design codes comparison

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DESIGN LOADS FOR INDUSTRIAL STEEL BUILDINGS IN THE ABSENCE OF LOCAL CODES IN THEKINGDOM OF SAUDI ARABIA

Dr. Mohammed Sajid Abbasi

Zamil Steel, Dammam

ABSTRACT: This study presents a comparison of various international codes of practice that are adopted inthe design of industrial steel buildings. The prominent codes widely used in engineering practice for theloading calculations in the Kingdom are UBC 1997 (Uniform Building Code), ASCE 7-95 (American Society ofCivil Engineers), BS 6399 & CP3 (British Standards) and MBMA 1996 (Metal Buildings ManufacturersAssociation). Since the Royal Commission Building Code is much similar to UBC code, it has not been dealtseparately. The loads considered are live, wind, snow and seismic that commonly govern building design.Although the philosophies on which the development of these codes is based are similar, there remainssignificant difference in the loading, making some codes more conservative than others. Thus the selectionof a code has a significant impact on the design and cost of a building project. ASCE 7-95 forms the basisfor the development of other American codes such as UBC 97 and MBMA 96 codes and thus it is regardedas the original reference code. ASCE 7-95 code is more rationale and comprehensive because itencompasses the various factors governing the design loads scientifically. UBC 97 is a much-simplified codeas compared to ASCE 7-95 thereby prescribing a conservative loading. MBMA 96 code is more pertinent tolow-rise industrial steel buildings and is far more practical than the remaining codes. British codes are basedon a quite distinct approach affording a more liberal loading over American codes. The need has long sincebeen felt for a local building code for the Kingdom and it may, preferably, be patterned closely to ASCE7-95 incorporating local environmental factors.

1. INTRODUCTION

During the past few decades, Saudi Arabia witnessed an unprecedented growth in all sectors resulting in aboom in the construction industry. To meet this challenge for rapid development, various local andmulti-national companies participated in the construction of industrial, commercial and residential projectson a mammoth scale. Amidst the burgeoning construction industry, various design and loading codes havebeen adopted. The choice of codes, for a particular project, was based on the origins and cultures of thecompanies involved in that project rather than a standardized local code.

In the absence of such a uniform nation-wide building code, prominent organizations in Saudi Arabia haveadopted the common prevailing international codes. Few examples can be cited: Saudi Aramco specifiesloading as per Uniform Building Code UBC [1], SCECO requires loading as per ASCE [2] (earlier ANSI) andthe Royal Commission developed its own code [3] which is much similar to UBC.

Most of the design consulting firms in the kingdom face the challenge of maintaining and training theirtechnical staff in order to cope with the usage of various codes requiring specialists in more than one codesimultaneously. The end-user is often confused as to what design code and loading is appropriate to agiven situation and what would be the material specifications. Such questions create confusion in theconstruction industry especially dealing with steel structures.

This paper deals with a comparison of various codes in order to assess the variation in the level ofconservativeness, simplicity and rationale relevant to the prevailing environmental conditions of theKingdom. This study will also provide a set of guidelines that will assist designers in choosing anappropriate code for a given project.

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As steel buildings are becoming more popular in the construction industry especially low-rise buildings,more emphasis will be given for such buildings in addressing this challenge. Four dominant loads: live,wind, snow and seismic loads are examined in this study.

2. LIVE LOAD

Roof live load depends on the tributary area of the member and the slope of the roof. Both UBC 97 [1](Section 1607.4) and MBMA 96 [4] (Section 3) have adopted similar roof live load calculations as specifiedby ASCE 7-95 [2] Section 4.9. The minimum roof live load as per these codes ranges between 0.57 kN/m2and 0.96 kN/m2.

BS 6399 Part 1: 1984 [5] prescribes an imposed roof load of 1.5 kN/m2 or a 1.8 kN concentrated loadwhichever produces the greater stress for roof slopes less or equal to 10o. Where deflection is the designcriterion, the concentrated load is assumed to act in the position which produces maximum deflection. Thisvalue is on a much higher side and is almost double the value recommended by other codes for mainframing. For roof slope greater than 10o, the imposed roof load is reduced to 0.75 kN/m2 or a 0.9 kNconcentrated load.

3. WIND LOAD

Unlike the live load, there is an apparent variation in the methods adopted by various codes for thecalculation of wind load. Since wind loading is the most dominant one, a detailed discussion is provided.

In all codes basic wind speed is used to calculate the velocity pressure. Then appropriate pressurecoefficients are used to determine the wind loading at windward and leeward roofs and walls. Velocitypressure is termed as stagnation pressure, qs, in UBC 97 code and dynamic pressure, q, in CP3 [6]. BothASCE 7-95 and MBMA 96 refer to it as velocity pressure and denote it by qz and q, respectively. Velocitypressure variation in different codes has been shown in Fig. 1 for basic wind speed of 180 km/h for thefollowing building configuration: building eave height of 7m, building width of 24m, gable slope of 1 in 20and building length of 48m.

3.1 Design Wind Pressure

The methods and formulae for the calculation of velocity pressure and design wind pressure forabove-mentioned codes are presented.

ASCE 7-95 Code: The velocity pressure, qh (N/m2), is calculated from the formula Eq. 6.1 of code given by:

qh = 0.613KhKhtV2I -----------------------------------(1)

where Kh is the velocity exposure coefficient that depends on both the building height and exposurecategory; Kht is a topographic factor; V is the velocity and I the importance factor. This formula does notuse the building height, H, directly since the velocity exposure coefficient Kh involves height of the building.

I, the importance factor is used as 1.0, accounting for moderate hazard to human life and damage toproperty. Kht is taken as 1.0 to represent a plain topography. Kh is obtained from Table 6-3 of the code forexposure category, C, that represents open terrain with scattered obstruction at mean roof height of 7.3mand found to be 0.932. Using these values in equation (1) will give rise to velocity pressure qh equal to 1.43kN/m2

Design wind pressure, p, is then calculated from Table 6-1 of the code:

p = qh[(GCpf)-(GCpi)] -----------------------------------(2)

where GCpf and GCpi are external and internal pressure coefficients read from figure 6-4 and Table 6-4 ofthe code respectively.

UBC 1997 Code: Wind stagnation pressure, qs, is given in Table 16-F of the code, calculated always at astandard height of 10m. Unlike the other codes, the actual building height effect is accounted for, at a laterstage while design wind pressure is evaluated. Wind stagnation pressure, qs, calculated for a wind velocityof 180 km/h by interpolation is equal to 1.54 kN/m2.


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Design wind pressure, P (upper case as per the code), is then calculated from equation 20-1 of the code asfollows:

P = CeCqqsIw -----------------------------------(3)

where Ce is the combined height, exposure and gust factor coefficient obtained from Table 16-G of thecode as 1.18, Cq is the pressure coefficient for the structure read from Table 16-H and Iw is the importancefactor equal to 1.0 as per Table 16-K of the code.

MBMA 1996: Velocity pressure, q, is directly related to the basic wind speed, V (km/h), and mean roofheight, H (m), as follows:

q = 2.456V2H2/710-5 -----------------------------------(4)

Using V equal to 180 km/h and H as 7.3 m will give rise to q equal to 1.4 kN/m2

Design wind pressure, p, is then calculated as:

p = Iwq (GCp) -----------------------------------(5)

where, Iw is the importance factor and GCp is the combined pressure coefficient read from Table 5.4(a) ofthe code.

CP 3 Chapter V Part 2: 1972: The British code calculates design wind speed, Vs, using basic wind speed, V,and factors S1, S2 and S3.

Vs = VS1S2S3 -----------------------------------(6)

where S1 is a topography factor, S2 is a combined factor for ground roughness, building size and height ofthe building. S3 is the statistical factor that depends on the desired degree of security.

S1 is equal to 1.0, accounting for plain ground. S2 for an eave height 7m and Class B-2 (open country withscattered windbreaks and main frame of moderate building size), is obtained from Table 3 of the code as0.796. S3 is taken as 1.0 for permanent structures. Using these values in equation (6) will result in designwind speed, Vs, equal to 143.3 km/h.

The dynamic pressure, q (N/m2), is then calculated as:

q = 0.613Vs2 (Vs in m/s) -----------------------------------(7)

Equation 7 will give the dynamic pressure, q, equal to 0.971 kN/m2.

The dynamic pressure ‘q’ is then multiplied by the appropriate pressure coefficients arriving at the designpressure using following expression:

p = (Cpe-Cpi) q -----------------------------------(8)

where, Cpe is the external pressure obtained Tables 7 and 8 of the code. Internal pressure coefficients, Cpi,for fully enclosed building are taken as +0.2 and –0.3.


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Fig (1) Velocity Pressure for Various Codes

In Fig.1 the wind stagnation pressure, qs, as per UBC 97 code attains the highest value since this pressureis initially calculated for a standard height of 10m. The effect of required height of 7m was incorporated inthe factor Ce that will be used for the calculation of design wind pressure. The values of velocity pressuresas per MBMA 96 and ASCE 7-95 are almost the same since the actual eave-height is used. Dynamicpressure using CP3 is much lowered due to ground roughness factor S2 which results in a design windspeed value as low as 143.3 km/h.

3.2 Design wind load for main framing

In this discussion fully enclosed and partially enclosed buildings are considered employing various codes.

3.2.1 Fully Enclosed Building:

Using appropriate coefficients, design wind loads (kN/m) are shown in Fig. 2 for each loading code. Forcomparison, these loads are calculated for the interior rigid frame of the building assuming fully a enclosedcondition for the wind-left case and considering a bay spacing of 8m between rigid frames. The plus andminus signs signify wind loads acting towards (pressure) and away (suction) from the external surfaces,respectively.

Two values of wind load as per ASCE 7-95 and CP3 indicate one for positive and the other for negativeinternal pressure. ASCE 7-95 code specifies +0.18 as internal pressure coefficients, GCpi, while CP3 assigns+0.2 & -0.3 for fully enclosed buildings. MBMA 96 and UBC 97 do not specify any internal pressurecoefficients for fully enclosed buildings.

As it is evident from Fig. 2, the final design wind loads as per these codes are not similar except that ASCE7-95 design wind loads with positive internal pressure compare well with those of MBMA 96. UBC 97 loadsare the most conservative while CP3 loads are much lower than other codes.

It is worth mentioning that an additional longitudinal force termed as ‘frictional drag’ should be consideredfor the corrugated wall and roof cladding, as per CP3 code, if building length to height ratio or length towidth ratio is greater than 4. Such a force is not addressed in other codes.


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ASCE 7-95 wind loading is based on the height, exposure conditions, topography factors, importance factor,velocity and roof slope; the code considers these factors comprehensively and in a rational manner. UBC 97is based on similar factors except the topography factor. All such factors are incorporated in the mostsimplified manner, as it is evident from the fact that factors like height, exposure condition and gusttogether are combined in one multiplier, Ce. MBMA 96 does not address topography and exposureconditions. CP3 uses a different set of factors from the above-mentioned codes such as ground roughnessand a statistical factor.

3.2.2 Partially Enclosed Building

UBC-97 defines a building partially enclosed when the building has more than 15% of the windward wallarea open and the area of openings on all other areas is less than half of that on the windward wall.MBMA-96 defines a building as partially enclosed when the total area of openings in a windward wallexceeds the sum of the areas of openings on other areas and also exceeds 5% of the area of that wall;furthermore the density of the openings in the balance of the building envelope does not exceed 20%. Asper ASCE, a building is regarded as partially enclosed when the total area of openings in a windward wallexceeds the sum of the areas of opening in the balance of the building envelope by more than 10% and thetotal area of opening in a windward wall exceeds 4 sq ft (0.37 m2) or 1% of the area of that wall whicheveris smaller, and the percentage of openings in the balance of the building envelope does not exceed 20%.

CP3 code does not give a precise definition of partially enclosed buildings and defines an enhanced internalpressure coefficient, Cpi, equal to 75% of the value of external pressure coefficient, Cpe, of windward wallwherein a dominant opening is likely to occur. This Cpi is equally applicable for all areas inside the building.The size of the dominant opening is estimated based on the development of minimum internal pressure of0.75 times the Cpe, the external pressure coefficient at opening. The procedure and a detailed explanationof CP3 code have been dealt in Wind Load Handbook [7].

Partially enclosed buildings experience larger wind loads than enclosed buildings due to additional internalpressure that reflects in an increase in the suction loads on the roof and leeward wall.

ASCE 7-95 sets the values of Cpi for partially enclosed buildings as +0.8 and –0.3. UBC 97 and MBMA 96codes prescribe additional suction coefficients of 0.4 and 0.5 for partially enclosed buildings respectively. Asper CP3 code, internal pressure coefficient, Cpi, is one value of 0.75Cpe which equals to 0.525 for eachsurface, assuming a dominant opening at the windward wall. The size of the dominant opening is estimatedby a procedure outlined in [7] as around 0.58% of the windward wall area.

This indicates that the most conservative definition provided for partially enclosed building is as that givenby ASCE 7-95 and CP3.

From Fig. 3 it can be observed that for partially enclosed conditions, wind loads are significantly differentwith various codes and the load values do not follow a regular pattern in order to establish a correlationbetween any two different codes. Except at windward wall, design wind loads as per ASCE 7-95, withpositive internal pressure coefficient, GCpi, of 0.8, are closer to those calculated using MBMA 96.

3.3 Design wind load for secondaries

Secondary members such as purlins, girts, eave struts, endwall posts & beams and claddings are treated


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with a different set of pressure coefficients generally a function of the tributary areas of these elements.Table 1 shows the variation in the design wind loads on purlins and girts at an interior zone for fullyenclosed condition for various codes using a basic wind speed of 180 km/h and a tributary area of 12 m2.

Secondary Design Wind Load (kN/m2) as per

Member ASCE UBC MBMA CP3

Purlins 1.39 2.36 1.68 1.10

GirtsPressure 1.46 2.18 1.40 0.94Suction 1.68 2.18 1.54 0.52

Table (1) Design wind load on purlins and girts

Table 1 shows that there is no similarity in the loading values obtained using different codes for thesecondary members too. However the trend is similar to that observed for main framing. Again the UBC 97code exhibits more conservative trend while CP3 code is the least conservative. ASCE 7-95 and MBMA 96loads are moderate and closer. The dynamic pressure for secondary members in CP3 code is different fromthat for primary framing, evaluated from Table 3 of the code using Class A. This variation in pressure isascribed to the reduced value of S2 that depends upon the tributary area of the members. Unlike othercodes, a separate set of pressure coefficients is not used for the secondary members in CP3 code.

Although the gulf region does not experience snow and seismic loads, a brief discussion would helpcomparing various codes in dealing with such loads.

4. SNOW LOAD

Snow loads are used in lieu of live loads if they govern over live loads. ASCE 7-95 and MBMA 96 codesincorporate appropriate coefficients for roof exposure condition, slope effect, smoothness of roof surfaceand building heating condition, resulting in the snow load calculations being more elaborate and rational. Adirect reduction in snow load was proposed in UBC 97 for snow load in excess of 0.96 kN/m2 (20psf) foreach degree of roof slope over 20 degrees as per equation 14-1 of the code. BS 6399 Part 3 [8] includesthe effects of roof slope, symmetry of loading and shape of roof by multiplying the site snow load with abuilding ‘shape coefficient’.

For a ‘total snow load’ (termed as ‘ground snow load’ in ASCE 7-95 and MBMA 96, ‘site snow laod’ in BS6399 Part 3) of 1.5 kN/m2 with a slope of 25o, roof snow loads are shown in Fig. 4. As seen in the figure,ASCE 7-95 and MBMA 96 codes give rise to quite similar roof snow loads since MBMA 96 adopted the samephilosophy and similar coefficients for determining the roof snow load from ground snow load. UBC 97 doesnot recognise factors other than roof slope resulting in a highly conservative value of roof snow load. Andthe load obtained by CP3 lies in the intermediate range.

Calculation of roof snow load for special considerations such as unbalanced roof snow load, partial loading,drift load has been addressed in a very rational way in ASCE 7-95 and MBMA 96.

5. SEISMIC LOAD


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The procedure outlined in MBMA 96 for the determination of seismic forces are in fact adopted from ASCE7-95 in a more simplified form, applicable to low rise metal buildings. The base shear in these codes, V, is aproduct of Seismic Design Coefficient, Cs, which is a function of Effective Peak Acceleration, Aa, andResponse Modification Factor.

The values of Aa are read from the peak acceleration contours provided for USA in ASCE 7-95. The valuesof Aa for other countries are not readily available. In the presence of peak acceleration data of a givencountry the procedure outlined in these codes can be precisely used. But in the absence of such data aseismic zone can be identified based on the seismic history and a method provided in UBC 97 code can beused more conveniently.

UBC 97 demarcates the seismic zone as 1, 2A, 2B, 3 and 4 with values of Seismic Zone Factor, Z, as 0.075,0.15, 0.2, 0.3 and 0.4, respectively. As per UBC 97, the Seismic Coefficient, Ca, depends on Z and the soilprofile type. Since United Kingdom is not a seismically active area, British Standards do not address seismicloads.

6. LOAD COMBINATIONS

Using the Strength Design or Load Resistance Factor Design [9] approaches there seems to be uniformity inthe UBC 97, ASCE 7-95 and MBMA 96 codes when addressing load combinations. With regard to AllowableStress Design [10], however there seems to be disparity among these codes.

Load combinations from MBMA 96 are much different from UBC 97 basic load combinations. HoweverMBMA 96 is much similar to the alternate basic load combinations of UBC 97 except the factor of 1/1.4applied to earthquake loads. MBMA 96 prescribe a completely new set of load combinations pertaining toearthquake loads.

Both UBC 97 and MBMA 96 allow for an increase in the allowable stresses by 33% for all load combinationsincluding wind and seismic loads. While ASCE 7-95 code does not permit such an increase in the allowablestresses for wind and seismic forces.

7. CONCLUSIONS

In this study load calculations as per ASCE 7-95, MBMA 96, UBC 97 and BS codes have been examined.Most particularly the design wind loads have been assessed for a given building configuration and windspeed. ASCE 7-95 is found to be the most comprehensive and rational code and forms the basis for thedevelopment of UBC 97 and MBMA 96 in most of the load cases. UBC 97 code is much simpler in itsapproach than ASCE 7-95 and thus is the most conservative of all codes. MBMA 96 has almost adopted thesame concepts of ASCE 7-95 and incorporated the results of research undertaken by MBMA 96 and industrygroups in order to come up with a code pertinent to the design of low-rise industrial steel buildings. Theloading as per MBMA 96 seems to be the most moderate amongst other codes. The British codes adopted adistinct approach from American codes and are found to result in more liberal loading.

For the task of development of a unified code for the Kingdom the author suggests the code should bepattered as per the guidelines provided in the commentary of ASCE 7-95 while applying the factorsgoverning environmental conditions. Pertaining to low-rise industrial buildings or pre-engineered buildings[11], the MBMA 96 code has the greater practical significance. UBC 97, though popular in the designindustry results in uneconomical designs because of its oversimplifications.

REFERENCES

[1] Uniform Building Code (1997) International Conference of Building Officials, 5360 Workman Mill Road,Whittier, CA.

[2] Minimum Design Loads for Buildings and Other Structures: ASCE 7-95 (1995) American Society ofCivil Engineers, New York.

[3] Madinat Yanbu Al-Sinaiyah Building Code, Directorate General for Yanbu Project (1982), RoyalCommission for Jubail and Yanbu, KSA.

[4] 1996 Low Rise Building Systems Manual, Metal Building Manufactures Association, 1300 Sumner Ave.


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Cleveland, Ohio 44115

[5] British Standard 6399: Part 1: 1984, Design loading for buildings Part 1. Code of practice for dead andimposed loads, British Standards Institution.

[6] CP3: Chapter V: Part 2: 1972, Code of Basic data for the design of buildings Chapter V. Loading Part2. Wind loads, British Standards Institution.

[7] Wind Loading Handbook by C. W. Newberry and K. J. Eaton, Department of the Environment BuildingResearch Establishment, London, Her Majesty’s Stationery Office 1974.

[8] British Standard 6399: Part 3: 1988, Loading for buildings Part 3. Code of practice for imposed roofloads, British Standards Institution.

[9] Manual of Steel Construction, Load Resistance Factor Design, Ninth Edition, American Institute ofSteel Construction, Inc. 1 East Wacker Drive, Suite 3100, Chicago, Illinois 60601

[10] Manual of Steel Construction, Allowable Stress Design, Ninth Edition, American Institute of SteelConstruction, Inc. 1 East Wacker Drive, Suite 3100, Chicago, Illinois 60601

[11] Metal Building Systems Design and Specifications by Alexander Newman, McGraw-Hill, 1997.

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