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# Applying loads to buildings - PowerPoint PPT Presentation

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

### 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)

Building Design – Fall 2007

• MWFRS – examples

• C&C - examples

Building Design – Fall 2007

• ..\..\..\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

Building Design – Fall 2007

• 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

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110

120

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• 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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• 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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• 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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• 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

Building Design – Fall 2007

• 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

Building Design – Fall 2007

• 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

Building Design – Fall 2007

• 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

• 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

• 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

Building Design – Fall 2007

• 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

• 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

• Open

• Partially enclosed

• Enclosed

• Definitions for these classifications are given in Sec 6.2 definitions

Building Design – Fall 2007

• 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

• 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

• 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

• 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

• 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

• 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

• Work one example using 2 methods and compare results

• Simplified procedure

• Low-rise building provisions

Building Design – Fall 2007

• 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

• 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

• 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

Building Design – Fall 2007

Building Design – Fall 2007

Building Design – Fall 2007

Building Design – Fall 2007

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

• Best guidance is:

Building Design – Fall 2007

• Force of moving water

Building Design – Fall 2007

Building Design – Fall 2007

• Against slender element like pile

Building Design – Fall 2007

Building Design – Fall 2007

• 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

• Correction – Δg should be Δt impact duration

Building Design – Fall 2007

Building Design – Fall 2007