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Class 5 Applying Loads to Buildings – Wind and FloodPowerPoint Presentation

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

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

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

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

Wave height determination

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

Homework 4

Building Design – Fall 2007

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