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Chapter 1: Types of structures and loads

Introduction

Fundamental of Structural Theory

Classification

Loads

Structural design

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Introduction

A structure refers to a system of connected parts used to support a

load....

Structure

Design of structures

Safety

Esthetics

Serviceability

Environment

Economy

Analysis of structures

Strength

Rigidity

Idealization of structures

Physical model

Mathematical model

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Fundamental of structural theory

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Classification of structures

Structural Elements

Tie rods: Structural members subjected to a tensile force are

often referred to as tie rods or bracing struts.

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Beams: are usually straight horizontal members used primarily

to carry vertical loads.

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Columns: Members that are generally vertical and resist axial

compressive loads are referred to as columns.

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Types of structures

Trusses:

Cables and arches:

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Frames: Frames are often used in buildings and are composed

of beams and columns that are either pin or fixed connected.

Surface structures: A surface structure is made from a

material having a very small thickness compared to its other

dimensions.

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Loads

The design loading for a structure is often specified in codes.

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Dead loads:

Dead load consist of the weights of the various structural

members and the weights of any objects that are

permanently attached to the structure.

The densities of typical materials used in construction are

listed in Table 1-2, and a portion of a table listing the

weights of typical building components is given in table 1-

3.

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15Dead load = 4.72 + 0.48 + 12.375 = 17.575 KN/m

Example: The floor beam is used to support

the 2m width of a lightweight plain concrete slabhaving a thickness of 10 cm. The slab serves as a

portion of the ceiling for the floor below, and

therefore its bottom is coated with plaster.

Furthermore, a 2.5m height, 30 cm thick light

weight solid concrete block wall is directly over

the top flange of the beam. Determine the loading

on the beam measured per meter of length of the

beam.

0.24

16.5 12.375 kN

0.48 kN

Solution

See tables

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Live loads:

Live loads can vary both in their magnitude and location.

They may be caused by the weights of objects temporally

placed on a structure, moving vehicles, or natural forces.

Various types of live loads will now be discussed.

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For some types of buildings having very large floor areas, many codes will

allow a reduction in the uniform live load for a floor, since it is unlikely that the

prescribed live load will occur simultaneously throughout the entire structure

at any one time.

(37.2) square meters, equal to four

0

57.425.0 L

AL

I

0

1525.0 L

AL

I

(FPS units)

(SI units)

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A two-story office building has interior columns that are spaced 7 m

apart in two perpendicular directions. If the (flat) roof loading is 100

kg/m2 determine the reduced live load supported by the spread-

footing foundation. Assume the ground floor is a slab on grade.

Problem 1-2

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SOLUTION: ANSI-based US Code

The roof loading is 100 kg/m2 (given)

At = (7 m)(7 m) = 49 m2

FRoof = (100 kg/m2)(49 m2) = 4.90 T

For the second floor, the live load is taken

from table 1-4: Lo = 250 kg/m2

Since AI = 4At = 4(49 m2) = 196 m2 > 37.2 m2 , the live load can be reduced.

Thus,

250494

57.425.0

4

57.425.0

57.425.0 00

L

AL

AL

tI

2/144250575.0 mkgL

The load reduction = (0.575)L0 > (0.50)L0 O.K. Therefore,

FFloor = (144 kg/m2) (49m2) = 7.044 t

Floor:

Roof:

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Ground floor:

For the ground floor, the live load is taken from table 1- 4:

Lo = 250 kg/m2 . No live load reduction is allowed.

FGround floor = (250 kg/m2)(49 m2) = 12.25 T

The total live load supported by the foundation is thus

F = FRoof + FFloor + FGround floor = 4.90 + 7.044 + 12.25 = 24.19 T

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Example 1-2b

A eleven-story office building has interior columns that are spaced 7 m apart

in two perpendicular directions. If the (flat) roof loading is 100 kg/m2 and floor

loading is 250 kg/m2 determine the reduced live load supported by a typical

interior footing using the US code.

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SOLUTION

For the US code based on ANSI:

At = (7 m)(7 m) = 49 m2

For the second floor, the live load is taken

from table 1- 4: Lo = 250 kg/m2.

Since 4At = 4(49 m2) = 196 m2 > 37.2 m2 , the

live load can be reduced. Thus,

250494

57.425.0

4

57.425.0

57.425.0 00

L

AL

AL

tI

2/144250575.0 mkgL

The load reduction here is (0.575)L0 > (0.40)L0 O.K. Therefore use 0.575 for all.

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Bridge Loads

12 ton truck 21 ton truck

For highway c the Association of State and Highway Transportation Officials

(ASHTO) Specification gives the expression for the impact factor (I) as

In which L is the length in meter of the portion of the span loaded to cause the

maximum stress in the member under consideration.

3.01.38

24.15

LI

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Wind Loads

Wind Pressure for Building

External Pressure

Internal Pressure

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External Pressure

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External Pressure: Formulation

p = q G C

Where, q = basic pressure at the height of 10 m

q = 0.613 KzKztKdV 2I With (q [N/m2], V [m/s])

V: the velocity of a 3 seconds gust of wind measured 10m above the

ground during a 50-year recurrence period. Values are obtained from a

wind map shown in figure 1-12.

I: the importance factor that depends upon the nature of the building

occupancy; for example, for buildings with a low hazard to human life,

such as agriculture facilities in a non-hurricane prone region, I = 0.87,

but for hospitals, I = 1.15

Kz: The velocity pressure exposure coefficient, which is a function of

height and depends upon the ground terrain. Table 1- 5 lists values for

a structure which is located in open terrain with scattered low-lying

obstructions

Kzt: a factor that accounts for wind speed increases due to hills and

escarpments, for flat ground Kzt = 1.

Kd: a factor that accounts for the direction of the wind. It is used

only when the structure is subjected to combinations of loads. For

wind acting alone, Kd = 1.

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p = wind pressure

G = gust factor (0.85, typical)

C = shape factor

p = q G C

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External Pressure on Main Wind-Resisting System

q = 0.613 KzKzt KdV 2I With (q [N/m2], V [m/s])

p = q G Cp

Cp: a wall or roof pressure coefficient (see Figures below).

qz: windward wall.

qh: Leeward wall.

33Cp: See figure 1-13 (textbook)

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Internal Pressure: Enclosed Building*

Internal Pressure: Partially Open Building*

* Reference: ASCE 7-98

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Wall pressure coefficient, Cp.

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Maximum negative roof pressure coefficient, Cp, for use with qh.

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Design Wind Pressure for Signs.

Here:F= qzGCfAf

If a structure is classified such that, the distance from

the ground to the bottom edge must be equal to or

greater than 0.25 times the vertical dimension, the wnd

will produce a resultant force acting on the face of the

sign which is determined from:

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Hydrostatic and Soil Pressure

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Snow loads:Like wind, snow loads in the ASCE 7-02 Standard are generally determined

from a zone map reporting 50-year recurrence intervals of an extreme snow

depth.

If a roof is flat, defined as having a slope of less than 5%, then the pressure

loading on the roof can be obtained by modifying the ground snow loading,

pg, by the following empirical formula:

Pf = 0.7CeCtIpg

Here:

Ce: exposure factor which depend upon the terrain. For example, for a fully

exposed roof in an unobstructed area, Ce = 0.8, whereas if the roof is

sheltered and located in the center of a large city, then Ce = 1.3

Ct: a thermal factor which refers to the average temperature within the

building. For unheated structures kept below freezing Ct =1.2, whereas if the

roof is supporting a normally heated structure, then Ct = 1.0.

I: the importance factor as it relates to occupancy. For example, I = 0.8 for

agriculture and storage facilities, and I = 1.2 for hospitals.

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If pg ≤ 20lb/ft2 (0.96 kN/m2), then use the largest value for pf, either computed

from the above equation or from pf = Ipg.

If pg > 20lb/ft2 (0.96 kN/m2), then use pf =I(20lb/ft2).

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Structural Design

Reinforced Concrete Structures

Steel Structures

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Example 1-3b

The building shown in the figure is used for industrial purpose and is located

outside of Nakhon Ratchasima, Thailand on flat terrain. When the wind is

directed as shown, determine the design wind pressure acting on the roof

and sides of the building using the ANSI / ASCE 7-95 Specifications. Use G =

0.85.

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The basic wind speed is V = 150 km/h = 41.67 m/s , and since the building is

used for industrial purposes, the importance factor is I = 1.0. Also, for flat

terrain, Kzt = 1. Therefore,

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