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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 5 applying loads to buildings wind and flood l.jpg

Class 5Applying Loads to Buildings – Wind and Flood


Wind loads l.jpg
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


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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


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Determine loads for:

  • MWFRS – examples

  • C&C - examples

Building Design – Fall 2007


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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

Building Design – Fall 2007


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C&C

Building Design – Fall 2007


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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

Building Design – Fall 2007


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Wind Speed Map Fig. 6-1

Building Design – Fall 2007


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Delaware wind speeds

110

120

Building Design – Fall 2007


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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

Building Design – Fall 2007


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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

Building Design – Fall 2007


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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

Building Design – Fall 2007


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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

Building Design – Fall 2007


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Exposure B (from ASCE 7)

Building Design – Fall 2007


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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

Building Design – Fall 2007


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Exposure D

Building Design – Fall 2007


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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

Building Design – Fall 2007


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Exposure C

Building Design – Fall 2007


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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

Building Design – Fall 2007


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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


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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


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Effects from topography

Building Design – Fall 2007


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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

Building Design – Fall 2007


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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

Building Design – Fall 2007


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Enclosure classification

  • Open

  • Partially enclosed

  • Enclosed

  • Definitions for these classifications are given in Sec 6.2 definitions

Building Design – Fall 2007


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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

Building Design – Fall 2007


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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%

Building Design – Fall 2007


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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

Building Design – Fall 2007


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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

Building Design – Fall 2007


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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

Building Design – Fall 2007


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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

Building Design – Fall 2007


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Design example

  • Work one example using 2 methods and compare results

  • Simplified procedure

  • Low-rise building provisions

Building Design – Fall 2007



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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


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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

Building Design – Fall 2007


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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

Building Design – Fall 2007


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Hydrostatic forces

Building Design – Fall 2007


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Hydrostatic force

Building Design – Fall 2007


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Buoyancy forces

Building Design – Fall 2007


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Buoyancy failure

Building Design – Fall 2007


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Velocity

  • Do not have good information about velocity of water moving during a flood except FIS

  • Best guidance is:

Building Design – Fall 2007


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Hydrodynamic forces

  • Force of moving water

Building Design – Fall 2007


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Wave height determination

Building Design – Fall 2007


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Breaking wave forces

  • Against slender element like pile

Building Design – Fall 2007


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Breaking wave forces on wall

Building Design – Fall 2007


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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

Building Design – Fall 2007


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Debris

  • Correction – Δg should be Δt impact duration

Building Design – Fall 2007


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Homework 4

Building Design – Fall 2007


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