PRINCIPLES OF STRUCTURAL

ENGINERRING

Assignment by

SUHAS

AH14121

ACM-B

PGP ACM 28th Batch (2014- 2016)

NATIONAL INSTITUTE OF CONSTRUCTION MANAGEMENT

AND RESEARCH HYDERABAD

1. Explain the philosophies and design issues of Reinforced Concrete

Structures.

The design of a structure may be regarded as the process of selecting proper materials and

proportioned elements of the structure, according to the art, engineering science and

technology. In order to fulfill its purpose, the structure must meet its conditions of safety,

serviceability, economy and functionality.

a.) Strength design method:

It is based on the ultimate strength of the structural members assuming a failure

condition, whether due to the crushing of concrete or due to the yield of reinforced steel

bars. Although there is additional strength in the bar after yielding (due to Strain

Hardening), this additional strength in the bar is not considered in the analysis or design

of the reinforced concrete members. In the strength design method, actual loads or

working loads are multiplied by load factor to obtain the ultimate design loads. The

load factor represents a high percentage of factor for safety required in the design. The

ACI code emphasizes this method of design.

b.) Working stress design:

This design concept is based on elastic theory, assuming a straight line stress

distribution along the depth of the concrete. The actual loads or working loads acting

on the structure are estimated and 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 (fc') and the yield strength (fy). Because of the differences

in realism and reliability over the past several decades, the strength design method has

displaced the older stress design method.

c.) Limit state design:

It 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 local damage. According to limit state design, reinforced concrete

members have to be analyzed with regard to three limit states:

1. Load carrying capacity (involves safety, stability and durability)

2. Deformation (deflection, vibrations, and impact)

3. The formation of cracks

The aim of LSD is to ensure that no limiting sate will appear in the structural member

during its service life.

Structure to be designed for the Limit States at which they would become unfit for their

intended purpose by choosing, appropriate partial safety factors, based on probabilistic

methods. Two partial safety factors, one applied to loading and another to the material

strength shall be employed.

Design issues:

The procedure for analysis and design of a given building will depend on the type of

building, its complexity the number of stories etc.

The design should be carried so as to conform to the following Indian code for reinforced

concrete design, published by the Bureau of Indian Standards, New Delhi.

The aim of design is achievement of an acceptable probability that structures being

designed shall, with an appropriate degree of safety:

Perform satisfactorily during their intended life.

Sustain all loads and deformations of normal construction & use

Have adequate durability

Have adequate resistance to the effects of misuse and fire

Structure and structural elements shall normally be designed by Limit State Method. Where

the Limit State Method cannot be conveniently adopted, Working Stress Method may be

used. Arrangement of loads should be carefully done

It’s the combination of design dead load on all spans with full design live load on two

adjacent spans & design dead load on all spans with full design live load on alternate spans.

When design live load does not exceed three-fourths of the design dead load, the load

arrangement may be design dead load and design live load on all the spans.

For determining the moments and shears at any floor or roof level due to gravity loads, the

beams at that level together with columns above and below with their far ends fixed may

be considered to constitute the frame. For lateral loads simplified methods are used for

symmetrical structures and rigorous methods for unsymmetrical structures

Unless more exact estimates are made, for beams of uniform cross-section which support

substantially uniformly distributed load over three or more spans which do not differ by

more than 15 percent of the longest, the bending moments and shear forces used in design

may be obtained using the coefficients. For moments at supports where two unequal spans

meet or in case where the spans are not equally loaded, the average of the two values for

the negative moment at the support may be taken for design.

Where a member is built into a masonry wall which develops only partial restraint, the

member shall be designed to resist a negative moment at the face of the support of W1/24

where W is the total design load and 1 is the effective span, or such other restraining

moment as may be shown to be applicable. For such a condition shear coefficient at the

end support may be increased by 0.05.

For monolithic construction, the moments computed at the face of the supports shall be

used in the design of the members at those sections. The shears computed at the face of the

Support shall be used in the design of the member at that section except as in above cases.

When the reaction in the direction of the applied shear introduces compression into the end

region of the member, sections located at a distance less than d from the face of the support

may be designed for the same shear as that computed at distance d.

The deflection shall generally be limited to the following:

The final deflection due to all loads including the effects of temperature, creep and

shrinkage and measured from the as cast level of the supports of floors, roofs and

all other horizontal members, should not normally exceed span/250.

The deflection including the effects of temperature, creep and shrinkage occurring

after erection of partitions and the application of finishes should not normally

exceed span/350 or 20mm whichever is less.

The minimum area of tension reinforcement shall not be less than As/bd = .85/fy.

The maximum area of compression reinforcement shall not exceed 0.04 bd. Compression

reinforcement in beams shall be enclosed by stirrups for effective lateral restraint.

The transverse reinforcement in beam shall be taken around the outer most tension &

compression bars. In T-beams and I-beams, such reinforcement shall pass around

longitudinal bars located close to the outer face of the flange.

The above mentioned issues should carefully be considered when designing a reinforced

concrete structure.

2. Explain in detail about preparation of bar bending schedule along with

one example of your choice.

Preparation of highly detailed reinforcement drawing and bar bending schedule is an

essential requirement in the construction field, all over the world except in India!! With

this we can achieve a high level of quality control at site and will be advantageous in

various aspects of construction. Unlike the practice of doing reinforcement cutting and

bending in a separate workshop, as in UK and USA, based on the bar bending schedule

prepared by engineers in the design office, the practice in India has been doing

reinforcement cutting and bending at the site only which is based on the bar bending

schedule prepared at site by site engineers or supervisors. In the present construction

industry, the preparation of the bar bending schedule for the reinforcement work is by site

engineers or the supervisors which is being done in India is becoming a bad practice. Due

to lack of quality supervisors and unskilled labors in the site, our quality control in bar

bending and laying is not up to the standard. Also proper cutting of bars is not done which

results in cutting wastage .The fact is that the site engineers or the supervisors are not fully

aware of all the design considerations that was made for the project, regarding the

anchoring, curtailment and lapping of reinforcement bars and their positions, particularly

in respect of ductile detailing for earthquake-resistant structures. So these schedules are to

be prepared by the engineers in the design office only.

Bar bending schedule is used to communicate the design requirement of reinforcement

steel to the fabricator and execution team and to enumerate the weight of each size of

steel.

Generally, civil engineers who are familiar in RCC structure are employed to prepare

Rebar bending schedule. Now a days software/programs are available to prepare BBS.

When compared to common quantity take offs, preparation of Rebar schedule needs

excessive time for calculation. These calculations include cutting length, deduction of

bend allowance within cutting length and summary of weight of each size separately.

Insertion of any omitted items or revisions in the drawing may lead to repeated

calculations and consume ample time.

Advantages of preparing a BBS are:

Scheduling and proper bending is strongly recommended for Fe 500. Fe 500 saves

10% compared to Fe 415 steel used presently. Cutting and bending in a cut and

bend factory avoids the wastage completely (5-7 %). With BBS, bars can be cut

with planning to reduce the wastage in a site with even the present setup.

There is a general tendency to group slabs and beams in the usual design methods.

In BBS, it is a must to detail every member separately to account finer geometry

and different forces coming on the structure in the modern design methods.

Instead of grouping members as all members are detailed separately gives

reduction in steel as every member is individually reinforced to resist what it has

Better estimation of steel.

Real time estimation data, with the design.

Better control on stock of steel actually required.

Theft and pilferage of steel can be reduced.

Economical order quantity for better project management

Bench marking quantity and quality requirements.

Optimize your design based on the quantity of steel.

Steel bending and cutting can commence even before the form work is done.

Steel bending can be done at a separate site, marked and then can be assembled at

site, if there is space limitations.

Project time can be reduced as the bars can be cut and bend before form work is

done.

What you see in the drawing is what you get at the site.

With a quality data set, other management software (ERP systems) can work on

it.

A paperless office concept in the construction industry and associated advantages.

Total length of bars calculated using Engineering formula, leaves nothing to

approximation.

Mechanization of bending and cutting is possible. (Cut and bend systems)

Reduces labor and time but increases the reliability.

As the works gets organized, smaller contractors can work on the project at lesser

rate.

Fig 1.1 : Calculation of lengths of different bars

Fig 1.2 The bar bending schedule was prepared from data collected during my summer internship in Cyberlife,

Aparna constructions (Beam of tower D). It was prepared in excel and is imported to this pdf.

3. Explain in detail about the build ability concept and give one example.

Buildability or "Constructability" has been used and evolved in the construction

management in the late 1970’s in United Kingdom, but its potential was not been fully

exploited in construction industry at the time.

Buildability is increasingly become an integral part of the construction industry in many

countries because it was a technique used to manage the construction process during the

pre-construction stage.

Buildabilty is the extent to which the design of a building facilitates ease of construction.

The second major aspect of research into buildability is that of heuristic design principles.

These are rules of thumb about the design of the building that an architect or building

designer might employ in order to ensure the good buildability of a project. Several

researchers have produced sets of such principles, usually from analysis of case studies of

buildings that achieved good buildability in comparison with case studies of buildings with

poor buildability. Different researchers have developed their principles in different ways

but there is much common ground in these proposed strategies. These strategies cover

issues such as access, timing, skill levels, repetition, tolerances and sequences in their study

of the construction industry, identified seven general principles of buildability:

•Carry out thorough investigation and design.

•Plan for essential site production requirements.

•Plan for a practical sequence of building operations and early enclosure.

•Plan for simplicity of assembly and logical trade sequences.

•Detail for maximum repetition and standardization.

•Detail for achievable tolerances.

• Specify robust and suitable materials

Buildable design often has a direct implication on quality achievement in a project. Projects

with better quality performance (as measured by CONQUAS- Construction Quality

Assessment System) invariably are also those that had adopted good buildable designs.

Examples of such projects, include commercial, residential, institutional and mixed

developments like The Esparis, Monterey Park, Savannah Park, The Pier, Icon, One Marina

and ITE Simei.

The “examples” from residential projects that adopted good design concepts such as flat

plate slab, curtain wall, drywall partitions, prefabricated bathrooms, screed-less floor, etc.

All these buildable systems facilitate ease of construction leading to good quality

workmanship

1st example in buildability--Flat plate slab:

The floor slab has no intermediate or secondary beams. This improves the construction

cycle time and productivity. The quality of the slab surface is also better as there are less

joints in the system formwork. The M&E system can be accommodated easily under the

slab since there are no intermediate beams causing obstructions.

2nd example in buildability-- Drywall partitions:

Drywall partitions have many advantages compared to conventional wet trade partitions.

Quality finish, speed of construction, ease of installation and reconstruction are the key

attractions.

3rd example in buildability-- Prefabricated bathrooms:

Bathrooms are prefabricated in a factory and installed on site. This innovative method

results in better tolerances and the workmanship is significantly better than conventional

bathroom construction. The off-site production often makes the manufacture of the

bathroom no longer a critical activity that may affect other construction works on site.

4th example in buildability-- Screed-less flooring:

A combination of suitable adhesives and a leveled concrete floor slab makes the system

practical. Using this method, screeding, one of the “messy” trade in flooring installation,

can be eliminated. Hollowness in flooring, which is caused mainly by incompatibility

between substrate and screed, can also be minimized.

4. Discuss about Economics of Structural design

Construction, like all businesses, is driven by time and money. During the early planning

stages of a project, the budget is determined, design team chosen and preliminary building

plan laid out. It is imperative to include all design team members when decisions

concerning the building layout are concerned.

Potential for cost control is determined by the choice of structural system and its layout

during the early planning of a project. Through diligent cost evaluation, the structural

design engineer is a key person who can save the client time and money.

Construction economy is almost always best served when there is a high repetition and

modularity in the original building layout. Other factors that affect the cost of the structural

system include framing direction, total number of structural members, and connection

selection.

As design professionals, as a minimum standard, we conform to applicable building codes

to determine building member sizes. As a result, the most economical member size is

chosen based upon required strength, material availability, serviceability guidelines, and

fabrication and construction considerations. The lightest structure does not necessarily

result in the most economical design.

We no longer build buildings like we used to, nor do we pay for them in the same way.

Buildings today are...life support systems, communication terminals, data manufacturing

centers, and much more. They are incredibly expensive tools that must be constantly

adjusted to function efficiently. The economics of building has become as complex as its

design.

Every owner wants a cost-effective building. In many respects the interpretation is

influenced by an individual's interests and objectives, and how they define "cost-effective".

Is it the lowest first-cost structure that meets the program?

Is it the design with the lowest operating and maintenance costs?

Is it the building with the longest life span?

Is it the facility in which users are most productive?

Is it the building that offers the greatest return on investment?

While an economically efficient project is likely to have one or more of these attributes, it

is impossible to summarize cost-effectiveness by a single parameter. Determining true

cost-effectiveness requires a life-cycle perspective where all costs and benefits of a given

project are evaluated and compared over its economic life.

In economic terms, a building design is deemed to be cost-effective if it results in benefits

equal to those of alternative designs and has a lower whole life cost, or total cost of

ownership For example, the HVAC system alternative that satisfies the heating and cooling

requirements of a building at the minimum whole life cost, is the cost-effective HVAC

system of choice. Components of the whole life cost include the initial design and

construction cost, on-going operations and maintenance, parts replacement, disposal cost

or salvage value, and of course the useful life of the system or building.

As most projects are authorized/funded without a means of increasing budgets, it is

essential that the project requirements are set by considering life-cycle costs. This will

ensure that the budget supports any first-cost premium that a life-cycle cost-effective

alternative may incur. Once a budget has been established, it is essential to continually test

the viability of its assumptions by employing cost management throughout the design and

development process. An aspect of cost management is a cost control practice called Value

Engineering (VE). VE is a systematic evaluation procedure directed at analyzing the

function of materials, systems, processes, and building equipment for the purpose of

achieving required functions at the lowest total cost of ownership.

In addition to first costs, facility investment decisions typically include projected cost

impacts of, energy/utility use, operation and maintenance and future system replacements.

At the beginning of each project, establish what economic tools and models will be used

to evaluate these building investment parameters. The methodologies of life-cycle cost

analysis (LCCA) will typically offer comparisons of total life-cycle costs based upon net

present values. Other methods usually used as supplementary measures of cost-

effectiveness to the LCCA include Net Savings, Savings-to-Investment Ratios, Internal

Rate of Return, and Payback.

The objective of a LCCA is to determine costs and benefits of design alternatives to

facilitate informed decision-making. Costs can be more readily quantified than benefits

because they normally have dollar amounts attached. Benefits are difficult because they

often tend to have more intangibles. In some cases, these non-monetary issues are used as

tiebreakers to quantitative analyses. In other instances, non-monetary issues can override

quantitatively available cost comparisons, for example, renewable energy application.

These cost-effectiveness principles serve as driving objectives for cost management

practices in the planning, design, construction, and operation of facilities that balance cost,

scope, and quality. Analyzing the environmental costs through Life Cycle Assessment

(LCA) can be complementary to the dollar cost implications of the design, materials

selection, and operation of buildings. The LCA methodology, which can enhance

information gleaned from an LCC, includes definition of goal and scope, an inventory

assessment, life-cycle impact assessment, and interpretation-an iterative process.

The above stated principles, if used can minimize the cost associated with the structure.

5. Explain how prestresssed concrete is alternative solution in economizing

the structural design and discuss about pre tensioning and post

tensioning techniques.

Prestressed concrete is a method for overcoming concrete's natural weakness in tension.

Prestressing tendons (generally of high tensile steel cable or rods) are used which produces

a compressive stress that offsets the tensile stress that the concrete compression member

would otherwise experience due to self–weight and gravity loads. Traditional reinforced

concrete is based on the use of steel reinforcement bars, rebar, and inside poured concrete.

Coming to the economics pre-stressing offers direct cost reduction over conventionally

reinforced slabs primarily by reducing concrete and rebar material quantities as well as

rebar installation labor. Typically, savings between 10%–20% in direct cost are achieved.

Followings are the factors which contribute to direct cost reduction:

Less concrete material

Reduction in slab thickness reduces total building height and cost

Less rebar

Less labor cost for installation of material

Reduced material handling

Simplified formwork leads to less labor cost

Rapid reuse of formwork leads to less formwork on jobsite

70 % less rebar

13 % less concrete

Elimination of all Hurdy Blocks

Unified structural slab system

Beams & drop caps were deleted, simplifying slab installation

25% less formwork

3 months shorter construction program

15% savings in site overhead and plant

As a rule, the break even mark between conventional and prestressed solutions is approx.

7m spans. In a typical slab with spans over 7 meters, the net savings in material cost can

range between 10%–20% of original RC alternative.

Hence using prestressing technique is much beneficial economically.

Prestressing can be accomplished in two ways: pre-tensioned concrete and post-tensioned

concrete.

a.) Pre-tensioned concrete:

Pre-tensioned concrete is cast around already tensioned tendons. This method produces

a good bond between the tendon and concrete, which both protects the tendon from

corrosion and allows for direct transfer of tension. The cured concrete adheres and

bonds to the bars and when the tension is released it is transferred to the concrete as

compression by static friction.

However, it requires stout anchoring points between which the tendon is to be stretched

and the tendons are usually in a straight line. Thus, most pretensioned concrete

elements are prefabricated in a factory and must be transported to the construction site,

which limits their size. Pre-tensioned elements may be balcony elements, lintels, floor

slabs, beams or foundation piles. An innovative bridge construction method using pre-

stressing is described in stressed ribbon bridge.

b.) Post-Tensioned Concrete

Bonded post-tensioned concrete is the descriptive term for a method of applying

compression after pouring concrete and the curing process (in situ). The concrete is

cast around a plastic, steel or aluminum curved duct, to follow the area where otherwise

tension would occur in the concrete element. A set of tendons are fished through the

duct and the concrete is poured.

Once the concrete has hardened, the tendons are tensioned by hydraulic jacks that react

against the concrete member itself. When the tendons have stretched sufficiently,

according to the design specifications (see Hooke's law), they are wedged in position

and maintain tension after the jacks are removed, transferring pressure to the concrete.

The duct is then grouted to protect the tendons from corrosion. This method is

commonly used to create monolithic slabs for house construction in locations where

expansive soils (such as adobe clay) create problems for the typical perimeter

foundation. All stresses from seasonal expansion and contraction of the underlying soil

are taken into the entire tensioned slab, which supports the building without significant

flexure.

Post-stressing is also used in the construction of various bridges, both after concrete is

cured after support by false work and by the assembly of prefabricated sections, as in

the segmental bridge. The advantages of this system over unbonded post-tensioning

are:

Large reduction in traditional reinforcement requirements as tendons cannot

de-stress in accidents.

Tendons can easily be 'weaved' allowing a more efficient design approach.

Higher ultimate strength due to bond generated between the strand and

concrete.

No long term issues with maintaining the integrity of the anchor/dead end.

Unbonded Post-Tensioned Concrete differs from bonded post-tensioning by providing

each individual cable permanent freedom of movement relative to the concrete. To achieve

this, each individual tendon is coated with a grease (generally lithium based) and covered

by a plastic sheathing formed in an extrusion process. The transfer of tension to the concrete

is achieved by the steel cable acting against steel anchors embedded in the perimeter of the

slab.

The main disadvantage over bonded post-tensioning is the fact that a cable can de-stress

itself and burst out of the slab if damaged (such as during repair on the slab). The

advantages of this system over bonded post-tensioning are:

The ability to individually adjust cables based on poor field conditions, e.g.,

shifting a group of 4 cables around an opening by placing 2 to either side).

The procedure of post-stress grouting is eliminated.

The ability to de-stress the tendons before attempting repair work.