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8/13/2019 One Way Joist Florr

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INTRODUCTION


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Design ObjectivesFor reinforced concrete structures, the

design objectives of the structural

engineer typically consist of the

following:1. To configure a workable and

economical structural system. This

involves the selection of the

appropriate structural types and laying

out the locations and arrangement of

structural elements such as columnsand beams.

2. To select structural dimensions,

depth and width, of individual

members, and the concrete cover.

3. To determine the required

reinforcement, both longitudinal andtransverse.

4. Detailing of reinforcement such as

development lengths, hooks, and

bends.

5. To satisfy serviceability requirements

such as deflections and crack widths

Loads

Analysis is performedon an idealized

structure .

1.Determinate

Structures.

2. Indeterminate

Structures.

3. Matrix Structural

Analysis

4. Finite Element

Analysis

Materials

Mechanics of materialsand structures

1. Steel Structures.

2. Wood Structures.

3. Bricks and Stones.

4. Concrete Structures.

5. Composite Structures

6. Reinforced ConcreteStructures.

7. Prestressed Concrete

Structures


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Design

The task of the structural engineer is to design a structure which satisfies

the needs of the client and the user. Specifically the structure should be

safe, economical to build and maintain, and aesthetically pleasing. But what

does the design process involve?Design is a word that means different things to different people. In

dictionaries the word is described as a mental plan, preliminary sketch,

pattern, construction, plot or invention. Even among those closely involved

with the built environment there are considerable differences in

interpretation. Architects, for example, may interpret design as being the

production of drawings and models to show what a new building willactually look like.

To civil and structural engineers, however, design is

taken to mean the entire planning process

for a new building structure, bridge, tunnel, road, etc., from outline

concepts and feasibility studies through mathematical calculations toworking drawings which could show every last nut and bolt in the project.

Together with the drawings there will be bills of quantities, a specification

and a contract, which will form the necessary legal and organizational

framework within which a contractor, under the supervision of engineers

and architects, can construct the scheme.


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There are many inputs into the

engineering design process as

illustrated by Fig.

1. client brief

2. experience3. imagination

4. a site investigation

5. model and laboratory tests

6. economic factors

7. environmental factors

Structural Design


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Allowable Stress Design

The design concept is based on the elastic

theory assuming a straight line stress

distribution along the depth of the

concrete section under service loads. The

members are proportioned on the basis

of certain allowable stresses in concrete

and steel. The allowable stresses are

fractions of the crushing strength of

concrete and yield strength of steel. This

method has been deleted from the ACI

Code. The application of this approach is

still used in the design of prestressed

concrete members under service load

conditions

Ultimate/Unified Strength

DesignThe unified design method (UDM) is

based on the strength of structural

members assuming a failure condition,

whether due to the crushing of the

concrete or to the yield of the reinforcing

steel bars. Although there is some

additional strength in the bars after

yielding (due to strain hardening), this

additional strength is not considered in

the analysis of reinforced concrete

members. In this approach, the actual

loads, or working loads, are multiplied by

load factors to obtain the factored design

loads. The load factors represent a high

percentage of the factor for safety

required in the design


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Alternative Design Method

A second approach for the design ofreinforced and prestressed concrete

flexural and compression members is called

the strength design method, or the

alternative provisions (ADM), as introduced

in the ACI Code, Appendix B. When this

method is used in the design, the designer

must adhere to all sections of Appendixes

Band C and substitute accordingly for the

corresponding sections of the Code.

Reinforcement limits, strength reduction

factors, load factors, and moment

redistribution are affected. The provisions

of this method satisfy the Code and are

equally acceptable

Strut and Tie MethodAn other approach for the design of

concrete members is called the strut and

tie method (STM). The provisions of this

method are introduced in the ACI Code,

Appendix A. It applies effectively in

regions of discontinuity such as supportand load applications on beams.

Consequently, the structural element is

divided into segments and then analyzed

using the truss analogy approach, where

the concrete resists compression forces as

a strut, while the steel reinforcement

resists tensile forces as a tie.


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Limit State DesignLimit state design is a further step in the

strength design method. It indicates the state

of the member in which it ceases to meet the

service requirements such as losing its ability

to withstand external loads, or suffering

excessive deformation, cracking, or local

damage. According to the limit state design,

reinforced concrete members have to be

analyzed with regard to three limiting states:

1. Load carrying capacity (safety, stability, and

durability)

2. Deformation (deflections, vibrations, and

impact)3. The formation of cracks.

The aim of this analysis is to ensure that no

limiting state will appear in the structural

member during its service life

Loads are those forces for which a given

structure should be proportioned. In

general, loads may be classified as deador live

Dead loads include the weight of the

structure (its self-weight) and any

permanent material placed on the

structure, such as tiles, roofing materials,and walls. Dead loads can be determined

with a high degree of accuracy from the

dimensions of the elements and the unit

weight of materials.

Live loads are all other loads that are notdead loads. They may be steady or

unsteady or movable or moving; they

may be applied slowly, suddenly,

vertically, or laterally, and their

magnitudes may fluctuate with time.

Loads


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Type of Live LoadsOccupancy loads caused by

the weight of the people,

furniture, and goods

Forces resulting from wind

action and temperature

changes

The weight of snow if

accumulation is probable

The pressure of liquids or

earth on retaining structures

The weight of traffic on a

bridge

Dynamic forces resulting

from moving loads (impact),

earthquakes, or blast loading

Live loads for highway bridges are specified by the

American Association of State Highway and

Transportation Officials (AASHTO) in its LRFD Bridge

Design Specifications

For railway bridges, the American Railway Engineering

and Maintenance-of-Way Association (AREMA) has

published the Manual of Railway Engineering , which

specifies traffic load

Environmental loads consist mainly of snow loads, wind

pressure and suction, earthquake loads (i.e., inertia

forces caused by earthquake motions), soil pressures

on subsurface portions of structures, loads from

possible ponding of rainwater on flat surfaces,and forces caused by temperature differentials. Like

live loads, environmental loads at any given time are

uncertain in both magnitude and distribution. ASCE

manual contains much information on environmental

loads, which is often modified locally depending, for

instance, on local climatic or seismic conditions.


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Occupancy LoadsThe minimum live loads for which the floors and roof of a building should be

designed are usually specified in the building code that governs at the site of

construction. Representative values of minimum live loads to be used in a wide variety

of buildings are found in Minimum Design Loads for Buildings and Other Structures ofAmerican Society of Civil Engineers (ASCE).

The table gives uniformly distributed live loads for various types of

occupancies; these include impact provisions where necessary. These loads are

expected maxima and considerably exceed average values.

In addition to these uniformly distributed loads, it is recommended that as an

alternative to the uniform load floors be designed to support safely certainconcentrated loads if these produce a greater stress.

Certain reductions are often permitted in live loads for members supporting

large areas, on the premise that it is not likely that the entire area would be fully loaded

at one time.

Tabulated live loads cannot always be used. The type of occupancy should be

considered and the probable loads computed as accurately as possible. Warehouses forheavy storage may be designed for loads as high as 500 psf or more; unusually heavy

operations in manufacturing buildings may require an increase in the 250 psf value

specified in Table ; special provisions must be made for all definitely located heavy

concentrated loads.


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For occupancies or uses not

designated in this book's chapter4,

the live load shall be determined in

accordance with a method

approved by the authority having

jurisdiction.


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Simple HousesFor structural members in

one and two-family dwellings

supporting more than one

floor load, the following floorlive load reduction shall be

permitted as an alternative to

Eq. 4.7-1:

L = 0.7 (Lo1 + Lo2 + )

Lo1, Lo2,

are theunreduced floor live loads

applicable to each of multiple

supported story levels

regardless of tributary area.

The reduced floor live load

effect, L, shall not be lessthan that produced by the

effect of the largest

unreduced floor live load on a

given story level acting alone.

Heavy Live Loads

Live loads that exceed 100 lb/ft2 shall not be reduced.

EXCEPTION:

Live loads for members supporting two or more floors

shall be permitted to be reduced by 20 percentLimitations on One-Way Slabs

The tributary area,At for one-way slabs shall not exceed

an area defined by the slab span times a width normal to

the span of 1.5 times the slab span.

Seismic forces may be found for a particular structure byelastic or inelastic dynamic analysis, considering expected

ground accelerations and the mass, stiffness, and damping

characteristics of the construction. However, often the

design is based on equivalent static forces . The base shear

is found by considering such factors as location, type of

structure and its occupancy, total dead load, and theparticular soil condition. The total lateral force is

distributed to floors over the entire height of the structure

in such a way as to approximate the distribution of forces

obtained from a dynamic analysis.


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Design Criteria

In achieving the design objectives, there are four general design criteria of SAFE that must be

satisfied:

1. Safety, strength, and stability. Structural systems and member must be designed with

sufficient margin of safety against failure.

2. Aesthetics. Aesthetics include such considerations as shape, geometrical proportions,

symmetry, surface texture, and articulation. These are especially important for structures of

high visibility such as signature buildings and bridges. The structural engineer must work in

close coordination with planners, architects, other design professionals, and the affected

community in guiding them on the structural and construction consequences of decisions

derived from aesthetical considerations.

3. Functional requirements. A structure must always be designed to serve its intended

function as specified by the project requirements. Constructability is a major part of the

functional requirement. A structural design must be practical and economical to build.

4. Economy. Structures must be designed and built within the target budget of the project.For reinforced concrete structures, economical design is usually not achieved by minimizing

the amount of concrete and reinforcement quantities. A large part of the construction cost

are the costs of labor, formwork, and false work. Therefore, designs that replicate member

sizes and simplify reinforcement placement to result in easier and faster construction will

usually result in being more economical than a design that achieves minimum material

Quantities.


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Design LoadsTo serve its purpose, a

structure must be safe against

collapse and serviceable in use.

Serviceability requiresthat deflections be adequately

small; that cracks, if any, be kept

to tolerable limits; that vibrations

be minimized; etc.

Safety requires that the

strength of the structure beadequate for all loads that may

foresee ably act on it.

If the strength of a

structure, built as designed, could

be predicted accurately, and if the

loads and their internal effects(moments, shears, axial forces)

were known accurately, safety

could be ensured by providing a

carrying capacity just barely in

excess of the known loads.

Uncertainty in Design1. Actual loads may differ from those assumed.

2. Actual loads may be distributed in a manner

different from that assumed.

3. The assumptions and simplifications inherent in anyanalysis may result in

calculated load effects-moments, shears, etc.-different

from those that, in fact, act in the structure.

4. The actual structural behavior may differ from that

assumed, owing to imperfect knowledge.

5. Actual member dimensions may differ from thosespecified.

6. Reinforcement may not be in its proper position.

7. Actual material strength may be different from that

specified.

Real Load = Design load x (1 + A )Real Strength = Design Strength x (1 B )

For safety real load= real strength

Design strength/design load = FOS=( 1+A)/(1-B)


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Advantages of Strength Design

1. The derivation of the strength design expressions takes into account the nonlinear shape of

the stressstrain diagram. When the resulting equations are applied, decidedly better estimates

of load-carrying ability are obtained.

2. With strength design, a more consistent theory is used throughout the designs of reinforced

concrete structures. For instance, with working-stress design the transformed-area or straight-line method was used for beam design, and a strength design procedure was used for columns.

3. A more realistic factor of safety is used in strength design. The designer can certainly estimate

the magnitudes of the dead loads that a structure will have to support more accurately than he

or she can estimate the live and environmental loads. With working stress design, the same

safety factor was used for dead, live, and environmental loads. This is not the case for strength

design. For this reason, use of different load or safety factors in strength design for the differenttypes of loads is a definite improvement.

4. A structure designed by the strength method will have a more uniform safety factor against

collapse throughout. The strength method takes considerable advantage of higher strength

steels, whereas working-stress design did so only partly. The result is better economy for

strength design.

5. The strength method permits more flexible designs than did the working-stress method. Forinstance, the percentage of steel may be varied quite a bit. As a result, large sections may be

used with small percentages of steel, or small sections may be used with large percentages of

steel. Such variations were not the case in the relatively fixed working stress method. If the

same amount of steel is used in strength design for a particular beam as would have been used

with WSD, a smaller section will result. If the same size section is used as required by WSD, a

smaller amount of steel will be required.

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