ICS 91.080 
 
Reference number  
DRS 114-2: 2020  
© RSB 2020  
  
 RWANDA 
STANDARD  DRS  
114-2  
 
Second edition  
2020 -mm-dd 
 
Structural design  —  
Part 2 : Actions on structures — wind 
actions  
 
 DRS 114-2: 2020  
©RSB 2020  - All rights reserved                  ii 
 In order to match with technological development and to keep continuous progress in industries, 
standards are subject to perio dic review. Users shall ascertain that they are in possession of the latest 
edition  
 
 
 
 
 
 
 
 
© RSB 2020  
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized 
in any form or by any means, electronic or me chanical, including photocopying and microfilm, without 
prior written permission from R SB. 
Requests for permission to reproduce this document should be addressed to : 
Rwanda Standards  Board  
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E-mail: info@rsb.gov.rw  
Website: www.rsb.gov.rw  
ePortal: www.portal.rsb.gov.rw   DRS 114-2: 2020  
iii                ©RSB 2020  - All rights reserved  
 Contents  Page 
Foreword  ................................ ................................ ................................ ................................ ............................  iv 
1 Scope  ................................ ................................ ................................ ................................ ......................  1 
2 Normative references  ................................ ................................ ................................ ............................  1 
3 Terms and definitions  ................................ ................................ ................................ ...........................  1 
4 Symbols (and abbreviated terms)  ................................ ................................ ................................ ........  2 
5 Design situations  ................................ ................................ ................................ ................................ ... 5 
6 Clause Modelling of wind actions  ................................ ................................ ................................ ........  6 
6.1 Nature  ................................ ................................ ................................ ................................ .....................  6 
6.2 Representations of wind actions  ................................ ................................ ................................ .........  6 
6.3 Classification of wind actions  ................................ ................................ ................................ ..............  6 
6.4 Characteristic values  ................................ ................................ ................................ ............................  7 
6.5 Models  ................................ ................................ ................................ ................................ ....................  7 
7 Wind velocity and velocity pressure  ................................ ................................ ................................ ... 7 
7.1 Basis for calculation  ................................ ................................ ................................ .............................  7 
7.2 Basic Val ues ................................ ................................ ................................ ................................ ..........  7 
7.3 Mean wind  ................................ ................................ ................................ ................................ ..............  8 
7.3.1  Variation with height  ................................ ................................ ................................ .............................  8 
7.3.2  Terrain roug hness  ................................ ................................ ................................ ................................ . 9 
7.3.3  Terrain topography  ................................ ................................ ................................ ..............................  11 
7.3.4  Numerical calculation of topography coefficients  ................................ ................................ ...........  11 
7.4 Wind turbulence  ................................ ................................ ................................ ................................ .. 12 
7.5 Peak velocity pressure ................................ ................................ ................................ ........................  13 
8 Wind actions  ................................ ................................ ................................ ................................ ........  14 
8.1 General  ................................ ................................ ................................ ................................ .................  14 
8.2 Wind zones in Rwanda ................................ ................................ ................................ ........................  15 
8.3 Wind zones classification  ................................ ................................ ................................ ...................  15 
8.4 Calculation procedures for the determination of wind actions  ................................ ......................  15 
8.5 Wind pressure on surfaces  ................................ ................................ ................................ ................  16 
8.6 Wind forces  ................................ ................................ ................................ ................................ ..........  16 
9 Structural factor  ................................ ................................ ................................ ................................ ... 19 
9.1 General  ................................ ................................ ................................ ................................ .................  19 
9.2 Determina tion of structural factor  ................................ ................................ ................................ ..... 19 
9.3 Importance factors  ................................ ................................ ................................ ..............................  19 
10 Simplified method to calculate wind forces on structures in Rwanda  ................................ ..........  20 
Annex  A (normative)  Wind Zones and areas affected by the regional wind velocity  ..............................  22 
 DRS 114-2: 2020  
©RSB 2020  - All rights reserved                  iv 
 Foreword   
Rwanda Standard s are prepared  by Technical Committees and approved by R wanda Standards Board (RSB) 
Board  of Directors in  accordance with the procedures of RSB, in compliance with Annex 3 of the WTO/TBT 
agreement on the preparation, adoption and application of standards.  
The main task of technical committees is to prepare natio nal standards.  Final Draft Rwanda S tandards 
adopted by Technical committ ees are ratified  by me mbers of RSB Board of Director s for publication and 
gazettment as Rwanda Standards.  
DRS 114-1 was prepared by Technical Committee RSB/TC 9, Civil engineering and building materials  
In the preparation of this standard, reference was made to the following standard s: 
1) ISO 4354: 1997, Wind actions on structures  
2) BS EN 1991 -1-4: 2005, Actions on structures — Part 1 -4, General actions — Wind actions  
The assistance de rived from the above source is hereby acknowledged with thanks.  
This second  edition  cancels and replaces the first  edition (RS  114-1: 2011 ), of which ha s been technically 
revised.  
DRS  114 consists of the following parts, under the general title Structural desig n —: 
 Part 1: Actions on structures — Densities, self -weight, imposed loads for buildings  
 Part 2: Actions on structures — Wind actions  
Part 3: Thermal actions on Building  
Committee membership  
The following organizations were represented on the Technical Committee on Civil engineering and building 
materials (RSB/TC 9) in the preparation of this standard.  
University of Rwanda – College of  Science and Technology (UR -CST)  
Institut d’ Enseignement Supérieur ( INES - Ruhengeri ) 
City of Kigali  
Green Effect Engineering  DRS 114-2: 2020  
v                ©RSB 2020  - All rights reserved  
 Integrated Polytechnic Regional Centre –Kigali (IPRC)  
NPD Ltd  
REAL Contractors Ltd  
Rwanda Housing Authority (RHA)  
Rwanda Transport  Development Agency (RTDA)  
Standard For Sustainability (SFS)  
Gasabo 3D  
Masss Design Group - Rwanda  
NPD Ltd  
Enterpris es-Generale -des-Constrictions - EGC  
TECOS Ltd  
Institution of Engineers Rwanda  
CIMERWA Ltd  
Rwanda Standards Board (RSB) – Secretariat  DRS 114-2: 2020  
©RSB 2020  - All rights reserved                  vi 
 Introdu ction   
Buildings and their components are to be designed to withstand wind loads. Calculating wind loads is  
important in design of the wind forces including structural members, components, and cladding, against shear,  
sliding, overturning, and uplift actio ns. 
Certain aspects necessary to determine wind actions on a structure are mainly dependent on the location and  
on the availability and quality of meteorological data and the type of terrain. Wind forces may fluctuate with  
time, but for most of the structu res, dynamic effect is small. Therefore, the wind load is treated as lateral static  
loads. However for tall and slender structures such as the high rise buildings, a dynamic structural wind  
analysis is required.  DRS 114-2: 2020  
1                ©RSB 2020  - All rights reserved  
 Structural design  — Part 2: Actions on stru ctures — Wind actions  
1 Scope   
This Draft Rwanda Standard describes the actions of wind on structures and specifies methods for calculating  
characteristic values of wind loads for use in designing buildings and civil engineering works for each of the  
loaded areas under consideration. This includes the whole structure or parts of the structure or elements  
attached to the structure.   
2 Normative references   
The following documents are referred to in the text in such a way that some or all of  their content constit utes 
requirements of this document.  For dated references, only the edition  cited applies. For undated references, 
the latest edition of the referenced document (including any amendments) applies.  
RS 112 , Basis of structural design  
3 Terms and definitions  
For the purposes of this document, the following terms and definitions apply.  
3.1 
fundamental basic wind velocity  
hourly mean wind velocity with an annual risk of being exceeded of 0, 02, irrespective of wind direction, at a 
height of 10 m above flat open ter rain and accounting for altitude effects (if required)  
3.2 
basic wind velocity  
fundamental basic wind velocity modified to account for the direction of the wind being considered and the 
season (if required)  
3.3 
mean wind velocity  
basic wind velocity modifi ed to account for the effect of terrain roughness and topography  
3.4 
pressure coefficient  
external pressure coefficients give the effect of the wind on the external surfaces of buildings; internal 
pressure coefficients give the effect of the wind on the in ternal surfaces of buildings.   
©RSB 2020  - All rights reserved                  2 
 The external pressure coefficients are divided into overall coefficients and local coefficients. Local coefficients 
give the pressure coefficients for loaded areas of 1 m2 or less e.g. for the design of small elements and 
fixings; overall coefficients give the pressure coefficients for loaded areas larger than 10 m2.  
Net pressure coefficients give the resulting effect of the wind on a structure, structural element or component 
per unit area.  
3.5 
external pressure  
pressure actin g on an external surface of a building caused by the direct action of the wind  
3.6 
internal pressure  
pressure acting on an internal surface of a building caused by the action of the external pressures through 
porosity and openings in the external surfaces of the building  
3.7 
net pressure  
pressure difference between opposite faces of a surface  
3.8 
building height  
height of a building or part of a building above its base  
3.9 
reference height  
reference height for a part of a structure is the datum height above  ground for the pressure coefficients and is 
defined with the pressure coefficients for that part  
3.10 
force coefficient  
force coefficients give the overall effect of the wind on a structure, structural element or component as a 
whole, including friction, if not specifically excluded  
4 Symbols (and abbreviated terms)   
For the purposes of this Rwanda standard, the following symbols apply.  
NOTE :   In this Part the symbol dot in expressions indicates the multiplication sign.  DRS 114-2: 2020  
3                ©RSB 2020  - All rights reserved  
 A                        area 
Afr                       area swept by the wind 
Aref                      reference area  
Ffr                       resultant friction force  
Fw                      resultant wind force  
H                        height of a topographic feature  
Iv                        turbulence intensity  
K                        mode shape factor; shape parameter  
Lu                      actual length of an upwind slope  
b                        width of the structure (the length of the surface perpendicular to the win d direction if not  
      otherwise specified)  
cd                        dynamic factor  
cdir                      directional factor  
ce(z)                   exposure factor  
cf                         force coefficient  
cfr                                     friction c oefficient  
cp                        pressure coefficient  
cprob                               probability factor  
cr                        roughness factor  
co                       topography factor  
cs                       size factor  
cseason                  seasonal factor  
d                        depth of the structure (the length of the surface parallel to the wind direction if not otherwise  
             specified)   
©RSB 2020  - All rights reserved                  4 
 e                        eccentricity of a force or edge distance  
h                        height of the structure  
k                         equivalent roughness  
kp                       peak factor  
kr                        terrain factor  
l                          length of a horizontal structure  
m                        mass per unit lengt h 
m1                      equivalent mass per unit length  
p                        annual probability of exceeda nce 
qb                       reference mean (basic) velocity pressure  
qp                                    peak velocity pressure  
t                         averaging time of the reference wind speed, plate thickness  
vm                       mean wind velocity  
vb,0                      fundamental value of the basic wind velocity  
vb                        basic wind velocity  
w                        wind pressure  
x                         horizontal distance of the site from the top of a crest  
z0                                     roughness length  
ze, zi                   reference height for external wind action, internal pressure  
zg                        distance from the gro und to the considered component  
zmax                    maximum height  
zmin                     minimum height  
zs                        reference height for determining the structural factor  DRS 114-2: 2020  
5                ©RSB 2020  - All rights reserved  
 Φupwind  slope  
ρair density  
σv                       standard deviation of the turbulence  
x-direction          horizontal direc tion, perpendicular to the span  
y-direction          horizontal direction along the span  
z                         height above ground  
z-direction          vertical direction  
Indices  
crit                      critical  
e                         external ; exposure  
fr                        friction  
i                          internal  
m                        mean  
p                         peak  
ref                       reference  
v                         wind velocity  
x                         along - wind direction  
y                         cross -wind direction  
z                         vertical direction  
 
5 Design si tuations  
5.1 The relevant design situations shall be selected taking into account the circumstances under which the 
structure is required to fulfill its function.   
©RSB 2020  - All rights reserved                  6 
 5.2 Design situations shall be classified as follows:  
a) persistent design situations, which ref er to the conditions of normal use;  
b) transient design situations, which refer to temporary conditions applicable to the structure, e.g .   
during execution or repair;  
c) accidental design situations, which refer to exceptional conditions applicable to the struc ture or to  its 
exposure, e.g. to fire, explosion, impact or the consequences of localized failure; and  
d) seismic design situations, which refer to conditions applicable to the structure when subjected to 
 seismic events.  
5.3 The selected design situations sh all be sufficiently severe and varied so as to encompass all conditions 
that can reasonably be foreseen to occur during the execution and use of the structure.  
5.4 Where in design windows and doors are assumed to be shut under storm conditions, the effect of these 
being open should be treated as an accidental design situation.  
5.5 Fatigue due to the effects of wind actions should be considered for susceptible structures.  
6 Clause  Modelling of wind actions  
6.1 Nature  
Wind actions fluctuate with time and act direct ly as pressures on the external surfaces of enclosed structures 
and, because of porosity of the external surface, also act indirectly on the internal surfaces.  They may also act 
directly on the internal surface of open structures. Pressures act on areas of  the surface  resulting in forces 
normal to the surface of the structure or of individual cladding components.  
Additionally, when large areas of structures are swept by the wind, friction forces acting tangentially to the  
surface may be significant.  
6.2 Represe ntations of wind actions  
The wind action is represented by a simplified set of pressures or forces whose effects are equivalent to the 
extreme effects of the turbulent wind.  
6.3 Classification of wind actions  
 
Unless otherwise specified, wind actions sh all be classified as variable static actions . DRS 114-2: 2020  
7                ©RSB 2020  - All rights reserved  
 6.4 Characteristic values  
The wind actions calculated using this standard are characteristic values . They are determined from the basic 
values of wind velocity or the velocity pressure.  
The basic values are characteristic  values having annual probabilities of exceedence of 0. 02, which is 
equivalent to a mean return period of 50 years.  
NOTE  All coefficients or models, to derive wind actions from basic values, are chosen so that the probability of the 
calculated wind actions  does not exceed the probability of these basic values.  
6.5 Models  
The effect of the wind on the structure (i.e. the response of the structure), depends on the size, shape and 
dynamic properties of the structure. This Part covers dynamic response due to a long -wind turbulence in 
resonance with the along -wind vibrations of a fundamental flexural mode shape with constant sign.  
The response of structures shall be calculated according to Clause 8 from the peak velocity pressure, qp, at 
the reference height in the u ndisturbed wind field, the force and pressure coefficients and the structural factor 
cscd (see Clause  9). qp depends on the wind climate, the terrain roughness and topography, and the reference 
height. qp is equal to the mean velocity pressure plus a contr ibution from short -term pressure fluctuations.  
Aero -elastic  response sh all be considered for flexible structures such as cables, masts, chimneys and bridges.  
7 Wind velocity and velocity pressure  
7.1 Basis for calculation  
The wind velocity and the velocity press ure are composed of a mean and a fluctuating component. The mean 
wind velocity vm shall be determined from the basic wind velocity vb depends on the wind climate as described 
in 7.2, and the height variation of the wind determined from the terrain roughnes s and topography as 
described in 7.3. The peak velocity pressure is determined in 7.5.  
The fluctuating component of the wind is represented by the turbulence intensity defined in 7.4.  
7.2 Basic Values  
7.2.1  The fundamental value of the basic wind velocity, vb,0, is the characteristic ten (10) minute mean wind 
velocity, irrespective of wind direction and time of year, at 10 m above ground  level in open terrain with low 
vegetation such as grass and isolated obstacles .  
NOTE  This terrain corresponds to terrain cat egory II in Table 1. 
7.2.2 The basic wind velocity shall be calculated from Expression ( 7.1). 
vb cdir cseason  vb,0                                                                     (7.1) 
where:   
©RSB 2020  - All rights reserved                  8 
 vb      the basic wind velocity, defined as a function of wind direction and time of year at 10 m above    
      ground  of terrain category II.  
vb,0       the fundamental value of the basic wind velocity, see (1) P  
cdir       the directional factor, see Note 1.  
cseason   the season factor, see Note 2.  
NOTE 1 The recommended value of the directional factor, cdir, for various wind is 1,0.  
NOTE 2 The reco mmended value of the season factor, cseason , is 1,0.  
NOTE 3    The hourly mean wind velocity  having the probability p for an annual exceedence is determined by multiplying 
the basic wind velocity vb in 4.2 .2 by the probability factor, cprob  given by Express ion (7.2).  


K  is the shape parameter depending on the coefficient of variation of the extreme -value distribution.  
n   is the exponent.  
NOTE  The recommended value for K is 0,2 and  0,5 for n. 
7.2.2 For temporary structures and for all structures in the execution phase, the seasonal factor cseason  
may be used. For transportable structures, which may be used at any time in the year,  cseason  should be taken 
equal to 1 .0. 
7.3 Mean w ind 
7.3.1  Variation with height  
The mean wind velocity vm(z) at a height z above the terrain depends on the terrain roughness and 
topography and on the basic wind velocity, vb, and sh all be determined using Expression ( 7.3) 
vm(z) cr (z) co(z) vb                                                                            (7.3) 
where:  
cr(z)  is the roughness factor, given in 7.3.2 
co(z)  is the topography factor, taken as 1,0 unless otherwise specified (See 7 .3.4) 
NOTE 1    If the  topography is accounted for in the basic wind velocity, the recommended value is 1,0.  DRS 114-2: 2020  
9                ©RSB 2020  - All rights reserved  
 7.3.2 Terrain categories  
Terrain category I  
Lakes or area with negligible v egetation and without obstacles  
Terrain category II  
Area with low vegetation such as grass and  isolated obstacles (trees, buildings)  
Terrain category III  
Area with regular cover of vegetation or buildings or with  isolated obstacles (such as villages, subu rban 
terrain, permanent forest)  
Terrain category IV  
Area in which at least 15 % of the surface  is covered with  buildings and their average height exceeds 15 m  
7.3.2  Terrain roughness  
7.3.2. 1 Wind at ground level is subject to boundary layer effect due to  interferences from hills, trees, 
structures, etc. as well as the  friction with the ground. This terra in roughness increases  turbulence and 
decreases resulting wind pressures. Wind data  collected at airports where the  terrain roughness is minimized  
apply in the design of buildings.  
7.3.2. 2 The roughness factor, cr(z), accounts for the variability of the me an wind velocity at the site of the 
structure due to the height above ground level ,the ground roughness of the terrain upwind of the structure in 
the wind direction considered .  
NOTE  The recommended procedure for the determination of the roughness factor at height z is given by 
Expression ( 7.4) and is based on a logarithmic velocity profile.  
 
Cr(Z) = C r (Zmin)            for Z  Zmin                                                         (7.4) 
Where  
z0  is the roughness length  
k r   terrain factor depend ing on the roughness length z 0 calculated using  
                                                 (7.5)  
©RSB 2020  - All rights reserved                  10 
 Where  
z0,II   = 0,05 m (terrain category II, Table 1)  
zmin   Is the minimum height defined in Table 1  
zmax  is to be taken as 200 m  
z0, zmin  depend on the terrain category. Recommended values are given in Table 1 depending on four 
representative terrain categories.  
Expression ( 7.4) is valid when the upstream distance with uniform terrain roughness is long enough to 
stabilise the profile sufficiently, see  (2). 
Table 1 — Terrain categories and terrain parameters  
Terrain category  z0 
m zmin 
m 
I   0.01 1 
II   0.05 2 
III   0.3 5 
IV   1 10 
3.2.3  The terrain roughness to be used for a given wind direction depends on the ground roughness and 
the distance with  uniform terrain roughness in an angular sector around the wind direction. Small areas (less 
than 10% of the area under consideration) with deviating roughness may be ignored. See Figure 1  DRS 114-2: 2020  
11                ©RSB 2020  - All rights reserved  
 
 
 Figure 1: Assessment of terrain roughness  
2.2.4  When a pressure or force coefficient is defined for a nominal angular sector, the lowest roughness 
length within any 30° angular wind sector should be used.  
2.2.5  When there is choice between two or more terrain categories in the definition of a given area, then 
the area with the lowest roughness length sh all be used.  
7.3.3  Terrain topography  
Where topography (e.g. hills, cliffs etc.) increases wind velocities by more than 5% the effects sh all be taken 
into account using the topography factor cO. 
7.3.4  Numerical calculation of topogra phy coefficients  
7.3.4. 1 At isolated hills and ridges or cliffs and escarpments different wind velocities occur dependent on the 
upstream slope =H/Lu in the wind direction, where the height H and the length Lu are defined in Figure 2 . 
7.3.4. 2 The effects of topography may be neglected when the average slope of the upwind terrain is less 
than 3°. The upwind terrain may be considered up to a distance of 10 times the height of the isolated 
topographic feature.   
©RSB 2020  - All rights reserved                  12 
 
 
Figure  2 - Illustration of increase of wind vel ocities over topography  
7.3.4. 3 The topography factor, co(z)=vm/vmf  accounts for the increase of mean wind speed over isolated  hills 
and escarpments (not undulating and mountainous regions). It is related to the wind velocity at  the base of the 
hill or es carpment . 
7.4 Wind turbulence  
7.4.1 The turbulence intensity lv(z) at height z is defined as the standard deviation of the turbulence divided 
by the mean wind velocity.   
NOTE 1 The turbulent component of wind velocity has a mean value of 0 and a standard devia tion σ v. Thestandard 
deviation of the turbulence σv may be determined using Expression ( 7.6).
σv. kr vb kI                                                                             (7.6) 
For the terrain factor kr , see Expression ( 7.5), for the ba sic wind velocity vb , see Expression ( 7.1) and for 
turbulence factor kI , see Note 2.  
NOTE  2 The recommended rules for the determination of lv(z) are given in Expression ( 7.7) 
                (7.7) 
where:  
kI is the turbulence factor. The recommended valu e for kI is 1,0.  
co is the topography factor as described in 7.3.3 
z0 is the roughness length, given in Table  1 DRS 114-2: 2020  
13                ©RSB 2020  - All rights reserved  
 7.5 Peak velocity pressure  
7.5.1  The peak velocity pressure qp(z) at height z, which includes mean and short -term velocity 
fluctuations, sh all be de termined.  
 NOTE 1 The recommended rule  for the determination of  qp(z)  is given in Expression ( 7.8). 
          (7.8) 
where:  
ρis the air density, which depends on the altitude, temperature and barometric pressure to be expected in 
the region during wind  storms  
ce(z) is the exposure factor given in Expression ( 7.9) 
                                                                  (7.9)                                
qb is the basic velocity pressure given in Expression ( 7.10) 
                                                                                 (7.10) 
NOTE 2 The recommended value for ρ is 1,25 kg/m3. 
NOTE 3 For flat terrain where cO(z) = 1,0 (see 7.3.3), the exposure factor ce(z) is illustrated in Figure 2 as a function of 
height above terrain and a function of terrain category as defined in Table 1.  
The exposu re factor accounts for the variability of the velocity pressure at the site of the structure due to:  
a) the height above ground level,  
b) the roughness of the terrain, and  
c) the shape and slope of the ground contours in undulating terrain.  
The value of the exposur e factor may vary with wind direction.  
  
©RSB 2020  - All rights reserved                  14 
 
 
Figure 3 — Illustration of the exposure factor Ce (z) for Co 1.0, Kr = 1.0  
The exposure factor accounts for the variability of the velocity pressure at the site of the structure due to:  
a) the height above ground leve l, 
b) the roughness of the terrain, and  
c) the shape and slope of the ground contours in undulating terrain.  
The value of the exposure factor may vary with wind direction.  
8 Wind actions  
8.1 General  
8.1.1 Wind actions on structures and structural elements shall be det ermined taking account of both  
external and internal wind pressures.  Wind actions which shall be considered in the design of a structure may 
produce the following:  
a) excessive forces or instability in the structure or its structural members or elements;  
a) b) excessive deflection or distortion of the structure or its elements;  DRS 114-2: 2020  
15                ©RSB 2020  - All rights reserved  
 b) repeated dynamic forces causing fatigue or structural elements;  
c) aero-elastic instability, in which motion of the structure in wind produces aerodynamic forces 
augmenting the motion;  
d) excess ive dynamic movements causing concern or discom fort to occupants or onlookers.  
8.2 Wind zones in Rwanda  
Wind zones are based on peak wind velocity and assumptions, such as the assu med height of a future 
building , wind direction, and a very general topographic and sheltering information.  
This standard divides Rwanda into two wind zones (see Table 2). 
Annex A shows the map of wind zones and areas affected by the regional wind velocity.  
8.3 Wind zones classification  
8.3.1  Wind zones are classified as low, moderate, hi gh and very high according to the basic wind velocity (Vb) 
of the region . 
Low:  Below 30  m/s 
Moderate: 30m/s  Vb  43 m/s 
High: 44m/s  Vb  50 m/s 
Very high: V b 50 m/s 
Table 2  — Wind zones in  Rwanda  
 
Zone  Basic Wind velocity  Areas  affected  
I 29 m/s Central and eastern regions  ( see districts affected i n 
annex A)  
II 40 m/s Northern, western  and southern regions  ( see districts 
affected i n annex A)  
8.3.2 The wind velocity is  sensed by a spinning anemometer which magnetically induces a voltage that is  
proportional to the wind velocity . This voltage is measured and plotted against time to give a history of wind 
velocities  in a chart called an anemogram. This  output is the instantaneous wind velocity  at any given time. 
The voltage is subject to a conversi on factor specific to the  anemometer for reporting purposes . 
8.4 Calculation procedures for the determination of wind actions  
The procedures for the determination of wind actions are shown in table 4  
©RSB 2020  - All rights reserved                  16 
 8.5 Wind pressure on surfaces  
8.5.1 The wind pressure acting on the external surfaces, we, shall be obtained from Expression ( 8.1). 
we qp (ze ) cpe                                               (8.1) 
where:  
qp(ze) is the peak velocity pressure  
ze  is the reference height for the external pressure  
cpe is the pressure coefficient for the external pressure  
 
NOTE  qp(z) is defined in  7.5 
8.5.2 The wind pressure acting on the internal surfaces of a structure, wi , shall be obtained from 
Expression ( 8.2) 
wi qp (zi ) cpi                                             (8.2) 
where:  
qp(zi)  is the peak velocity pressure  
zi  is the reference hei ght for the internal pressure  
cpi  is the pressure coefficient for the internal pressure  
NOTE  qp(z) is defined in 7.5 
8.5.3 The net pressure on a wall, roof or element is the difference between the pressures on the  opposite 
surfaces taking due account of  their signs. Pressure, directed towards the surface is taken as  positive, and 
suction, directed away from the surface as negative. Examples are given in Figure 5.  
8.6 Wind forces  
8.6.1  The most common concept of wind is that it produces a force in the directi on it moves that is 
proportional to its wind velocity.  This action is referred to as downwind force.  
Force = Pressure x Projected Area  
8.6.2 The projected area is the area exposed to the wind flow normal to the wind direction. Wind from all 
directions must  be considered to determine the maximum area and maximum force.  
8.6.3 For the purpose of calculating the total wind load on a structure, the wind pressure is applied to the 
orthogonal projection of the structure onto a plane perpendicular to wind direction . 
 DRS 114-2: 2020  
17                ©RSB 2020  - All rights reserved  
 
 
Figure 5 - Pressure  on surfaces  
 
8.6.4 The wind force per unit area is assumed to act statically in a direction normal to the surface of the 
structure or element, except where otherwise specified, e.g. with tangential frictional forces. Both internal and 
external forces should be considered.  
8.6.5 For some structures it may be appropriate to represent the wind forces by their resultants. These 
resultants shall include along -wind (drag), crosswind (lift), torsional and overturning actions.  
8.6.6 The w ind forces for the whole structure or a structural component should be determined:  by 
calculating forces using force coefficients  or by calculating forces from surface pressures . 
8.6.6 The wind force Fw acting on a structure or a structural component may be determined directly by  
using Expression (8.3)  
Fw cscd cf qp (ze ) Aref                                  (8.3) 
Or by vectorial summation over the individual structural elements by using  Expression ( 8.4) 
                (8.4) 
where:  
cscd     is the structural factor as defined in Section 9 
cf         is the force coefficient for the structure or structural element, taken equal to 1for all structures.  
qp(ze) is the peak velocity pressure (defined in 7.5) at reference height ze taken equal to the total height of 
a building  
Aref      is the reference area of the structure or structural element .  
©RSB 2020  - All rights reserved                  18 
 8.6.7 The wind force, Fw acting on a structure or a structural element may be determined by vectorial  
summation of the forces Fw,e, Fw,i and Ffr calculated from the external and internal pressures using  Expressi ons 
(8.5) and ( 8.6) and the frictional forces resulting from the friction of the wind parallel to the  external surfaces, 
calculated using Expression (8.7).  
External forces:  
 
                                     (8.5) 
Internal forces  
                                                           (8.6) 
Friction forces  
                                                        (8.7) 
where:  
cscd  is the structural factor as defined in Section 9 
we     is the external pressure on the individual surface at height ze, given in Expression ( 8.1) 
wi      is the internal pressure on the individual surface at height zi, given in Expression ( 8.2) 
Aref    is the reference area of the individual surface  
Cfr     is the friction coefficient  
Afr     is the area of external surfac e parallel to the wind  
 
NOTE 1  For elements (e.g. walls, roofs), the wind force becomes equal to the difference between the  external and 
internal resulting forces.  
NOTE 2 Friction forces Ffr   act in the direction of the wind components parallel to extern al surfaces.  
8.6.8 The effects of wind friction on the surface can be disregarded when the total area of all surfaces  
parallel with (or at a small angle to) the wind is equal to or less than 4 times the total area of all  external 
surfaces perpendicular to the wind (windward and leeward).  
 8.6.8 In the summation of the wind forces acting on building structures, the lack of correlation of wind 
pressures between the windward and leeward sides may be taken into account.  DRS 114-2: 2020  
19                ©RSB 2020  - All rights reserved  
 9 Structural factor  
9.1 General  
The structural  factor cscd shall take into account the effect on wind actions from the non simultaneous 
occurrence of peak wind pressures on the surface ( cs) together with the effect of the vibrations of the structure 
due to turbulence ( cd). 
NOTE  The structural factor cscd  may be separated into a size factor cs and a dynamic factor cd, based on 9.3.  
9.2 Determination of structural factor  
cscd may be determined as follows:  
a) For buildings with a height less than 15 m ; the value of cscd may be taken as 1.  
b) For facade and roof e lements having a natural frequency greater than 5 Hz, the value of cscd may be 
 taken as 1.  
c) For framed buildings which have structural walls and which are less than 100 m high and whose  height  is 
less than 4 times the in -wind depth, the value of cscd may be taken as 1.  
d) For chimneys with circular cross -sections whose height is less than 60 m and 6,5 times the  diameter, 
 the value of cscd may be taken as 1.  
e) Alternatively, for cases a), b), c) and d) above, values of cscd may be derived from 9.3.1.  
9.3 Importance  factors  
The importance factors  account for a higher wind load on structures that would be important during an 
emergency compared to other buildings and temporary structures. These values range from 0. 87 to a 
maximum of 1. 25 (see Table 3). 
Table 3 — Import ance factors for higher wind load on structures  
 
Nature of occupancy  Category  Importance factor  
Buildings and other structures that represent a low hazard 
to human life in the event of failure. Includes agricultural 
facilities, certain temporary facilitie s and minor storage 
facilities  I 0.87 
All buildings and other structures  
except those listed in Categories I, III and  
IV. II 1.00 
Buildings and other structures that represent a substantial 
hazard to human life in the event of failure.  
Includes churches,  schools, jails and other highly III 1.15  
©RSB 2020  - All rights reserved                  20 
 occupied structures.  
Buildings and other structures designated as essential 
facilities.  
Includes hospitals, emergency  
facilities, power stations, etc.  IV 1.25 
Table 4 — Calculation procedures for the determinatio n of wind actions  
 
Parameter  Subject reference  
peak velocity pressure qp  
basic wind velocity vb  7.2 (2)P  
terrain category  Table 1  
characteristic peak velocity pressure qp  7.5 (1)  
turbulence intensity Iv  7.4 
mean wind velocity vm  7.3.1 
topograp hy coefficient co(z)  7.3.3 
roughness coefficient cr(z) 7.3.2 
Wind pressures, e.g. for cladding, fixings and structural parts   
external wind pressure: we=qp cpe 8.4(1) 
internal wind pressure: wi=qp cpi 8.4(2) 
Wind forces on structures, e.g. for over all wind effects   
structural factor: cscd  9 
wind force Fw calculated from force coefficients  8.5 (2) 
wind force Fw calculated from pressure coefficients  8.5 (3) 
 
10 Simplified method to calculate wind forces on structures in Rwanda  
The design pressure f or a given structure c an be determined considering it s geographical location, exposure 
(height and surrounding setting), and use. The exterior pressure coefficients are established based on the 
component size and location on the building. The internal pres sure coefficients are determined based on the 
potential for wind being blown into or sucked out of the building.  
Table 5 specifies the minimum wind pressures to be considered for structural design according to regional 
wind velocity.  
The force obtained sha ll be multiplied by an important factor based on the expected use of the structure (see 
Table 3). DRS 114-2: 2020  
21                ©RSB 2020  - All rights reserved  
  
 
Table 5 — Minimum design wind pressure  
 
Height (m)  W (kN/m2) 
Zone A  Zone B  
H10 0.4 0.70 
10<H15 0.55 1.00 
15<H30 0.80 1.35 
30<H50 1.05 1.60 
50<H75 1.35 2.00 
75<H100 1.65 2.50 
 
 
 
 
 
 
 
 
 
 
  
©RSB 2020  - All rights reserved                  22 
 Annex  A 
(normative)  
 
Wind Zones and areas affected by the regional wind velocity  
A wind zone refers to the wind forces that act on a building on a particular building site. The wind zone is  
mainly characterized by follo wing features :  
 The wind region the site is in   
 Exposure of the site - whether the site is sheltered or exposed   
 Topography (steepness or slope of the land) - whether the land is gentle, moderate or extreme   
Once the wind zone has been established , the des igner uses this information to determine the bracing 
requirements of the proposed building.  Figure A1(To be developed) shows the map of the two wind zones in 
Rwanda.  
 
Figure A.1  — Map of wind zones in Rwanda  DRS 114-2: 2020  
23                ©RSB 2020  - All rights reserved  
 Table A.1 - Districts affected by the regional wind velocity  
Zone I  Zone II  
Rulindo district  Rubavu district  
Gasabo District  Nyabihu district  
Nyarugenge district  Rutsiro district  
Kicukiro district  Karongi district  
Kamonyi district  Ngororero district  
Muhanga District  Nyamasheke district  
Ruhango d istrict  Rusizi district  
Nyanza district  Musanze district  
Huye district  Burera district  
Gisagara district  Gicumbi district  
Bugesera district  Gakenke district  
Rwamagana district  Nyamagabe district  
Ngoma district  Nyaruguru district  
Kirehe district   
Kayonza district   
Gatsibo district   
Nyagatare district   
 
 
 
 
 
 
 DRS 114-2: 2020  
 
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©RSB 2020 - All right s reserved