lb

Vessel hydrotest weight=15938 kN= 3586050 lb

Site location : Fort McMurray

Site class : Class D

Vessel content is flammable hydrocarbon

Structure importance category : High

 

 

Determine If Vessel Is Rigid Nonbuilding Structure

Vessel linear weight W = 4518225 lb / 108.760 ft = 41543.1 lb/ft

Vessel fundamental period??? ?= 0.527 s >> 0.06 s? ? the vessel is a flexible Nonbuilding Structure

 

 

 

 

Determine If Nonbuilding Structure Wt Is More Than 25% of Comb Wt

 

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Vessel Support Steel Frame

 

Determine RdxRo Value

RdxRo value of combined system shall be taken as the lesser? RdxRo value of the nonbuilding structure or the supporting structure ? Use RdxRo = 1.5x1.3 as Conventional Construction

 

Modeling Techniques In STAAD

1.       Model the vertical vessel as seven segments of beam element, break the 33.15m into? 6x5m + 1x3.15m =33.15m

 

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Define Vessel Support Base as Rigid Diaphragm

Apply Vessel Content Mass As Linear Load

 


 

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NOTES

 

It?s incorrect to conceive that in Fort McMurray area the wind load will govern structural design and the seismic load is negligible compared to wind load. In this case

seismic base shear is 766 kN? vs wind base shear 279 kN ??? 766 / 279 = 2.7 times

seismic overturn moment is 15072 kNm? vs wind overturn moment 4620 kNm?? 15072 / 4620 = 3.3 times

 

It?s also risky to assume that the vendors? calculation will take care of the seismic design. The vendor?s seismic calculation always assumes the vessel base is fixed, as the vendor never has intension to get the boundary condition of support structure. In this case, when vessel weight exceeds 25% of combined weight, the vessel and supporting structure shall be modeled together in a combined model to get the accurate response of seismic load.

 


 

 

 

 

 

 

 

 


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