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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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Presentation Transcript
wind loads
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
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
Determine loads for:
  • MWFRS – examples
  • C&C - examples

Building Design – Fall 2007

mwfrs
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

slide6
C&C

Building Design – Fall 2007

asce design methods
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
Wind Speed Map Fig. 6-1

Building Design – Fall 2007

delaware wind speeds
Delaware wind speeds

110

120

Building Design – Fall 2007

wind speed measuring standards
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 k d
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
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
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
Exposure B (from ASCE 7)

Building Design – Fall 2007

exposure d
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 d16
Exposure D

Building Design – Fall 2007

exposure c
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 c18
Exposure C

Building Design – Fall 2007

caution
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 k z
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 k zt
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
Effects from topography

Building Design – Fall 2007

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

Building Design – Fall 2007

slide26
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
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
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
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
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
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
Design example
  • Work one example using 2 methods and compare results
  • Simplified procedure
  • Low-rise building provisions

Building Design – Fall 2007

flood loads
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
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
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
Hydrostatic forces

Building Design – Fall 2007

hydrostatic force
Hydrostatic force

Building Design – Fall 2007

buoyancy forces
Buoyancy forces

Building Design – Fall 2007

buoyancy failure
Buoyancy failure

Building Design – Fall 2007

velocity
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
Hydrodynamic forces
  • Force of moving water

Building Design – Fall 2007

wave height determination
Wave height determination

Building Design – Fall 2007

breaking wave forces
Breaking wave forces
  • Against slender element like pile

Building Design – Fall 2007

breaking wave forces on wall
Breaking wave forces on wall

Building Design – Fall 2007

effect of scour and erosion
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
Debris
  • Correction – Δg should be Δt impact duration

Building Design – Fall 2007

homework 4
Homework 4

Building Design – Fall 2007