Class 5 Applying Loads to Buildings

1
Class 5Applying Loads to Buildings Wind and
Flood
2
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)

3
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)

4
Determine loads for

• MWFRS examples
• CC - examples

5
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

6
CC
7
ASCE Design Methods

• Simplified procedure
• Analytical procedure the design process
  mentioned above follows this approach
• Were going to work a problem with same givens
  through both approaches and see how the results
  compare

8
Wind Speed Map Fig. 6-1
9
Delaware wind speeds
110
120
10
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

11
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

12
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

13
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

14
Exposure B (from ASCE 7)
15
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

16
Exposure D
17
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

18
Exposure C
19
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

20
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.

21
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

22
Effects from topography
23
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

24
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 lt 150 ft unless
  structural system is extremely flexible

25
Enclosure classification

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

26
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

27
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

28
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

29
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

30
CC 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

31
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

32
Design example

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

33
(No Transcript)
34
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

35
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

36
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

37
Hydrostatic forces
38
Hydrostatic force
39
Buoyancy forces
40
Buoyancy failure
41
Velocity

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

42
Hydrodynamic forces

• Force of moving water

43
Wave height determination
44
Breaking wave forces

• Against slender element like pile

45
Breaking wave forces on wall
46
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

47
Debris

• Correction ?g should be ?t impact duration

48
Homework 4