PURPOSE PURPOSE
This practice provides recommended procedures for calculation of wind forces on This practice provides recommended procedures for calculation of wind forces on various types of equipment, supporting structures & buildings as
various types of equipment, supporting structures & buildings as per the Canadianper the Canadian codes. This practice does not address tornadoes.
codes. This practice does not address tornadoes. This practice is intended to be
This practice is intended to be used in conjunction with NBCC and is used in conjunction with NBCC and is not annot an independent document.
independent document. This document mostly deals with the This document mostly deals with the types of structures,types of structures, commonly encountered
commonly encountered in petrochemical facilities. The principles underlaid in in petrochemical facilities. The principles underlaid in thethe ASCE
ASCE Task Committee Task Committee on Wind on Wind Loads for Loads for Petrochemical Facilities Petrochemical Facilities are included are included inin this practice.
this practice.
This practice is a companion to
This practice is a companion to Structural Engineering practice 000 215 1215 WindStructural Engineering practice 000 215 1215 Wind Load Calculation.
Load Calculation.
SCOPE SCOPE
This practice includes the following topics: This practice includes the following topics: SCOPESCOPE APPLICATIONAPPLICATION DEFINITIONSDEFINITIONS
GENERAL DISCUSSIONGENERAL DISCUSSION
VERTICAL VESSELSVERTICAL VESSELS
HORIZONTAL VESSELSHORIZONTAL VESSELS
ENCLOSED STRUCTURESENCLOSED STRUCTURES
OPEN EQUIPMENT STRUCTURESOPEN EQUIPMENT STRUCTURES
LOAD COMBINATIONSLOAD COMBINATIONS
OTHER CONSIDERATIONSOTHER CONSIDERATIONS
REFERENCESREFERENCES
APPLICATION APPLICATION
The details, principles and methods co
The details, principles and methods contained in this practice ntained in this practice will be used for thewill be used for the calculation of wind loads as per C
calculation of wind loads as per C anadian codes. Whenever client or localanadian codes. Whenever client or local
jurisdiction requirements differ or are incomplete, this practice shall be used as much jurisdiction requirements differ or are incomplete, this practice shall be used as much
as feasible and the more c
as feasible and the more conservative shall be adonservative shall be adopted. opted. This practice requires the useThis practice requires the use of general proce
of general procedures detailed in dures detailed in NBCC – part 4 NBCC – part 4 (1995) Minimum Design (1995) Minimum Design Loads Loads forfor Buildings and Other Structures and Commentary ‘B’-wind l
Buildings and Other Structures and Commentary ‘B’-wind l oads.oads.
DEFINITIONS DEFINITIONS
Reference Wind Speed (
Reference Wind Speed ( V V )) : : Reference wind speed is a Reference wind speed is a measure of the hourlymeasure of the hourly mean wind speed taken at sites (usually airports) chosen in most cases to be mean wind speed taken at sites (usually airports) chosen in most cases to be representative
representative of a height of 10 of a height of 10 m in an open expom in an open exposure. This is determined bysure. This is determined by extreme value analysis of meteorological observations of hourly mean wind
extreme value analysis of meteorological observations of hourly mean wind speeds.speeds. Flexible Buildings & Other
Flexible Buildings & Other StructuresStructures : : A structure is considerA structure is considered as slender ed as slender andand flexible when the ratio of heig
flexible when the ratio of height to least horizontal dimension exceht to least horizontal dimension exceeds 5 or eds 5 or thethe fundamental natural frequ
GENERAL DISCUSSION GENERAL DISCUSSION
National Building Code of Canada National Building Code of Canada
NBCC forms the basis of this practice with due reference to ASCE Task Committee NBCC forms the basis of this practice with due reference to ASCE Task Committee parameters for wind loads on petrochemical faciliti
parameters for wind loads on petrochemical faciliti es and Structural Engineeringes and Structural Engineering practice 000 215 1215 Wind Load Calculation.
practice 000 215 1215 Wind Load Calculation. Reference Wind Pressure
Reference Wind Pressure
The Reference Wind Pressure (q) is directly available from NBCC, Appendix “ C ” The Reference Wind Pressure (q) is directly available from NBCC, Appendix “ C ” for many Canadian locations. The values of
for many Canadian locations. The values of q are tabulated for three different q are tabulated for three different levelslevels of probability being exceeded per year. (1 in 10,
of probability being exceeded per year. (1 in 10, 1 in 30 & 1 in 1 in 30 & 1 in 100).100). q
q == C C V V 22
The factor C depends on atmospheric pressure and
The factor C depends on atmospheric pressure and air temperature. For Canadianair temperature. For Canadian conditions C = 50 X 10
conditions C = 50 X 10-6-6forfor V V in km/hr.in km/hr. Wind pressure ‘p’
Wind pressure ‘p’ is calculated from is calculated from the formulathe formula p
p
=
=
q.Cq.Cee.C.Cgg.C.Cppwhere
where p is the specified external pressure p is the specified external pressure acting statically and in a acting statically and in a direction normaldirection normal to the surface
to the surface q
q = = reference reference wind wind pressure pressure ( ( KPa KPa )) C
Cee = = exposure factor exposure factor depending depending on height on height for different for different categories of categories of
exposure exposure C
Cgg = = Gust Gust Effect FEffect Factoractor
C
Cpp = = external pressure coefficient averaged external pressure coefficient averaged over the area of theover the area of the
surface considered surface considered Exposure Factor C
Exposure Factor Cee
The ex
The exposure factor posure factor ‘C‘Ce’ reflectse’ reflectschanges in wind speed with height and effect of changes in wind speed with height and effect of terrainterrain
category classified
category classified as as A, B & A, B & C.C. For the windward face C
For the windward face Ceecorresponds to that for the height ‘Z’ corresponds to that for the height ‘Z’ and thereforeand therefore
increases with height. For the leeward f
increases with height. For the leeward f ace Cace Ce ise isevaluated at half the height ‘H’ of theevaluated at half the height ‘H’ of the
building. building.
Exposure Category Exposure Category Exposure A:
Exposure A: Open level terrain with only Open level terrain with only scattered buildings, scattered buildings, trees or othertrees or other obstructions, open water or shorelines thereof. This
obstructions, open water or shorelines thereof. This is the exposure on which theis the exposure on which the reference wind speeds are based.
reference wind speeds are based. C
Cee = = (Z (Z /10)/10)0.280.28 CCee>1.0>1.0
Where
Where Z Z = = height height above above ground ground in in meters.meters. Exposure B:
Exposure B: Suburban & Urban aSuburban & Urban areas , wooded terrain reas , wooded terrain or centers of large towns.or centers of large towns. C
Exposure C: Centers of large cities with heavy concentrations of tall buildings. At least 50% of the buildings should exceed 4 storeys.
Ce = 0.4 (Z /30) 0.72
Ce>0.40
Speed up Over Hills & Escarpments
Buildings on a hill ( with a maximum slope > 1 in 10 ) particularly near a crest may be subject to significantly higher wind speeds than buildings on level ground. The exposure factor shall be multiplied by a ‘speed up factor’ as given in table B-1 of NBCC commentary ’ B’ – Wind Loads.
Gust Effect Factor Cg
Gust effect do not consider effects of across wind response, vortex shedding, instability due to galloping or flutt er or dynamic torsional effects.
Gust factor ‘Cg’ is defined as the ratio of the maximum effect of the loading to the
mean effect of the loading.
This factor accounts for the dynamics of wind fluctuations and the load amplification introduced by the building dynamics.
The total response may be considered as a superposition of a background component with out any structural dynamic magnification and a resonant component due to excitation close to the natural frequency of the structure.
A general expression for the peak loading effect (Wp) is given by
p
p g
W
And the expression for Cg, which is the ratio of peak loading to the mean loading, can
be identified as: -) / ( 1 p g g C
Where the mean loading effect
the root mean square loading effect
p
g statistical peak factor for the loading effect
For majority of the structures the resonant component is small and the dynamic factor can be simplified by considering the background component only.
Simple Procedure:
The form of the fluctuating wind loading effect ( ) varies with the excitation – whether due to gusts, wake pressures or motion induced forces. For a large class of smaller structures only the added loading due to gusts must be dealt with and simplified methods are adequate.
For small structures and components having relatively high rigidity, a simplified set of dynamic gust factors is given in commentary ‘B’ of NBCC.
Cg= 2.5 for cladding elements and small structural components
= 2 for structural systems including anchorages to foundations. Where combined gust factor & pressure coefficients ( CpCg) are given in the figures
& tables of the Commentary ‘B’ of NBCC, it is not required to determine gust factors separately.
Detailed Procedure:
For structures that are tall, slender, lightweight, flexible or lightly damped, the ‘Resonant’ component of the total response may be dominant and hence the value of Cg is obtained by the detailed procedure.
The value of
/
is expressed as:) ( / B sF C k eH
k = a factor related to the surface roughness coeff. of terrain = 0.08 For exposure A
= 0.10 For exposure B = 0.14 For exposure C
CeH = Exposure factor ( Ce ) at the top of the building of
height ‘H’
B = Background turbulence factor as obtained from Fig. B-3 of Commentary ‘B’ as a function of W/H where
W= Width of windward face of the building H = Height of the windward face of the building s = Size reduction factor as obtained from Fig. B-4 of
Commentary ‘B’ as a function of W/H and the Reduced frequency 0 H / V H where
0
= Natural frequency of vibration (Hz) H
V = Mean wind speed at top of structure (H) in ‘m/s’ = V C e H
F = Gust energy ratio at the natural frequency of the Structure, as obtained from Fig. B-5 of Commentary ‘B’ as a function of wave No. ( 0 / V H )
Critical damping ratio
= 0.01 for Steel framed building = 0.02 for concrete framed building
= 0.0016 to 0.008 for closed circular unlined welded steel stacks
= 0.0048 to 0.0095 for lined welded steel stacks
= 0.0095 to 0.019 for unlined reinforced concrete Stacks Note:
(i) The lower values of are appropriate for foundations on rock or piles. Average values are appropriate for foundations on
compacted soils. Higher values are appropriate for vessels supported by elevated structures or soft soils
(ii) It is also recommended that the User checks the value of adopted by the Mechanical Engineer and the relevant mechanical software for the design of the vertical vessel The Peak factor (
g
p) is obtained from Fig. B-6 of Commentary ‘B’ as a function of . B sF sF 0Where = average fluctuation rate Pressure Coefficient
Pressure coefficients are usually determined from wind tunnel experiments. They are non-dimensional ratios of wind induced pressures on a building to the dynamic pressure of the wind speed at reference height.
Whenever the sign of plus or minus is specified, check both positive and negative values to obtain controlling loads. Sign convention is as follows:
+ (Plus sign) means positive pressure acting toward the surface. - (Minus sign) means negative pressure acting away from the surface. Figures B-7 to B-27 of NBCC commentary ‘B’ on wind loads covers information on external and internal pressure coefficient required for the design of cladding and the structures as a whole for a variety of building geometry. Fig B-14 gives pressure coefficients for flat roofed buildings greater in height than in width. In Figure B7 to B13, peak pressure coefficients have been determined directly from wind tunnel test and composite values of (
C
p. C
g.
) are obtained incorporating the aerodynamic shapefactors. VERTICAL VESSEL
Vertical vessels must be designed for along-wind response caused by straight wind (drag forces). Flexible vessels must also consider across-wind response caused by vortex shedding (lift forces). The design procedure herein is also appropriate for determining design wind forces on stacks and chimneys. A vertical vessel (or a stack or chimney) will behave like a cantilever beam.
General Procedure
For rounded structures, the pressure varies with the wind velocity depending on the Reynolds number (Re) expressed by the equation:
Re = d qC e 2.7106
Where d = Diameter of sphere / cylinder in (m).
The roughness of rounded structure is of importance. Common well-laid brickwork without parging can be considered as having a “moderately smooth” surface. Surfaces with ribs projecting more than 2% of dia. are considered “very rough”. As per fig B-18 of NBCC-1995 commentary ‘B’ on Wind Loads, for a vertical vessel Total Force F =
q.C
e.C
g.C
f .AWhere A = D. h ( where h = height above the ground ) F = Design wind force
C
g = Gust effect factorC
f = Force coefficient from fig. B-18 of Commentary ‘B’D = Basic vessel diameter, equal to vessel internal
diameter plus 2 times the wall thickness plus 2 times the insulation thickness ( m ).
Wind on Appurtenance
The general procedure for vertical vessel requires modification to account for vessel appurtenances such as ladders, piping & platforms.
As per the ASCE Task Committee on Wind loads for Petrochemical Facilities: (i) To account for wind on ladders and piping, the Cf factor as determined in general
procedure is increased by WIF (Wind Increase Factor) Cfm = Cf (WIF)
The following values of WIF may be used. (Reference Structural Engineering practice 000.215.1215 Wind Load Calculation)
D (inches) WIF
24 to 30 1.5
36 to 48 1.4
54 to 72 1.3
78 and greater 1.2
(ii) For Wind on platforms:
F = qCeCg Cf A
C
f = 2.0A = Projected area of supporting steel
In absence of information, the criteria to estimate the number and size of platform as given in Structural Engineering practice 000.215.1215 Wind Load Calculation may be adopted.
(iii) For Wind on handrails: F = qCeCg Cf A
C
f = 2.0C
g = 2.5 ( as per NBCC Simple Procedure)A = Projected area (generally a value of 0.80 sq.ft./ft may be adopted) Across Wind Response – Vortex Shedding
When the wind blows across a slender, prismatic, cylindrical body vortices are shed alternatively from one side and then the other, giving rise to fluctuating force acting at right angles to the wind direction along the length of the body.
The wind speed (
V
H) at the top of the structure when the frequency of vortexshedding equals the natural frequency ‘ ’ of structure: D
S 1 VH
Where = Natural frequency of the s tructure (Hz)
The natural frequency can be calculated by ‘WINPLUS’ or refer Structural
Engineering practice 000.215.1215 Wind Load Calculation section on ‘Fundamental Frequency’
V
H = Mean wind speed at the top of structure (H) when the frequency ofvortex shedding equals the natural frequency ‘ ’ of the structure (resonance condition)
D = Basic vessel diameter S = Strouhal number
As per NBCC Commentary ‘B’, value of
V
Hcan be approximated by:(i) D20.5(m2 /s) VH 6 D (For
R
e
2
10
5 and6
1
S
) (ii) 0.5m2 / s D20.75(m2 /s) D s) / 1.5(m D 3 V 2 H For(
2
10
5
R
e
2
.
5
10
5)
(iii) D2 0.75(m2 /s) VH 5 D For (R
e
2
.
5
10
5 and5
1
S
)Across Wind Response Evaluation Consideration
The evaluation of a vessel for across wind response is not clearly addressed in the NBCC. The following criteria have been picked from various references.
(i) As per Structural Engineering practice 000 215 1215 Wind Load Calculation, Across Wind Response is not a concern i f
1
.
3
V
V
H
C
e H(Also refer Attachment 07 of Structural Engineering practice 000 215 1215 Wind Load Calculation)
This is close to the criteria adopted by Mechanical for Design of Vertical Vessels. As per the Mechanical Criter ia, Across Wind Response is not a concern if
1
.
25
V
V
H
C
e H(ii) As per ASTM STS-1-2000 Clause 5.22(3), Across Wind R esponse can be ignored if
1
.
2
V
V
H
C
e H(iii) There is also a strong opinion that the design procedure outlined in (i) and (ii) above is appropriate for determining design wind forces on stacks and chimneys. For structures like the vertical vessels as typical to the refinery and petrochemical facility, Across Wind Response is not a concern if
H e
C
V
V
H
(iv) As per another school of thought, vertical vessels, as typical to the refinery and petrochemical facility, generally have access platforms and other appurtenances attached on the outside. These act as strakes and dampners, which prevent the formation of vortices and hence Across Wind R esponse is not a concern at all.
Where V C e H = Hourly mean wind speed at the top of the structure being designed.
It is recommended that the crit eria outlined in (iii) above be followed.
It is also recommended that the Us er checks the criteria adopted by the Mechanical Engineer and the relevant mechanical software for the design of the vertical vessel. It may not be appropriate to design the vessel foundation for a force greater than (or less than) the force for whi ch the vessel and its anchorage are designed by the mechanical group.
In any case, the criteria adopted for Across Wind Response Evaluation consideration should be acceptable to the Client.
Vortex Shedding Force
approximated by an equivalent static force per unit length (
F
L), acting over the top 1/3 of the structure. D q M D C H 2 2 1 L C F
= Critical damping ratio
= Aspect ratio H/DH
q
= Velocity pressure corresponding toV
H where VHis in m/s (resonancecondition) 2
60
.
0
V
H
(Pa)M
= Average mass per unit length over top 1/3 of structure (kg
/m) = Density of air 3 / 2 . 1 kg m 1C
= 3 for
16
=
4
3
for
16
2C
= 0.60 In addition:(i) If VHis low (VH< 10m/s) and
12
(very slender structure), vortexinduced motion is significantly increased. In such a case adopt C1 = 6
C2 = 1.2
(ii) For tapered structures with a diameter variati on exceeding 10% over the top
third, adopt C1 = 3
C2 = 0.6
And no increase in these coefficients is r equired for low value of VH
(iii) If M D C 2 2
then large amplitude motion upto one diameter may result . Adopt appropriate remedial measures.
Design Wind Force
The wind loads shall be esti mated for both the Along Wind Response as wel l as the Across Wind Response. There are no clear guidelines in the NBCC to suggest
whether the Along Wind and the Across Wind loads shall be considered separately or shall be considered as a combined action.
The following have been picked from various references.
(i) ASME STS-1-2000 Clause 5.2.2(a) point (1) under the heading ‘Wind Responses – Vortex Shedding’ states
“ The vortex shedding loads need not be combined with along wind loads.” Along wind and Across wind moment and shear shall be taken separately.
Adopt more critical of Along or Across Wind Response loads.
(ii) Texts available suggest that the Along wind and Across wind loads shall be taken separately and as a combined action.
As a practice, the Across wind loads have been combined with the Along wind loads at wind speed VH. (where VH. is the Mean wind speed at the top
of structure (H) when the frequency of vortex shedding equals the natural frequency ‘ ’ of the structure).
The Resultant load is expressed by the SRSS of the Across Wind loads and the Along Wind loads at VH.. The more critical of the Along Wind load and
the Resultant load is considered as the Design load.
In absence of more definite information, it is recommended to follow (ii) above. It is also recommended that the User checks the crit eria adopted by the Mechanical Engineer and the relevant mechanical software for the design of the vertical vessel In any case, the criteria adopted should be acceptable to the Client
HORIZONTAL VESSEL General Procedure
For horizontal vessels, no check for dynamic properties is required.
As per the ASCE T ask Committee on Wind loads for Petrochemical Facilities, Projected diameter of the vessel is equal t o the outer diameter of vessel plus 1.5 ft (0.46 m) to account for ladders, nozzles and pipes of diameter 8 inches (including insulation) or smaller.
F = qCeCg Cf A
Note: It is assumed that all forces are applied on Centerline.
For horizontal vessel and structures having high rigidity, as per the simple procedure adopt:
2.0 Cg Transverse wind:
For wind perpendicular to the longitudinal axis of the vessel use coefficient for walls above ground as per figure B-17 of Commentary ‘B’.
Multiply the Cf factor from figure B-17 by 0.70 t o account for the cylindrical shape
of vessel. Generally for
10
h
l
and
40
oLongitudinal wind:
For wind in the longitudinal direction adopt Cf from figure B-17
On the conservative side, the end of the vessel is considered flat. For determining the force coefficient Cf the vessel and the pier are considered together and the force
coefficient for walls on ground is adopted. Generally for
1
h
l
and
50
oCf = 1.5, may be adopted (all values are from Table B-17)
Appurtenances :
(i) For wind on Pipes larger than 8” refer figure B-22 of Commentary ‘B’ (ii) For wind on Piers, Cf = 1.5
(iii) For wind on Platforms, follow the procedure as outlined under ‘Vertical Vessel’ (iv) For Ladders (if required to be considered separate)
As per the ASCE Task Committee on wind loads for Petrochemical Facilities
2.0
C
f
0.1524
A sq. m /m, i.e. 0.50 sq.ft./ft. (for ladder without cage) 0.2286
A sq. m /m, i.e. 0.75 sq.ft./ft. (for ladder with cage) ENCLOSED STRUCTURES
The general procedure for enclosed structure requires Cgto be calculated as per the
detailed procedure if the fundamental frequency (first mode) of vibration of the structure is less than 1 Hz or if the height to width ratio exceeds 5.
While calculating Cg, the value of the critical damping ratio (
) should be selectedas appropriate for the structural system.
The pressure coefficient can be obtained from Fig. B-14 of NBCC Commentary ‘B’. The net specified pressure due to wi nd on part or all of a surface of a building shall be the algebraic difference of the external pressure (p = qCeCgCp)and the specified
internal pressure or suction ( pi = qCeCgCpi)
where Cp = external pressure coefficient
Cpi = internal pressure coefficient
pi = specified internal pressure acting statically and in a direction
normal to the surface as a pressure or suction OPEN EQUIPMENT STRUCTURES
It is not conservative to assume that an upper bound to wind force on an open structure is given by the force on that structure as if it were enclosed. ASCE Task Committee on Wind loads for Petrochemical Facilities comments that model test of
open buildings have demonstrated that wind force on an open st ructure can exceed wind force on that structure when subsequently enclosed.
Open equipment structures support equipment and piping within an open structural frame, generally unenclosed by siding or other shielding appurtenances. Open equipment structures include:
Open Equipment Structures Pipe racks or cable tray racks Framed or trussed towers
Structural frames supporting appurtenances
The main wind force resisting system includes, wind forces acting on the structural frame and the appurtenances such as ladders, handrails etc.
f g e C C C q p
The wind forces on vessels, piping and cable t rays located on or attached to the structure shall be calculated separately and added to t he wind forces acting on the main wind force resisting system.
Adopt the applicable value of Cgas per simple or detailed procedure conforming to
NBCC Commentary ‘B’. Wind Loads with no shielding
Wind loads force coefficient for design of individual components, cladding and appurtenance (excluding vessels, piping and cable trays) shall be calculated as per figure B-23 of NBCC Commentary B.
The force coefficient is applicable for st ructural members of infinite lengths and this is multiplied by the reduction factor ‘k’, for finite length of members. (If member projects from large plate or walls, the reduction factor ‘k’ should be calculated for slenderness based on twice the actual length.)
Wind load is calculated on the Effective soli d area exposed to the wind. (Elements such as cladding, bracing, ladders, stairs and handrails can be considered as part of the solid area.)
ASCE Task Committee on Wind Loads for Petrochemical Facilities – Recommended guidelines
For the main force resisting system T he ASCE Task Committee on Wind L oads for Petrochemical Facilities suggests value of Cf to be used for piperack and other
similar structures based on no shielding (except as defined for piping and cable tray). For all structural members Cf = 1.8 or alternately Cf = 2.0 below the first level and Cf
= 1.6 for members above the first level. Loads on Frame and Shielding
In addition to the use of f igure B-23 of NBCC Commentary B as explained above (Wind load with no shielding), for framing members that are located behind each other in the direction of the wind, the shielding effect may be taken into account. The
windward member and the unshielded part of the l eeward member should be
designed for the full pressure (q). The shielded part of the leeward member should be designed for the reduced pressure (qx) as per figure B-25 of Commentary B.
The solidity ratio
A
A
s
Where A = Gross area (envelope area) of the frame.
As = Effective Solid area (Elements such as cladding, bracing, ladders,
stairs and handrails can be considered as part of the solid area.) The effective 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. (As per t he ASCE Task Committee on Wind Loads for Petrochemical Facilities, the presence of
flooring or decking does not cause an increase of solid area beyond the inclusion of the thickness of the deck.)
ASCE Task Committee on Wind Loads for Petrochemical Facilities – Recommended guidelines
Refer Attachment 01. The structure is idealized as two sets of orthogonal frames. The maximum wind load on each set of fr ame is calculated independently.
The force coefficient is defined for wi nd forces obtained normal to the frames, irrespective of the actual wind direction. It accounts for entire structure in the direction of wind. The value obtained for each axes of the structure is the maximum force coefficient for the component of force acting n ormal to the frames for all horizontal wind angles.
The force coefficient CDgwas developed from wind tunnel test for use on the gross
area of the structure. These are converted to force coefficient applied to the solid area as per the equation.
Dg f
C
C
Where CDg = Force coefficient for a set of frames.
= Solidity ratio,A
A
s
As = Effective solid area of the windward frame (Elements such as
cladding, bracing, ladders, stairs and handrails can be considered as part of the solid area.)
A = Gross area (envelope area) of the windward frame. The force coefficient (CDg) depends on
(i) Frame spacing ratio SF /B, where ‘SF‘ is measured center to center and ‘B’ is
measured from outside edge to outside edge.
(ii) N = Number of framing lines normal to the nominal wind direction. CDgis obtained from graph between ‘’ and N for various values of SF /B. Refer
The force coefficient CDgwere developed for structure with a vertical aspect ratio
(Height/Width, perpendicular to the wind flow direction) = 4.
The coefficient CDgfrom the graphs will be slightly conservative for relatively
shorter structures and slightly non-conservative for relatively taller structures. The force coefficient CDgis applicable for frames of typical s harp edged steel shapes
such as wide flanged shapes, channels and angles.
The graphs are available for SF / B varying from 0.10 to 0.50 and
varying from0.10 to 0.35. (Linear interpolation may be used for values of SF /B not given in the
graphs.)
For the range of values falling outside the graphs, the force coefficient
C
f can be obtained directly from the following equations. (Reference Structural Engineering practice 000 215 1215 Wind Load Calculation)For N = 2 to 4 Cf = 1.8 + 1.4 N - (1.0 + 1.2 N)0.45 -0.06
For N = 5 to 7 Cf = 3.0 + 1.2 N - (1.2 + 1.2 N)0.45 -0.02 (N-1)
Where= SF/ B.
(i) Expressions above are based on data for 0.10 0.50 from ASCE Wind Load on Petrochemical Facilitiesand for SF / B = 1.0 with N = 2
from ASCE 7-95.
(ii) They also agree well with test data reported by Whitbread for parallel trusses normal to wind. His data are for 2N5 and
0.5SF/ B 4.0.
(iii) For smaller solidity ratios, neglect shielding and use Cf = 2.0 for each
member in each frame. For larger solidity ratios, use these expressions with caution
Area of Frames
For structures with frames of equal solidity, the effective solid area should be taken as the solid area of windward f rame.
Where the solid area of windward frame exceeds the solid area of the other frames, As = Solid area of windward frame.
If the solid area of windward frame is less than the solid area of the other frames, As = Average of the solid area of all the frames.
Pipe Racks and Cable Tray Racks
Pipe racks or cable tray racks are s pecialized open equipment structures whose principal function is to support horizontal runs of piping, cable trays, or both. Calculate wind forces on the structure as described above -- wind forces on piping and trays are calculated separately.
If the rack is significantly longer than its width, only wind force in the transverse direction of the rack need be considered. Wind load in the longitudinal direction may
not be critical. For short r acks with small pipe anchor loads, effects of longitudinal wind force shall be evaluated.
Pipes
The ASCE Task Committee on Wind Loads for Petrochemical Facilities recommends that, for piperack and other similar structures, the tributary area for pipi ng should be based on the diameter of the largest pipe, plus 10% of the width of the piperack. This sum is multiplied by the length of pipes (bent spacing) to determine the tributary area.
The force coefficient Cf = 0.70 should be used as a minimum. The force coefficient
can also be obtained from figure B-22 of NBCC Commentary B. The value of Cg= 2.0 can be adopted as per the simple procedure.
Cable Trays
The ASCE Task Committee on Wind Loads for Petrochemical Facilities recommends that the tributary area for Cable trays should be based on the height of the largest tray, plus 10% of the width of the piperack. This sum is multiplied by the length of the tray (bent spacing) to determine t he tributary area.
For cable trays the Force coefficient Cf = 2.0 should be used. The force coeffi cient
can also be obtained from figure B-17 of NB CC Commentary B for
L
10
h
(or applicable) for walls above ground. Use Cg= 2.0 as per the si mple procedure.
Note:
The above is applicable for the cable tray configuration as shown in Figure 1 of Attachment 02. For configurations shown in Figures 2 and 3 of the same attachment, estimation of the tributary area by the above approach will yield very conservative values.
Based on common practice, for configurations as shown in Figures 2 and 3 of Attachment 02, the following is recommended.
(i) The top cable tray layer shall be considered separate and as defined for Figure 1 above.
(ii) The remaining portion of the cable tray rack s hall be considered as a frame, with or without the shielding effect. If shielding is considered, the frame spacing shall be W/2 and W for Figure 2 and 3 respectively.
Shielding of equipment
For wind load on equipment inside structures, fol low the recommendation of ASCE ‘Wind Load on Petrochemical Facilities’ as detailed in practice 000.215.1215 Wind Load Calculation
LOAD COMBINATIONS
000.215.00910, Structural Engineering Criteria, unless applicable local codes or Client requires otherwise.
Enclosed Structures
For wind loads on enclosed structures, use ful l and partial loading as described in NBCC - 1995
Open Equipment Structures
For wind loads on open equipment structures, follow the recommendation of ASCE ‘Wind Load on Petrochemical Facilities’ as detailed in practice 000.215.1215 Wind Load Calculation.
OTHER CONSIDERATIONS Drift Control
PIP STC 01015 addresses allowable drift limits for structures in petrochemical facilities, and provides for the following limits:
For pipe racks Height / 150
For process structures, pre-engineered metal buildings, and personnel access
platforms Height / 200
For structures with bridge cranes The smaller of 2 inches or Height / 200 For occupied buildings which may be damaged by excessive drift Height / 400 Also refer Appendix I of CAN/C SA-S16.1-94.
Overturning Stability
Follow 000.215.1215 Wind Load Calculation. Shielding
No reduction in wind loads shall be made for the shielding effects of vessels or structures adjacent to the one being designed. NBCC - 1995 does not permit consideration of possible shielding of one building or structure by another unless verified by tests.
REFERENCES
ASCE (American Society of Civil Engineers). Wind Loads on Petrochemical Facilities. New York, 1997.
ASME (American Society of Mechanical Engineers) STS-1-2000. Steel Stacks. PIP (Process Industry Practices) STC 01015. Structural Design Criteria. Austin, TX, 1998.
NBCC (National Building Code of Canada) Commentary B Wind Loads Structural Engineering Practice 000 215 1215 Wind Load Calculation.
ATTACHMENTS
Attachment 01: XXXXX03
Typical Arrangement of Frames for calculation of force coefficient, as per ASCE Task Committee on Wind Load for Petrochemical Facili ties, considering shielding. Attachment 02: XXXXX03
Arrangement of Cable Trays Attachment 03: XXXXX03
Typical Arrangement of Frames for Calculation of Force Coefficient, as Per ASCE Task
Committee on Wind Load for Petrochemical Facilities, Considering Shielding
SF
B
SF SF
Number of Frames, N
Plan View of Framing
Nominal Wind Direction
