WindRoof Calculator on Internet (WindRCI)rci-online.org/wp-content/uploads/2008-cts-proceedings... · · 2016-10-13interpretations of code language. Calculation of cover wind uplift

shy

WindshyRoof Calculator on Internet (WindshyRCI) Feel the Wind on Your Laptop

A Baskaran PhD PEng S Molleti PhD H Yes MEng

National Research Council Canada Ottawa Ontario Canada

Proceedings of the RCI 23rd International Convention Baskaran Molleti and Yew 25

ABSTRACT Wind effect on roofs is a complex phenomenon Poor wind design is one of the

common factors in roofing failures RICOWIs wind investigation program on Hurricanes Charley and Katrina also confirmed this and challenged designers to develop proper roof wind design tools To address this issue the National Research Council of Canada developed a Web-based roof wind design calculator It is named Wind-RCI (Wind-Roof Calculator on Internet) Wind-RCI can minimize possible mis-interpretations of code language Calculation of cover wind uplift design loads is a function of various parameters such as roof type slope wind speed building height roof area building terrain building type and openings As such it involves several procedural steps Wind-RCI integrates these procedural steps into a simple Web-based calculator Demonstrating the viability and flexibility of the Wind-RCI is the objective of this paper

SPEAKER Dr ldquoBasrdquo Baskaran is a group leader and senior research officer at the National Research Council of Canada Institute for Research in Construction (NRCIRC) He has been immersed for 25 years in researching the wind effects on building envelopes through wind tunnel experiments and computer modeling He also acts as adjunct professor at the University of Ottawa Mr Baskaran is a member of RCI ASCE SPRI RICOWI ICBEST and CIB technical committees His work in the area of wind engi-neering and building envelopes has received national and international recognition He has an outstanding research record with more than 150 publications in refereed journals and conference proceedings Being a professional engineer Baskaran received his masters degree in engineering and PhD from Concordia University Montreal Canada Both research topics focused on the wind effects on buildings and earned best dissertation award from the Canadian Society of Civil Engineers

Contact Information Phone oacute 613-990-3616 E-mail oacute basbaskarannrcca

Baskaran Molleti and Yew shy 26 Proceedings of the RCI 23rd International Convention


INTRODUCTION Natural wind hazard damages

have historically been dramatic incurring loss of life and property worldwide Wind-induced roof failure is one of the major contrib-utors to insurance claims and it is rising (AAWE 1997) Recently members of the Roofing Commit-tee on Weather Issues (RICOWI) completed two major wind investi-gations projects documenting extensive roof damages (Figure 1) providing factual data and chal-lenging designers to develop prop-er roof wind design tools (RICOWI 2006 and 2007) Simi-larly the Federal Emergency Management Agency (FEMA) also published reports summarizing the observations conclusions and recommendations of the Mit-igation Assessment Team (MAT) in response to recent hurricanes [FEMA 488489 (2005) and 549 (2006)]

Wind flow around buildings creates both negative and positive fluctuations over a roofing sys-tem The wind effect on roofing and its response is dynamic Wind pressure distribution varies spa-tially over a roof and can have high suction at the corner and perimeter due to formulation of vortex and flow separations Figure 2 illustrates the wind pres-sure variation on a building roof This data represents wind tunnel measurements carried out by the National Research Council Canada (NRC) in the 30 x 30 ft (9 x 9) wind tunnel These tests used full-scale roofing components [10 x 10 ft (3 x 3 m) in size] at differ-ent building heights for wind directions perpendicular to the building face (normal wind) and at

45 degrees (oblique wind) A PVC roofing system was tested with pressure taps fitted in the PVC single-ply membrane to measure the u n s t e a d y p r e s s u r e loads on the roof (Savage et al 1996)

To calcu-late wind up-lift loads on roof coverings designers use wind standards or building codes such as the National Building Code of Canada (NBC-2005) and the American Society of Civil Engineers (ASCE 7-05) Code pro-visions are a collection of facts and knowledge based on the

Figure 1 ndash Roof damage during a high wind event

experimental data measured by testing models in wind tunnels or full-scale measurement data which are obtained from instru-mented structures field observa-tions and consensus of expert opinions

Figure 2 ndash Spatial windshyinduced pressure distribution over a roof system

Proceedings of the RCI 23rd International Convention Baskaran Molleti and Yew shy 27

Wind load calculation for roof coverings involves several proce-dural steps since the design pres-sure is a function of various para-meters such as roof type slope wind speed building height roof area building terrain building type and openings Interpreting wind standards to identify the various parameters is indeed a time-consuming process and it can often lead to misinterpreta-tion and misinterpolation of code language

Considering this complexity the RCI Foundation (RCIF) has offered a research grant to NRC to develop a roof wind design tool This tool has been named Wind-RCI (Wind - Roof Calculator on In-ternet) Wind-RCI was developed based on the National Building Code of Canada (NBC - 2005) and therefore it is applicable for all Canadian provinces and territo-ries Nevertheless this paper also presents and discusses the Na-tional Roofing Contractors Asso-ciations (NRCA) recently devel-oped wind load calculator based on ASCE 7-05 We will first de-scribe the roof covering design pressure calculation followed by the functionality of the Wind -RCI Through case studies we shall demonstrate how this tool integrates several procedural steps involved in the wind load calculation into a simple Web load calcula-tor

WINDshyRCI NBC (2005) is a model

code that sets out techni-cal provisions for the design of buildings in Canada Wind-RCI com-putes the wind design using NBC 2005s Users Guide for Structural Commentary NBC 2005 specifies that for structur-al components and cladding the design wind pressure is the algebraic sum of the external pres-

sure and internal pressure across the specific component This can be mathematically expressed as shown in Equation 1

p = Iwq(CeCgCp shy CeCgiCpi) (1)

Where

p = design pressure Iw = importance factor

q = reference velocity pressure

Cg = gust factor

Cgi = internal gust factor

Equation 1

To obtain the design pressure as per Equation 1 a six-step pro-cedure was developed by Baska-ran and Smith (2005) as follows

1 Define corner zone

2 Calculate dynamic pres-sure

3 Calculate external pres-sure component

4 Calculate internal pres-sure component

5 Calculate design pressure (Equation 1)

6 Load diagram

Table 2 ndash Capability of the WindshyRCI for parameter investigation

Ce = exposure factor

Cp = external pressure coefficient

Cpi = internal pressure coefficient

Wind-RCI integrates these procedural steps into a simple Web-based calculator Table 1 gives a quick snapshot of this Web tool for a hypothetical building located in Toronto Ontario The Wind-RCI is capable of performing calculations for various parame-ters as listed in Table 2 This includes building height ranging from low-rise to high-rise roofs with various slope configurations from low-sloped to steep-sloped and building categories both in terms of occupancies and open-ings Thus the Wind-RCI is capa-ble of providing a complete tool for determining the wind design pres-sures of a roof


Table 1 ndash Snapshot of WindshyRCI




RoofWindDesigner (WWWROOFWINDDESIGNERCOM)

Similar to Wind-RCI the RoofWindDesigner is also a Web-based roof wind load calculator RoofWindDesigner applies the American Society of Civil Engineers (ASCE) Standard No-05 ldquoMinimum Design Loads for Buildings and Other Structuresrdquo It was developed by the NRCA in cooperation with the Midwest Roofing Contractors Association (MRCA) and the Northeast Roofing Contractors Association (NERCA) Apart from computing the roof covering design pressures it also recommends the roof covering uplift resistance by multiplying the calculated design pressures with a minimum safety factor of 2 (ASTM D6630) It is worth noting that the RoofWindDesigner is lim-ited to calculating wind uplift on low-rise buildings [the mean roof height h must be less than or equal to 60 feet (h eth 60 ft)] More-over one can use the RoofWind-Designer only for low-slope config-urations (roof incline less than 75 degrees which is about 1-1212 slope)

Parametric Investigations Using WindshyRCI

To demon-strate the viability and flexibility of the Wind-RCI var-ious parameters that can influence design pressures are studied These parameters can be grouped into two segments

1 One that is pertinent to the up-coming wind oacute ef-fects of wind speed and ter-rain expo-

sure and

2 One that is pertinent to the building oacute effects of building height openings and roof slope

Using the Wind-RCI all the parametric investigations are car-ried out through case studies For discussion purposes the paper presents design pressures only for the roof-corner zones and not for the edge and field zones Based on this approach it is easier to understand the effect of the influ-encing parameters on the roof-design pressures Designers can develop a similar inference for the edge and field zones with only variation in the magnitude of the design pressure This is due to the fact that the intensity of pressure moderates as one moves away from the corner towards the field as discussed in Figure 2

Effect of Wind Speed on Wind Uplift Pressures

NBC 2005 uses mean hourly wind speed to determine the refer-ence dynamic pressures q The dynamic pressures are based on the probability of being exceeded

per year of 1 in 50 This is tabu-lated for major Canadian cities in Appendix C of the NBC (2005) It is worth mentioning that there is no wind speed map in NBC (2005) that is similar to the ASCE 7 (2005) Nevertheless one can esti-mate the wind speed for a given dynamic pressure using the fol-lowing equation

q = CV2 (2)

Where

q = reference wind pressure

C = factor which depends on atmospheric pressure and air temperature

V = wind speed

Equation 2

To investigate the effect of wind speed a 50-ft (15-m) build-ing is selected The building has an importance factor of 1 open-exposure condition and category 1 as an internal opening provi-sion Beta version of the Wind-RCI can perform calculations for the provinces of British Columbia

Figure 3 ndash Effect of wind speed on wind uplift pressures


Ontario and Quebec For each province using the Appendix C of NBC (2005) two cities are select-ed such that they represent the minimum and maximum ie qmin

and qmax dynamic pressure levels

Equation 2 was used to calculate the respective wind speeds With these assumptions Figure 3 illus-trates the effect of wind speed on the design pressures As shown in Figure 3 Harrington-Harbor in Quebec has the highest mean hourly wind speed of 89 mph (142 kmph) and therefore its roof cor-ner zone has its highest design pressures of 113 psf (54 kPa) while Armstrong in Ontario expe-riences the lowest mean hourly wind speed of 48 mph (77 kmph) and has a corner design pressure of 33 psf (16 kPa) Therefore the data clearly indicate that the roof cladding design pressures are directly proportionate to the upcoming wind speed For the benefit of US readers equivalent 3-second gust wind speeds are also calculated from the Canadian

mean hourly wind speed and included in Figure 3

Effect of Exposure on Wind Uplift Pressures

ldquoExposurerdquo refers to the ground roughness surrounding the building The exposure factor characterizes the turbulence gen-erated by the ground roughness therefore it can directly reflect

(a) Open exposure (b) Urban exposure

Figure 4 ndash Exposure types as per NBC (2005)

changes in wind speed NBC 2005 defines exposure in terms of open and rough (urban) as shown in Figure 4 Designers often face challenges in identifying the ap-propriate exposure condition dur-ing the wind load calculations Building exposure can change from the design stage to construc-tion stage when the surrounding neighborhood develops and

Figure 5 ndash Effect of exposure on wind uplift pressures


Figure 6 ndash Effect of building height on wind uplift pressures

changes the design building land-scape In those situations it may be useful for the designer to vali-date the original wind load esti-mation

Wind-RCI makes this process much easier To demonstrate the exposure effect in addition to the open country exposure calcula-tions are performed using the Wind-RCI for urban exposure keeping the building configura-tion and other assumptions the same as that previously men-tioned Figure 5 shows such cal-culated wind uplift pressures

Comparison of the data in Figure 5 clearly indicates that the building located in open-exposure conditions has higher wind-design pressure when compared to the building in urban exposure With all the parameters kept con-stant the data also indicate that the roof constructed in open-exposure condition has 45 per-cent more roof-corner design pressure in comparison to the building in urban-exposure con-dition Even though the data are

presented for the roof-corner zone the trend will be similar for the other two zones oacute namely edge and field

Effect of Building Height on Wind Uplift Pressures

NBC (2005) provides for two sets of building heights h lt= 66 ft (20 m) and h gt66 ft (20 m) To demonstrate the capabilities of the Wind-RCI two additional building heights (30 and 70 ft) are selected This can provide an indi-cation of how much the wind-uplift load changes when there is change of 20 ft (6 m) from the base configuration building which is 50 ft (15 m) as presented above Figure 6 shows the com-puted design pressures for these three buildings As per the NBC 2005 30-ft (10-m) and 50-ft (15-m) buildings are low-rise build-ings while the 70-ft (20-m) build-ing is a high-rise building The graphical plot in Figure 6 shows an increase in the design pressure as the building height increases This is due to the fact that in-crease in height will increase the

wind speed As presented in Equashytion 2 the dynamic pressure will increase by the square of the wind speed

Changes in building height also change the corner zone (icirc) shape as depicted in Figure 7

Effect of Building Openings on Wind Uplift Pressures

As discussed under Equation 1 CeCgiCpi refers to the internal

pressure component The wind design pressure is calculated based on the algebraic sum of the external- and internal-pressure components The magnitude of this internal-pressure component depends on the amount and dis-tribution of the openings in a building It is rather difficult to quantify the size and distribution of openings for a given building due to variation in construction process and components used To account for these uncertainties NBC 2005 provides ranges for the internal pressure coefficient in three categories as follows


Figure 7 ndash Comparison of corner zone ndash lowshyrise vs highshyrise building

igrave Category 1 without large or significant openings Cpi -015 to 00

igrave Category 2 significant openings that can be closed during storms Cpi -045 to 03

igrave Category 3 large open ings through which gusts are transmitted Cpi -07 to 07

With Wind-RCI one can easily modify from one category to other during wind uplift calculations Previous examples above used Category 1 for internal pressure coefficient selection Wind-RCI repeated the computations by selecting the other two categories Figure 8 shows the calculated wind uplift pressures

The building openings can influence the air intrusion (Dre-

gger 1991 Baskaran et al 2003 Molleti 2005) into a roofing assembly Buildings with large openings and roof assemblies with air permeable components can significantly increase the design pressures This relation-ship is clearly shown in Figure 8 with all the parameters kept con-stant the corner design pressure increased by 10 and 26 when a Category 1 building changed to Category 2 and 3 respectively

A buildings internal pressure characteristics can also change during its service life This can be a small variation when the HVAC requirement changes or moderate when building envelopes are refurbished Also the internal pressure can change drastically and suddenly during catastrophe event such as windowdoor fail-ure during high wind events (hur-ricanes) Baskaran et al (2007)

investigated several such failures as part of the RICOWIs wind investigation program In those scenarios a roof assembly de-signed as Category 1 can experi-ence significantly higher uplift due to sudden increase in the internal pressure component Therefore it is quite critical for the designer to understand this transition and design the roof assembly accordingly Based on the field investigation data Bas-karan et al (2007) recommends that building with high probabili-ty of envelope (such as garage doors) failures during high wind events can be designed with the Category 3 internal pressure coef-ficient value This conservative approach is a good design prac-tice for wind mitigation measures provided that glazing and doors are not designed for windborne debris


Effect of Roof Slope on Wind Uplift Pressures

B u i l d i n g aerodynamics changes when there is change in the roof slope When the u p c o m i n g streamlines of wind hit the roof the pres-sure distribu-tion on the roof surface depends upon the extent to which the roof slope modi-fies the airflow This can alter the wind pres-sure distribu-tion and magni-tude It is worth mentioning that the distribution presented in tions Wind-RCI can perform cal- differences between the Wind-RCI Figure 2 corresponds to a low culations for various roof slope and RoofWindDesigner where the

slope-roof configuration (iuml lt = configurations as listed in Table 2 later is limited to only low-slope

100) as do all the above calcula- Note that this is one of the major roof configuration To demon-

Figure 8 ndash Effect of building openings in wind uplift design pressures

Figure 9 ndash Effect of roof slope on wind uplift pressures


strate the effect of roof slope two additional roof slopes [medium (100 lt iuml lt = 300) and steep (270 lt iuml lt = 450)] are selected and calcu-lations are repeated with the Wind-RCI All other parameters are kept similar to that of those listed above

Figure 9 shows the effect of roof slope on the computed design pressures It is clear from Figure 9 that the roof slope modifies the design pressure and it is not directly proportional to the increase in the roof slope This observation is different from the observation of previous parame-ters oacute namely the exposure height and building openings had on the design pressure One can-not assume a linear relationship between roof slope and design pressure ie increasing or de-creasing the roof slope cannot necessarily increase or decrease the design pressures Alternative-ly whenever there is change in the roof slope new calculations are warranted As presented the Wind-RCI makes that process easier

CONCLUSION Wind-RCI is developed based

on the National Building Code of Canada 2005 (NBC) NBC (2005) is a model code that sets out tech-nical provisions for the design of buildings in Canada In the Wind-RCI wind design loads are com-puted using NBC 2005s Users Guide for Structural Commentary A professional engineer certified the released beta version Some of the benefits of this Web-based tool are as follows

igrave Obtaining design loads at the palm of a designers hand

igrave Minimizing the complexity of the referenced code and the involved calculations

igrave Offering designers a faster reliable tool for wind cal-culations

The design pressures calculat-ed by the Wind-RCI should be multiplied by an appropriate safe-ty factor to obtain the required system resistance Wind-RCI also has certain limitations It does not provide calculations for the topo-graphicterrain influences on a building such as a structure sit-uated on hills and escarpment Also it cannot calculate design pressures for hipped roofs for post disaster building configura-tions and for roofs with over-hangs

ACKNOWLEDGEMENTS The presented research is

being carried out for a consor-tium the Special Interest Group for Dynamic Evaluation of Roofing Systems (SIGDERS) formed from a group of partners interested in roofing design These partners include

Manufacturers Atlas Roofing Corporation Canadian General Tower Ltd Carlisle SynTec GAF Materials Corporation Firestone Building Products Co IKO Indus-tries Ltd Johns Manville Sika-Sarnafil Soprema Canada Ste-vens Roofing Tremco and Tru-fast

Building Owners Canada Post Corporation Department of Na-tional Defence Public Works and Government Services Canada

Industry Associations Cana-dian Roofing Contractors Asso-ciation Canadian Sheet Steel Building Institute National Roof-ing Contractors Association RCI Inc and Roofing Contractors Association of British Columbia

REFERENCES American Association for

Wind Engineering (AAWE) report on the ldquoWorkshop on Large-scale Testing Needs in Wind Engineeringrdquo Washington DC 1997

American Society of Civil Engineers Standard ASCE 7-2005 ldquoMinimum Design Loads For Buildings And Other Structuresrdquo

Baskaran A Molleti S and Sexton ldquoWind Uplift Per-formance of Mechanically Attached Roofing Systems with Vapor Barrierrdquo 9th Conference on Building Science and Technology Vancouver British Colum-bia Canada February 27-28 2003

Baskaran A and TL Smith ldquoA Guide for the Wind Design of Mechanically Attached Flexible Mem-brane Roofsrdquo National Re-search Council of Canada Ottawa Ontario Canada K1A 0R6 2005

Baskaran A Molleti S and Booth RJ ldquoUnderstand-ing Air Barriers in Mechan-ically Attached Low-Slope Roofing Assemblies for Wind Upliftrdquo Pp 443-450 in Proceedings of the 3rd International Building PhyshysicsScience Conference Montreal August 2006

Baskaran A Molleti S Roodvoets D Under-standing Low-slope Roofs under Hurricane Charley from Field to Practice 6th Symposium on Roofing Research and Standards Development Tampa Flor-ida December 01 2007 pp 1-25

Dregger Phil D ldquoRole of Air Retarders Deserves Closer Scrutinyrdquo Professional Roofing p 46-49 October 1991

Federal Emergency Manage-ment Agency (FEMA)-488 ldquoMitigation Assessment Team Report Hurricane Charley in Floridardquo 2005


Federal Emergency Manage-ment Agency (FEMA)-489 ldquoHurricane Ivan in Ala-bama and Florida Obser-vations Recommendations and Technical Guidancerdquo 2005

Federal Emergency Manage-ment Agency (FEMA)-549 ldquoHurricane Katrina in the Gulf Coast Mitigation As-sessment Team Report Building Performance Ob-servations Recommenda-tions and Technical Guid-ancerdquo 2006

Molleti S ldquoPerformance Evaluation of Mechanically Attached Roofing Systemsrdquo PhD thesis University of Ottawa Ottawa Canada 2006

National Roofing Contractors Association ldquoHurricane Charley A Preliminary Re-portrdquo Professional Roofing NRCA October 2004

NRC (National Research Council) 2005 National Building Code of Canada Part 5 Ottawa National Research Council of Cana-da Ottawa Ontario Cana-da K1A 0R6

Roofing Industry Committee on Weather Issues Inc Hurricanes Charley and Ivan Wind Investigation Report Powder Springs GA 2006

Roofing Industry Committee on Weather Issues Inc Hurricane Katrina Wind Investigation Report Pow-der Springs GA pp 183 August 01 2007

Savage MG Baskaran A Cooper KR and Lei W ldquoPressure Distribution Da-ta Measured During the November 1994 Wind Tun-nel Tests on a Mechanically Attached PVC Single-Ply Roofing Systemrdquo IAR Re-port LTR-A-003 National Research Council Canada 1996


ABSTRACT Wind effect on roofs is a complex phenomenon Poor wind design is one of the

common factors in roofing failures RICOWIs wind investigation program on Hurricanes Charley and Katrina also confirmed this and challenged designers to develop proper roof wind design tools To address this issue the National Research Council of Canada developed a Web-based roof wind design calculator It is named Wind-RCI (Wind-Roof Calculator on Internet) Wind-RCI can minimize possible mis-interpretations of code language Calculation of cover wind uplift design loads is a function of various parameters such as roof type slope wind speed building height roof area building terrain building type and openings As such it involves several procedural steps Wind-RCI integrates these procedural steps into a simple Web-based calculator Demonstrating the viability and flexibility of the Wind-RCI is the objective of this paper

SPEAKER Dr ldquoBasrdquo Baskaran is a group leader and senior research officer at the National Research Council of Canada Institute for Research in Construction (NRCIRC) He has been immersed for 25 years in researching the wind effects on building envelopes through wind tunnel experiments and computer modeling He also acts as adjunct professor at the University of Ottawa Mr Baskaran is a member of RCI ASCE SPRI RICOWI ICBEST and CIB technical committees His work in the area of wind engi-neering and building envelopes has received national and international recognition He has an outstanding research record with more than 150 publications in refereed journals and conference proceedings Being a professional engineer Baskaran received his masters degree in engineering and PhD from Concordia University Montreal Canada Both research topics focused on the wind effects on buildings and earned best dissertation award from the Canadian Society of Civil Engineers

Contact Information Phone oacute 613-990-3616 E-mail oacute basbaskarannrcca


















Where



Cg = gust factor


Equation 1







6 Load diagram
















sure and






q = CV2 (2)

Where



V = wind speed

Equation 2





























































































Where



Cg = gust factor


Equation 1







6 Load diagram
















sure and






q = CV2 (2)

Where



V = wind speed

Equation 2



















































































Where



Cg = gust factor


Equation 1







6 Load diagram
















sure and






q = CV2 (2)

Where



V = wind speed

Equation 2






















































































sure and






q = CV2 (2)

Where



V = wind speed

Equation 2




















































































sure and






q = CV2 (2)

Where



V = wind speed

Equation 2


















































































sure and






q = CV2 (2)

Where



V = wind speed

Equation 2





















































































































































































































































































































































Documents

WindRoof Calculator on Internet (WindRCI)rci-online.org/wp-content/uploads/2008-cts-proceedings... · · 2016-10-13interpretations of code language. Calculation of cover wind uplift