TÍNH TẢI GIÓ THEO TIÊU CHUẨN- ASCE7

TÍNH TẢI GIÓ

Wind Loads:

The ASCE 7 Provisions

CE 694R – Fall 2007

T Bart Quimby, P.E., Ph.D.

Quimby & Associates

Permitted Design Methods

Method 1—Simplified Procedure

Important Definitions

Basic Wind Speed

Building open, enclosed, partially enclosed

Low-Rise Building

See ASCE 7-05 6.2

Exposure Categories

Exposure A – Deleted in ASCE 7-02 and later

Extremely sheltered Large city centers with tall buildings.

Flat, unobstructed areas and water surfaces outside

hurricane prone regions This category includes smooth mud flats, salt flats, and unbroken ice that extend 5,000 ft or 20 times the building height in the upwind direction.

See ASCE 7-05 6.5.6 & C6.5.6 (See images!)

Determining Exposure

Wind Direction & Sectors (ASCE 7-05

6.5.6.1)

the exposure of the building or structure shall

be determined for the two upwind sectors

extending 45 o either side of the selected wind direction.

the exposure resulting in the highest wind

loads shall be used to represent the winds

from that direction.

ASCE 7-05 Wind Pressures

The basic form of the pressure equation:

p = qGC

Where

p = a wind pressure on a surface

q = velocity pressure This is the pressure due to a moving fluid on a flat plate

G = gust factor The gust factor accounts for dynamic

interaction between the flowing air and the structure

C = pressure coefficient The pressure coefficient accounts for varying pressure across a surface.

Velocity Pressure, q

q z =Velocity Pressure = 0.00256K z K zt K d V2 I (lb/ft 2 )

Constant 0.00256

V = Basic wind speed in mph

I = Importance Factor (i.e different MRI)

The Velocity Coefficient

Based on the average density of air at sea level.

P' 1

2 DV 2' 1

2 [

0.0765 32.2 ][

5280

3600]

2V 2'0.00256V 2

See ASCE 7-05 C6.5.10

Basic Wind Speed, V

Obtained from Wind Speed maps in ASCE

7-05 Figure 6-1

Determined by localized research using

approved probabilistic methods

“The basic wind speed shall be increased

where records or experience indicate that the wind speeds are higher than those reflected

in Fig 6-1.” (ASCE 7-05 6.5.4.1)

See ASCE 7-05 6.5.4

The Importance Factor, I

Velocity Pressure Exposure

Coefficients, Kz and Kh

Modifies basic wind pressure for heights

other than 33 ft and exposures other than

exposure C

Can compute K directly from equations in the

commentary for any height and/or exposure

Good for spreadsheet or computer

programming.

For elevations less than 15 ft, use K15.

For elevations above gradient height use K .

See ASCE 7-05 6.5.6.6, Tables 6-2 and 6-3, and C6.5.6.6

Kz & Kh Computation

Kz = 2.01(z/zg)2/a

K Com putation

0.00 0.50 1.00 1.50 2.00 2.50

Elevation, z (ft)

Exposure B Exposure C Exposure D

When z > zg use z = zg

When z < 15 use z = 15 ft

Constants

Kzt Multipliers by Equation

See ASCE 7-05 Figure 6.4

Directionality Factor, Kd

This factor shall only be

applied when used in

conjunction with load

combinations specified

in Sections 2.3 and 2.4.

The wind load factors

changed when the

directionality factor was

extracted.

See ASCE 7-05 6.5.4.4 and

Table 6-4

The Gust Factor, G

Factor accounting for:

Gustiness and turbulence

Gust Factor, G

For stiff buildings and stiff structures

G = 0.85

For flexible buildings and other structures

Calculate “by a rational analysis that

incorporates the dynamic properties of the main wind-force resisting system.”

See ASCE 7-05 6.5.8

Pressure Coefficients, C

The pressure coefficients are based on

The enclosure category of the structure

The location on a structure for which a pressure is to

be computed.

The pressure coefficients have been determined

experimentally from wind tunnel studies done on

regular shaped structures

The coefficient represents the ratio between measured pressure and the computed basic velocity pressure.

C' P measured1

DV2

A building that is neither open nor partially enclosed.

See ASCE 7-05 6.2 & 6.5.9

Location of Pressure

ASCE 7 provides means for computing forces on

various surfaces.

The building envelope surfaces experience pressure

on both sides (i.e external and internal).

Internal Pressure Coefficients, GCpi

Internal pressure is fairly easy because the air is

relatively stagnant and the shape of the structure

does not affect it’s magnitude.

As gusting is not a concern internally, the gust factor and the pressure coefficient are combined.

GCpi

The magnitude of the internal pressure coefficient is strictly dependent on the enclosure classification.

The pressure can be both positive or negative (i.e

suction) depending on the direction of the wind

relative to opening for partially enclosed or enclosed buildings.

Both internal pressures must be considered.

See ASCE 7-05 6.5.11.1 & Figure 6-5

Internal Pressure

External Pressure Coefficients, Cp

As external surfaces are subject to “flowing” air, the pressure varies considerably on the building surface depending on structural configuration and direction of the wind.

Coefficients also depend on whether the resulting

forces are to be used to design/analyze:

Main Wind-Force Resisting Systems

Structural elements that support large areas exposed

to the wind

Components & Cladding

Structural elements that support small areas exposed

to the wind

See ASCE 7-05 6.5.11.2 & Figures 6-6, 6-7, and 6-8

Buildings with Roofs Consisting of Flat Surfaces

ASCE 7-05 Figure 6-6 gives the external

coefficients of wall and roof surfaces

See ASCE 7-05 Figure 6-6

Buildings with Roofs Consisting of

Flat Surfaces – Wall Cp

Wall pressure depends on whether the wall is

Same regardless of building plan dimensions

See ASCE 7-05 Figure 6-6

Buildings with Roofs Consisting of

Flat Surfaces – Roof Cp

Dependent on direction of wind relative to

ridge

Coefficients are given for various conditions Interpolation is used to find values of

conditions between those given.

See ASCE 7-05 Figure 6-6

Wind Normal to Ridge

Wind NORMAL to ridge

Values given for different

building height to length ratios and roof slope angles.

Windward roof surfaces

Can be both positive and negative on some slopes Both need consideration as separate load cases.

Leeward roof surfaces

All negative.

See ASCE 7-05 Figure 6-6

Wind Parallel to Ridge

Domed Roofs

Pressure distributions are fairly complex.

Two load cases to be considered.

See ASCE 7-05 Figure 6-7

Arched Roofs

Pressure coefficient depends on rate of rise

of the arch

Pressure varies by along the arch.

See ASCE 7-05 Figure 6-8

Components & Cladding

Elements of the structure that support local

peak loads need to be designed for these

pressures

The magnitude of the force is dependent on

the wind area tributary to the component

The smaller the tributary area of a component the more likely to see relatively high pressures

on their tributary areas.

Some Local Effects

Wind

around a

corner

Wind at a Corner

Uplift on Roof

Images from FEMA Multi Hazard Seminar

Typical

Roof

Chart

Finding Net Pressure

The net pressure is the vector sum of the

internal and external pressures

Typical form:

p = qGCp – qi(GCpi)

Note the sign… positive pressure externally

opposes positive pressure internally (i.e they act in opposite directions)

See ASCE 7-05 6.5.12

Sample Problem

V = 120 mph

Exposure C

Enclosed