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Class 5 Applying Loads to Buildings – Wind and Flood

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## Class 5 Applying Loads to Buildings – Wind and Flood

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**Wind loads**• References are ASCE 7 – Chapter 6 and the Guide to the Use of the Wind Load Provisions of ASCE 7 • Design process is to determine: • Basic wind speed from Figure 6-1 • Directionality factor (Kd) • Importance factor (I) • Exposure category and velocity pressure coefficient (Kz) • Topographic factor (Kzt) • Gust effect factor (G) • Enclosure classification • Internal pressure coefficients (GCpi) • External pressure coefficients (Cp) Building Design – Fall 2007**Wind pressures and loads**• Then calculate wind pressure q • Use q to find wind load p or F Basic wind pressure equation is: q = 0.00256 Kz Kzt Kd V2 I (psf) Building Design – Fall 2007**Determine loads for:**• MWFRS – examples • C&C - examples Building Design – Fall 2007**MWFRS**• ..\..\..\presentations\Design of Buildings in Coastal Regions Workshop\Reference material\FEMA 499 Home Builder's Guide Technical Fact Sheets\hgcc_fact10 Load Paths.pdf Building Design – Fall 2007**C&C**Building Design – Fall 2007**ASCE Design Methods**• Simplified procedure • Analytical procedure – the design process mentioned above follows this approach • We’re going to work a problem with same givens through both approaches and see how the results compare Building Design – Fall 2007**Wind Speed Map Fig. 6-1**Building Design – Fall 2007**Delaware wind speeds**110 120 Building Design – Fall 2007**Wind speed measuring standards**• 3-sec peak gust • 33 ft (10m) above the ground • Exposure C • Hurricane coastline event frequency is between 50 – 100 years MRI Building Design – Fall 2007**Directionality Factor Kd**• For most buildings Kd = 0.85 • Accounts for reduced probability that max winds will come from any particular direction • And reduced probability that max pressure coefficient will occur for any given wind direction Building Design – Fall 2007**Importance Factor**• I = 1.0 for Category II buildings which include residential and most commercial • I = 1.15 for both Category III and IV buildings which are high occupancy or critical use Building Design – Fall 2007**Exposure Category**• B – prevails upwind 2600 ft or 20 x bldg height • Described as urban and suburban areas, wooded or closely spaced obstructions • Exposures developed from surface roughness • ASCE Commentary discusses Building Design – Fall 2007**Exposure B (from ASCE 7)**Building Design – Fall 2007**Exposure D**• Prevails upwind 5000 ft or 20 x bldg height • Described as flat, unobstructed areas and water surfaces outside hurricane prone regions • Includes mud and salt flats, unbroken ice Building Design – Fall 2007**Exposure D**Building Design – Fall 2007**Exposure C**• Applies to all cases that are not Exposure B or D • Includes open terrain with scattered obstructions generally less than 30 ft tall • Airports are good examples Building Design – Fall 2007**Exposure C**Building Design – Fall 2007**Caution!!**• Wind speed maps are based on an Exposure C • All the tables and simplified wind design pressures are all based on Exposure B • Requires conversion to get pressures at Exposure C, • However, Exposure B is the most prevalent terrain condition Building Design – Fall 2007**Velocity Pressure Coefficient Kz**• Values provided in Table 6-3 • Values can be interpolated between heights above ground • Note that Kz = 1.0 for Exposure C at 33 ft which is the base for the wind speeds • Note there is no difference in coefficient between 0 and 15 ft. and in Exposure B no difference for 0 to 30 ft. Building Design – Fall 2007**Topographic factor Kzt**• There is a wind speed-up effect at isolated hills, ridges and escarpments in any exposure category • Must account for speed-up under 3 conditions (see Section 6.5.7.1) • If site conditions do not meet ALL the conditions in Section 6.5.7.1, then Kzt = 1 Building Design – Fall 2007**Effects from topography**Building Design – Fall 2007**Gust Effect Factor G**• For rigid structures G = 0.85 or calculated by Formula 6-4 • By definition, rigid structure is one whose fundamental frequency n1 is ≥ 1 hz • n1 = 1/Ta (the building period) • From earthquake design Ta = Cthx where h is height of building, Ct and x are coefficients based on shear wall strategies Building Design – Fall 2007**Determining height for a rigid building**• For most structural systems, Ct = 0.02 and x = .75, so if min. n1 = 1.0 then Ta must = 1.0 • Solving for h in Ta = Cthx or 1 = 0.02h.75 • h = (1/0.02)1.333 • h = 183.96 ~ 184 ft • Use G = 0.85 for any building < 150 ft unless structural system is extremely flexible Building Design – Fall 2007**Enclosure classification**• Open • Partially enclosed • Enclosed • Definitions for these classifications are given in Sec 6.2 definitions Building Design – Fall 2007**Open**• Building that has EACH wall at least 80% open • Examples of openings – doors, operable windows, air intake exhausts, gaps around doors, deliberate gaps in cladding, louvers Building Design – Fall 2007**Partially enclosed**• Building that complies with both conditions: • Total area of openings in wall that receives positive external pressure exceeds sum of areas of openings in balance of building envelope by more than 10% • Total area of openings in wall exceeds 4 ft2 or 1% of area of wall whichever is smaller and % of openings in balance of building envelope does not exceed 20% Building Design – Fall 2007**Enclosed**• Building that does not comply with either open or partially enclosed definitions • Importance of enclosed building • In order to qualify, openings must be impact-resistant • Required in wind-borne debris regions which are within hurricane prone areas where wind speed is 110 mph or greater and within 1 mile of coast or where wind speed is 120 mph or greater Building Design – Fall 2007**MWFRS Pressures**• GCp external pressure coefficients found in Figures in Chapter 6 (depends on the method you select to determine loads) • GCpi internal pressure coefficient found in Figure 6-5 and is a function of enclosed condition Building Design – Fall 2007**C&C Pressures**• GCp external pressure coefficients based on effective wind area and are function of building geometry • Use graphs to determine coefficients such as Figures 6-11A-D Building Design – Fall 2007**Important design concepts**• Wind loads are normal to the surface yet in order to perform load combinations for vertical and horizontal loads, the wind components must be determined • Wind loads acting toward the surface (windward) are ‘positive’ and loads acting away from the surface (leeward) are ‘negative’ • In design, we are looking for the very largest loads irrespective of windward/leeward acting Building Design – Fall 2007**Design example**• Work one example using 2 methods and compare results • Simplified procedure • Low-rise building provisions Building Design – Fall 2007**Flood loads**• References are ASCE 7 – Chapter 5, ASCE 24 and USACE Shore Protection Manual • Two primary flooding sources – riverine (mapped by FEMA as A Zones) and coastal (mapped as V Zones) • Regulatory elevation is the 1% or 100-year flood Building Design – Fall 2007**Flood Design Method**• Determine flood source – riverine or coastal • Determine depth of flooding • Determine flood parameters important to design – could include: • Depth (hydrostatic and buoyancy) • Velocity • Waves • Erosion • Scour • Debris Building Design – Fall 2007**Flood Depth**• Source of information is FEMA Flood Map – provides flood elevations • Need ground elevation – USGS Quad map or survey information • MUST add some factor of safety called freeboard • Flood depths too difficult to precisely quantify Building Design – Fall 2007**Hydrostatic forces**Building Design – Fall 2007**Hydrostatic force**Building Design – Fall 2007**Buoyancy forces**Building Design – Fall 2007**Buoyancy failure**Building Design – Fall 2007**Velocity**• Do not have good information about velocity of water moving during a flood except FIS • Best guidance is: Building Design – Fall 2007**Hydrodynamic forces**• Force of moving water Building Design – Fall 2007**Wave height determination**Building Design – Fall 2007**Breaking wave forces**• Against slender element like pile Building Design – Fall 2007**Breaking wave forces on wall**Building Design – Fall 2007**Effect of scour and erosion**• Both scour and erosion lower the ground elevation increasing water depth • Both reduce soil support for foundations • Pile embedment • Soil for shallow footings • Consider effects of both and for multiple storms Building Design – Fall 2007**Debris**• Correction – Δg should be Δt impact duration Building Design – Fall 2007**Homework 4**Building Design – Fall 2007