AS/NZS 1170.2:2002 Digest

This Digest of the Joint Australian/New Zealand Standard AS/NZS 1170.2:2002 contains cue cards for solving the procedures and formulas of the code. If you are truly interested, you will appreciate my short style - you will think between the lines. If you are not, no amount of verbosity on my side should make any difference.

The lines written in bold courier shall be processed as appropriate. Lines marked with **** are for data input, where you may change the given default values. The word "process" means to select (highlight) the relevant text and click the spinning globe in the Engineers' Compendium window. To work with the cue card, you need MATHSERV, which you may download here.

You may also evaluate wind actions online by clicking here.

This Digest or the Online Program should NOT be referenced without having access to the code. They are NOT a substitute for the code but are my personal interpretation of the code. Clause and Table numbers refer to AS/NZS 1170.2:2002 unless otherwise noted.

Procedure For Determining Wind Actions

  1. determine the design wind speeds from Section 3 - Regional Wind Speeds, and Section 4 - Site Exposure Multipliers;
     
  2. determine design wind pressures from Section 5 - Aerodynamic Shape Factors, and Section 6 - Dynamic Response Factors;
     
  3. calculate wind actions from Section 2 - Calculation of Wind Actions, being the vector sum of the forces calculated from the design wind pressures applied to surface areas or structural elements.

Regional Wind Speeds, VR (Clause 3.2)

Regional wind speeds are based on annual maximum 3-second gust speeds, corrected for site exposure to 10 m high in terrain category 2, allowing for a range of 'annual probability of exceedance' as required by ISO 2394:1998 - General principles on reliability for structures.
R is the inverse of annual probability of exceedance of the wind speed, i.e. R approximates the average recurrence interval in years. Select from the following range the values that are appropriate for serviceability and ultimate limit state in your given situation and process the next two lines: R = {5, 10, 20, 50, 100, 200, 500, 1000, 2000} 

**** Serviceability, R[1] = 20 
**** Ultimate limit state, R[2] = 1000

Australia and New Zealand are divided into non-cyclonic regions A, W and B, and cyclonic regions C and D. The following regional wind speeds have been observed for non-cyclonic regions (process only one of the following three equations, as appropriate): 

Region A, VR = round(67 - 41*R^-0.1)
Region W, VR = round(104 - 70*R^-0.045)
Region B, VR = round(106 - 92*R^-0.1)

For cyclonic regions C and D, process separately for serviceability wind speeds and ultimate limit states wind speeds.

Region C, serviceability, VR[1] = round(122 - 104*R[1]^-0.1)
          ultimate, VR[2] = round(1.05*(122 - 104*R[2]^-0.1))
           
Region D, serviceability, VR[1] = round(156 - 142*R[1]^-0.1)
          ultimate, VR[2] = round(1.1*(156 - 142*R[2]^-0.1))

Wind direction multiplier, MD (Clause 3.3)

In Australia and New Zealand for regions B, C and D, the wind direction multiplier is 0.95 for determining resultant forces and overturning moments on complete buildings, and 1.0 for all other cases, including cladding.

For regions A and W, smaller multipliers (as low as 0.8) are permitted on some of eight orthogonal directions, however, using 1.0 for any direction is on the safe side.
 

Terrain and height multiplier, MZC (Clause 4.2)

Terrain can be divided into categories according to the roughness length, Z0, of the terrain in the near and far vicinity of the structure.

"Roughness length" is defined as the theoretical quantification of the turbulence-inducing nature of a particular type of terrain on airflow. It is expressed in m and is 0.002 for terrain category 1 ("TC1"), 0.02 for TC2, 0.2 for TC3 and 2.0 for TC4. The roughness length of intermediate terrain categories can be calculated from: 

Z0 = 2*10^(TC - 4)

For example, TC2.5 would have a roughness length of Z0 = 2*10^-1.5 := 0.06325

TC1 (Z0=0.002) comprises open terrain with few or no obstructions (flat snow fields, sandy deserts, etc.) and water surfaces at serviceability wind speeds.

TC2 (Z0=0.02) comprises water surfaces, open terrain (such as airports), grasslands with few, well scattered obstructions of heights from 1.5 to 10 m.

TC3 (Z0=0.2) is terrain with numerous closely spaced obstructions 3 to 5 m high such as areas of suburban housing. Developing urban areas that comprise a mix of cleared, unoccupied land interspersed with new housing would be between TC2 and TC3 depending on the degree of development (for example TC2.5 Z0=0.063).

TC4 (Z0=2.0) is terrain with numerous large, high (10 to 30 m), and closely spaced obstructions such as large city centres and well-developed industrial complexes.

Intermediate roughness lengths are: forests (1.0=TC3.7), high density metropolitan areas (0.8=TC3.6), small town centres (0.4=TC3.3), level wooded country (0.2=TC3), few trees and long grass (0.06=TC2.5), crops (0.04=TC2.3), isolated tress and short grass (0.02=TC2), large areas of cut grass such as golf courses (0.008=TC1.6), and stony deserts (0.005=TC1.4).

If there is a variation in terrain within the "averaging distance" upwind of the structure, then the terrain and height multiplier is a weighted average. The averaging distance (AD) is 1000 m for structures less than 50 m high, 2000 m for 50 to 100 m high, 3000 m for 100 to 200 m high, and 4000 m for structures more than 200 m high.

The value is weighted by allowing for the lag distance, XI, at each TC change, where:
Z0R = the larger of the two roughness lengths at a boundary between roughnesses;
Z = the level at which the multiplier applies. 

**** your applicable array Z0R = {2.0, 0.2} 
**** your applicable Z = 50
then calculate array XI = Z0R*(Z/(0.3*Z0R))^1.25
weighted average of multipliers

Multipliers MZC are given in Tables 4.1(A) and 4.1(B), based on rigorous analysis. The following expressions are good approximations to the values in Tables 4.1(A) and 4.1(B) for values between 10 m and 100 m, where Z = the level at which the multiplier applies = reference height above average local ground level. 

**** set array of Z = {10,15,20,30,40,50,75,100}
process only one of the following equations as appropriate:
Table 4.1(A), TC2: MZC = 0.1*log(Z) + 0.0001*Z + 0.77
Table 4.1(A), TC3: MZC = 0.125*log(Z) + 0.00025*Z + 0.55
Table 4.1(B), TC1 and 2: MZC = 0.15*log(Z) + 0.0003*Z + 0.67
Table 4.1(B), TC 3 and 4: MZC = 0.2*log(Z) + 0.0007*Z + 0.43

For values outside the range of these formulas please refer to the standard. If averaging applies, process the following expressions with values obtained above and relevant to your given situation:

**** averaging distance AD = 2000 
**** array of multipliers, MZ = { }
**** corresponding distances, D = { }
adjusted distances, X[1] = D[1] - XI[1]
X[2] = D[2] + XI[1] - XI[2]
X[3] = AD - D[1] - D[2] + XI[2]
MZC = sum(MZ*X)/AD

Shielding multiplier, MS (Clause 4.3)

The shielding multiplier shall be 1.0 where the average upwind ground gradient is greater than 0.2 or where shielding effects are ignored, otherwise it is a function of the shielding parameter as given in Table 4.3 of the code.

The shielding parameter, S, is determined as follows:

**** height of structure being shielded, HT = 50
**** average height of shielding buildings > HT, HS = 60
radius of sector containing shielding buildings, R = 20*HT := 1000
**** number of shielding buildings within 45º x R, NS = 4
**** average breadth of shielding buildings, BS = 30
average spacing parameter, LS = H*(10/NS+5) := 375.
shielding parameter S = LS/sqrt(HS*BS) := 8.8388
interpolate Table 4.3, MS = .9 + .1*(S-6)/6 := 0.9473

Topographic multiplier, MT (Clause 4.4)

To determine the topographic multiplier, MT, you must know some specific details about the site as follows: 

**** height of the hill, ridge or escarpment being tested, H = 
**** horizontal distance upwind to half height below crest, LU = 
**** distance of structure from crest, upwind or downwind, X = 
**** reference height of structure above local ground level, Z = 
then length scale for vertical variation, L1 = max(.36*LU, .4*H)
length scale for horizontal variation, upwind, L2U = 4*L1
ditto, downwind if escarpment, L2D = 10*L1

Different rules apply for sites in New Zealand and Tasmania over 500 m above sea level, where
MT = MH*MLEE*(1 + 0.00015*E). Elsewhere, MT is the larger of MH and MLEE. MLEE is 1.0 except in New Zealand lee zones. Lee zones extend 30 km from the leeward crest of the initiating range. For the first 12 km, MLEE is 1.35, then it changes linearly to 1.0 at 30 km.

Where X=0 and Z=0, MH is a function of the upwind slope, H/(2*LU), given in Table 4.4. The following procedure applies: 

slope TMP = H/(2*LU)
if TMP < 0.05 then
  MH = 1.0
elseif TMP > 0.45 then
  MHU = 1 + .71*(1-X/L2U)
  MHD = 1 + .71*(1-X/L2D)
else
  MHU = 1 + (H/(3.5*(Z+L1))*(1-X/L2U)
  MHD = 1 + (H/(3.5*(Z+L1))*(1-X/L2D)
end if

The procedure calculates both MHU, which is MH for upwind X in all cases and downwind X for hills and ridges, and MHD, which is MH for downwind X for escarpments. You will know from your given situation which of the values applies. In the majority of cases, MT = MH as calculated above. For sites in New Zealand and Tasmania that are over 500 m above mean sea level, and for sites that are in lee zones, the following additional calculations are required: 

**** select the appropriate MH = 
**** site elevation above sea level, E = 
**** applicable MLEE = 
(either 1.0 or 1.35 or linear interpolation)
then MT = MH*MLEE*(1+0.00015*E)
alternatively, MT = max(MLEE, MH)

Now that all required values are in the MATHSERV data base, we can summarize Step 1 - Design Wind Speeds in m/sec: 

Serviceability wind speed, VS = VR[1]*MZC*MS*MT
Ultimate limit state wind speed, VU = VR[2]*MZC*MS*MT
Ultimate overturning wind speed, VO = VR[2]*.95*MZC*MS*MT

Step 2 - Determine Design Wind Pressures

This is where AS/NZS 1170.2:2002 excels over previous versions of wind loading codes. It allows the evaluation of aerodynamic shape factors for:

  1. enclosed rectangular buildings;
  2. circular bins, silos and tanks;
  3. freestanding walls, hoardings, canopies and roofs;
  4. exposed structural members, frames and lattice towers;
  5. flags and circular shapes;

Please note that the above links take you to the online version of this service. To continue with your understanding of the code, please don't click the links.

(a) Enclosed Rectangular Buildings

To encapsulate the provisions of Sections 5 in this digest is not practical and may, in any case, breach copyright. Therefore, please refer to the online program (opens in a new window).

The principle of Section 5 is to determine internal and external pressure coefficients for several load zones, and coefficients for frictional drag. The coefficients are then multiplied with factors for various effects. You end up with aerodynamic shape factors, CFIG, for a large number of load zones and load types, which must be combined for worst effect on the structure.

Section 6 applies only if the natural first mode fundamental frequency of the structure is less than 1 Hertz. An approximate method for calculating the first mode natural frequency (N) of a rectangular plan multistorey building is given as follows:

**** structure height in m, H = 
first mode natural frequency in Hertz, N = 46/H

The code does not cover dynamic response when the frequency is less than 0.2 Hertz, which indicates a 230 m cutoff for structure height. If your structure height is between 46 and 230 m, click here for Section 6 cue card (opens in separate window).

To be continued - please be patient.
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