Class 5 Applying Loads to Buildings – Wind and Flood

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Class 5 Applying Loads to Buildings – Wind and Flood 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 (K d ) Importance factor (I)

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

• 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

• 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)

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• MWFRS – examples
• C&C - examples

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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

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C&C

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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

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Wind Speed Map Fig. 6-1

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Delaware wind speeds

110

120

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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

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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

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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

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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

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Exposure B (from ASCE 7)

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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

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Exposure D

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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

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Exposure C

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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

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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

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Effects from topography

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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

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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

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Enclosure classification
• Open
• Partially enclosed
• Enclosed
• Definitions for these classifications are given in Sec 6.2 definitions

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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

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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%

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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

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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

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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

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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

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Design example
• Work one example using 2 methods and compare results
• Simplified procedure
• Low-rise building provisions

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• 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

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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

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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

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Hydrostatic forces

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Hydrostatic force

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Buoyancy forces

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Buoyancy failure

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Velocity
• Do not have good information about velocity of water moving during a flood except FIS
• Best guidance is:

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Hydrodynamic forces
• Force of moving water

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Wave height determination

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Breaking wave forces
• Against slender element like pile

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Breaking wave forces on wall

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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

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Debris
• Correction – Δg should be Δt impact duration

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Homework 4

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