Transcript ADVANCED STEEL DESIGN

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:
1.
2.
3.
4.
5.
6.
7.
8.
9.
7/17/2015
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
2
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)
7/17/2015
Building Design – Fall 2007
3
Determine loads for:
 MWFRS – examples
 C&C - examples
7/17/2015
Building Design – Fall 2007
4
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
7/17/2015
Building Design – Fall 2007
5
C&C
7/17/2015
Building Design – Fall 2007
6
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
7/17/2015
Building Design – Fall 2007
7
Wind
Speed
Map
Fig. 61
7/17/2015
Building Design – Fall 2007
8
Delaware wind speeds
110
120
7/17/2015
Building Design – Fall 2007
9
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
7/17/2015
Building Design – Fall 2007
10
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
7/17/2015
Building Design – Fall 2007
11
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
7/17/2015
Building Design – Fall 2007
12
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
7/17/2015
Building Design – Fall 2007
13
Exposure B (from ASCE 7)
7/17/2015
Building Design – Fall 2007
14
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
7/17/2015
Building Design – Fall 2007
15
Exposure D
7/17/2015
Building Design – Fall 2007
16
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
7/17/2015
Building Design – Fall 2007
17
Exposure C
7/17/2015
Building Design – Fall 2007
18
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
7/17/2015
Building Design – Fall 2007
19
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.
7/17/2015
Building Design – Fall 2007
20
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
7/17/2015
Building Design – Fall 2007
21
Effects from topography
7/17/2015
Building Design – Fall 2007
22
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
7/17/2015
Building Design – Fall 2007
23
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
7/17/2015
Building Design – Fall 2007
24
Enclosure classification




Open
Partially enclosed
Enclosed
Definitions for these classifications are
given in Sec 6.2 definitions
7/17/2015
Building Design – Fall 2007
25
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
7/17/2015
Building Design – Fall 2007
26
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%
7/17/2015
Building Design – Fall 2007
27
Enclosed
 Building that does not comply with either open
or partially enclosed definitions
 Importance of enclosed building
 In order to qualify, openings must be impactresistant
 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
7/17/2015
Building Design – Fall 2007
28
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
7/17/2015
Building Design – Fall 2007
29
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
7/17/2015
Building Design – Fall 2007
30
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
7/17/2015
Building Design – Fall 2007
31
Design example
 Work one example using 2 methods and
compare results
 Simplified procedure
 Low-rise building provisions
7/17/2015
Building Design – Fall 2007
32
7/17/2015
Building Design – Fall 2007
33
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 100year flood
7/17/2015
Building Design – Fall 2007
34
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
7/17/2015
Building Design – Fall 2007
35
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
7/17/2015
Building Design – Fall 2007
36
Hydrostatic forces
7/17/2015
Building Design – Fall 2007
37
Hydrostatic force
7/17/2015
Building Design – Fall 2007
38
Buoyancy forces
7/17/2015
Building Design – Fall 2007
39
Buoyancy failure
7/17/2015
Building Design – Fall 2007
40
Velocity
 Do not have good information about velocity of
water moving during a flood except FIS
 Best guidance is:
7/17/2015
Building Design – Fall 2007
41
Hydrodynamic forces
 Force of moving water
7/17/2015
Building Design – Fall 2007
42
Wave height determination
7/17/2015
Building Design – Fall 2007
43
Breaking wave forces
 Against slender element like pile
7/17/2015
Building Design – Fall 2007
44
Breaking wave forces on
wall
7/17/2015
Building Design – Fall 2007
45
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
7/17/2015
Building Design – Fall 2007
46
Debris
 Correction – Δg should be Δt impact duration
7/17/2015
Building Design – Fall 2007
47
Homework 4
7/17/2015
Building Design – Fall 2007
48