Loading for most of the structures are obtained from the relevant British Standards, the manufacturers data and similar sources. CIVL473 Fundamentals of Steel Design CHAPTER 2 Loading and Load Combinations Prepared By Asst.Prof.Dr. Murude Celikag a) The relevant British Standards for Loading BS 6399 Design Loading for Buildings Part 1: Dead and Imposed Loads (1984) Part 2: Wind Loads (1995) Part 3: Imposed Roof Loads (1988) b) The old British Standard for Wind Loading British standards Institute CP3: Chapter V: Part 2 1) DEAD LOADS Own weight of the steel member (kg/m of the steel section) Other permanent parts of the building, not normally movable. (e.g. concrete floor slabs, brick/block walls, finishes, cladding). Weight is calculated either from density of material (kg/m 3 ) or specific weight (kn/m 3 ). Typical values of common structural materials Material Density (kg/m 3 ) Specific Weight (kn/m 3 ) Steel Reinforced Concrete Brickwork Timber 7850 2420 2000-2300 500-900 77 23.7 20-23 5-9 Dr.Murude Celikag 1
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2) IMPOSED LOADS All imposed loads are based on experience within the construction industry and the statistical analyses of observed cases. Includes the following temporary loads: Snow on roofs People Furniture Equipment, such as cranes and other machinery Semi-permanent partitions that are movable Typical values of imposed loads Building Usage Imposed Loading (kn/m 2 ) Residential Offices Educational Theatres Warehousing Industrial Workshops 1.5 2.5-5.0 3.0 4.0 2.4 per meter height 5.0 3) WIND LOADS Wind loads to be used are based on the British Standards Institute CP3: Ch V: Part 2. Basic wind speed, appropriate to the location of the building, is selected and reduced to a design wind speed using factors which take into consideration topography, surrounding buildings, height above ground level, component size and period of exposure. Figure 1: (a) Variation of wind velocity with distance above ground surface; (b) variation of wind pressure specified by typical building codes tor windward side of building. Dr.Murude Celikag 3
Figure 2: Influence of shape on drag factor: (a) curved profile permits air to pass around body easily (drag factor is small); (b) wind trapped by flanges increases pressure on web of girder (drag factor is large). Figure 3 (a) Uplift pressure on sloping roof; - The wind speed along path 2 > that along path 1 because of the longer path - Increased velocity reduces pressure on top of roof, creating a pressure differential between inside and outside of building - The uplift is a function of the roof angle, θ. (b) Increased velocity creates a negative pressure (suction) on side and leeward face, direct pressure on windward face AA Dr.Murude Celikag 4
Vortex Shedding. As wind moving at constant velocity passes over objects in its path, the air particles are retarded by surface friction. This process is called vortex shedding. As the air mass moves away, its velocity causes a change in pressure on the discharge surface. If the period (time interval) of the vortices leaving the surface is close to that of the natural period of the structure, oscillations in the structure will be induced by the pressure variations. With time these oscillations will increase and shake a structure vigorously. Photo 1: Failure of the Tacoma Narrows Bridge showing the first section of the roadway as it crashes into Puget Sound. The breakup of the narrow, flexible bridge was produced by large oscillation induced by the wind. Figure 4: Vortices discharging from a steel girder. As vortex speeds off, a reduction in pressure occurs, causing girder to move vertically. http://www.pbs.org/wgbh/nova/bridge/tacoma3.html http://www.enm.bris.ac.uk/research/nonlinear/tacoma/tacnarr.mpg http://www.youtube.com/watch?v=j-zczjxsxnw http://video.nationalgeographic.com/video/player/environment/energyenvironment/wind-power.html Dr.Murude Celikag 5
Figure 6: Typical wind load distribution on a multi-story building. Figure 7: Variation of wind pressure on sides of buildings. Figure 5: Structural systems to resist lateral loads from wind or earthquake, (a) Reinforced concrete shear wall carries all lateral wind loads. (b) Shear and moment diagrams for shear wall produced by the sum of wind loads on the windward and leeward sides of the building in (a), (c) Plan of building showing position of shear walls and columns, (d) Cross-bracing between steel columns forms a truss to carry lateral wind loads into the foundations. Dr.Murude Celikag 6
The design wind speed is equated to a dynamic pressure q(kn/m 2 ). Owing to building and roof shape, opening is walls, etc., pressures and suctions, both external and internal, will arise. Pressure Coefficients external (C pe ) and internal (C pi ) may be used as shown in the example. Force on any element = (C pe -C pi )q x area of element Pressure Coefficients external (C pe ) and internal (C pi ) may be used as shown in the example. 4) LOAD COMBINATIONS Loads in any structure must be arranged during the design so that the maximum forces or moments are achieved at specific points of the structure. Figure 8: (a) Wind acts on windward side of building; (b) wind forces applied by windward wall to edge of roof and floor slabs; (c) side view showing resultant wind forces on each shear wall. Therefore, all realistic load combinations must be considered to ensure that all peak values have been calculated at every point. One load arrangement would be sufficient to produce maximum moments and forces for simple cases. Dr.Murude Celikag 7
Load Factors and Combinations Partial Safety Factor for Loads Loading Load Factor, f Dead load, D 1.4 Dead load restraining uplift 1.0 Imposed load, L 1.6 Wind load, W 1.4 Combined loading (D+L+W) 1.2 1.4DL+1.6LL 1.0DL+1.4WL E 1.0DL+1.4WL W Example Wind Loading 2.65m 7.35m B A C Frames at 6m centres 30m Pitch 10 o Basic wind speed 50 m/s Length of building 48m, Open country with scattered wind brakes S 1 and S3 are both taken as 1.0, S 2 is 0.88 for a height of 10m. Design wind speed is V s =S 1 S 2 S 3 V= 1.0x1.0x0.88x50 = 44 m/s 2 Dynamic pressure, q=kv s = 0.613x44 2 = 1.2 kn/m 2. D E 1.0DL+1.4WL N 1.0DL+1.4WL S 1.2DL+1.2LL+1.2WL E 1.2DL+1.2LL+1.2WL W 1.2DL+1.2LL+1.2WL N 1.2DL+1.2LL+1.2WL S Internal Pressure Coefficients (Cpi) is taken as +0.2 (maximum) and -0.3 (minimum). External pressure is obtained from Table 8 (CP 3). Dr.Murude Celikag 8
Example (continued) C pe for frame member AB BC CD DE Wind on side 0.7-1.2-0.4-0.25 Wind on end -0.5-0.6-0.6-0.5 1.2 0.4 0.6 0.6 0.7 B A C D E 0.25 0.5 B A C D E 0.5 0.2 0.3 a) Wind on side b) Wind on end c) Internal pressure d) Internal suction 1.4 0.6 0.9 0.1 0.8 0.8 0.3 0.3 0.5 0.45 1.0 0.05 0.7 0.7 0.2 0.2 Case 1 (a+c) Case 2 (a+d) Case 3 (b+c) Case 4 (b+d) C pe -C pi for frame member Case AB BC CD DE 1. Wind on side + internal pressure 2. Wind on side + internal suction 3. Wind on end + internal pressure 4. Wind on end + internal suction 0.5 1.0-0.7-0.2-1.4-0.9-0.8-0.3-0.6-0.1-0.8-0.3-0.45 0.05-0.7-0.2 Dr.Murude Celikag 9
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C C B D B D A E A E a) Wind on side b) Wind on end c) Internal pressure d) Internal suction Case 1 Case 2 Case 3 Case 4 C pe for frame member Case AB BC CD DE c) Internal pressure 1. Wind on side 2. Wind on end 0.2 C pe -C pi for frame member Case AB BC CD DE 1. Wind on side + internal pressure 2. Wind on side + internal suction 3. Wind on end + internal pressure 4. Wind on end + internal suction Dr.Murude Celikag 16