Durability of structure , Factors affecting durability of Building , Requirements for durability, Exposure Condition

DURABILITY - a durable steel structure is one that platform satisfactorily the desired function in the working environment under anticipated exposure condition during its service life, without deterioration of the cross-sectional area and loss of strength due to corrosion.
the material used, the detailing, fabrication, erection and surface protection measures should all address the corrosion protection and durability requirements.
                                   Requirements for Durability
As per Cl. 15.2.1 of IS-800-2007 Shape, Size ,Orientation of Members Connections and details
The design, fabrication and erection details of exposed structures should be such that good drainage of water is ensured. standing pool of water , moisture  accumulation and rundown of water for extended duration shall be avoided.
the details of connections should ensure that:
a) All exposed surfaces are easily accessible for inspection and maintenance;
b)All surfaces, not so easily accessible are completely sealed against ingress of moisture.
As per Cl. 15.2.2 of IS-800-2007 Exposure Condition
The general environment , to which a steel structure is exposed building its working life is classified into five levels of severity.
Sn.          Environmental classifications                  Exposure Conditions

i)  mild                            Surfaces normally protected against exposure                                                          to weather or aggressive condition as in interior of                                                building, except when located in coastal areas

ii) Moderate                               Structural Steel  Surfaces:

                                                   a) exposed to condensation and rain   

                                                  b)Continuously under water

                                                 c) exposed to non-aggregate soil/underwater

                                                 d) Sheltered from saturated salt air in coastal areas

iii) Severe                    Structural Steel  Surfaces:

                                              a) exposed to severe frequent rain

                                              b) exposed to alternate wetting and drying

                                               c) severe condensation

                                               d)Completely immersed in sea water

                                                e) exposed to saturated salt air in coastal area

iv) very severe                      Structural Steel  Surfaces exposed to:

                                                                     a) sea water spray

                                                                      b) corrosive fumes

                                                                      c) aggressive subsoil or groundwater

v) Extreme                              Structural Steel  Surfaces exposed to:

                                                           a) tidal zones and splash zones in the sea

                                                           b) aggressive liquid or solid chemicals

                                   
                        Factors affecting durability of the Building/ Structure 

factors that affect the durability of the buildings under condition relevant to their intended life are listed below.
a) Environment
b) Degree of exposure
c) Shape of the member and structural detail
d) Protective measure
e) Ease of Maintenance

Camber/ Cross Fall,Geometric Design and Alignment of Camber or Cross Fall

Camber/ Cross Fall
The reason for that is because most roads have a camber to them that helps water drain off of them rather than pooling up in the center of the road. 
The camber is any curve on a surface, and in this case refers to upward curve from the edge of a road towards the center.

As per IRC SP:73:2015 Clause; 2.8.1 - The crossfall on straight sections of road carriageway, paved shoulders and paved
portion of median shall be 2.5 percent for bituminous surface and 2.0 percent for cement
concrete surface.

 

As per IRC SP:73:2015 Clause; 2.8.2 - The cross fall for earthen shoulders on straight portions shall be at least 0.5 percent steeper than the slope of the pavement and paved shoulder subject to a minimum of 3.0 percent. On super elevated sections, the earthen portion of the shoulder on the outer side of the curve shall be provided with reverse crossfall of 0.5 percent so that the earth does not drain on the carriageway and the storm water drains out with minimum travel path.

As per IRC SP:73:2015 Clause; 2.8.3 -The two-lane roads shall be provided with a crown in the middle. On horizontal curves, the carriageway shall be super elevated.

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Geometric Design and Alignment
As per IRC SP:73:2015 Clause; 2.9.1 -Geometric design shall conform to IRC:73 except as otherwise indicated in this Manual. While designing the horizontal alignment, the following general principles shall be kept in view:
i) Alignment should be fluent and it should blend well with the surrounding topography.
ii) On new roads, the curves should be designed to have largest practical radius,
but in no case less than ruling value corresponding to ruling design speed.
iii) As a normal rule, sharp curves should not be introduced at the end of long tangent since these can be extremely hazardous.
iv) The curves should be sufficiently long and they should have suitable transitions to provide pleasing appearance.
v) Reverse curves shall be avoided as far as possible. Where unavoidable,sufficient length between two curves shall be provided for introduction of requisite transition curves.
vi) Curves in the same direction, separated by short tangents known as broken back curves, should be avoided as far as possible.
vii) To avoid distortion in appearance, the horizontal alignment should be coordinated carefully with the longitudinal profile.
viii) Hair pin bends on hilly terrain should be avoided as far as possible.

Total Station Survey,Total Station, Electronic Tachometer (ET)

In field survey, use of electronics-based instruments is now so widespread that it would be difficult to imagine any contemporary site surveying without it.
 The recent applications of electronics in surveying instruments have enabled surveyors to collect and process field data much more easily and to a higher precision than is possible using routine instruments.

Definition of Total Station

 A total-station is an optical instrument used as a primary contrivance for modern surveying.
 It is a combination of an electronic theodolite (transit), an electronic distance meter (EDM) and software running on an external computer known as a data collector.
 When these instruments are combined and interfaced with EDMS and electronic data collectors, they become total-stations or electronic tacheometers (ET).

Methodology

With a total-station one may determine horizontal and vertical angles together with slope distances from the instrument to points to be surveyed.

 With the aid of trigonometry and triangulation, the angles and distances may be used to calculate the coordinates of actual positions (X, Y, and Z or northing, easting and elevation) of surveyed points, or the position of the instrument from known points, in absolute terms. These are operated using a multi-function keyboard which is connected to a microprocessor built into the instrument.
 The microprocessor in the total-station can not only perform a variety of matnematical operations-for example, averaging multiple angle measurements, averaging muitiple distance measurements, calculation of rectangular coordinates, calculation Slope corrections, distances between remote points, remote object elevations, atmospheric and instrumental corrections but in some cases, can also store observations directly using an internal memory.
  Many total-stations also enabled with a GPS interface.
GPS technology has advantageously been used in total-stations.
 The use of GPS enhances the capability of a total-station as the line of sight is not required between points to be measured, and as compared to a traditional total-station, high precision for the measurement is enhanced especially in the vertical axis compared with GPS. These reduce the consequences of each technology's disadvantages, ie, GPS for poor accuracy in the vertical axis and lower accuracy without long occupation periods, and total-station which requires line of sight observations and must be set up over a known point or within a line of sight of two or more known points.


Modern Technology

Most modern total-station instruments measure angles by means of electro-optical scanning of extremely precise digital bar-codes etched on rotation glass cylinders or discs within the instrument.
 The best-quality total-stations are capable of measuring angles down to 0.5 arc-second.
The low-cost construction-grade total-stations can generally measure angles up to 5 or 10 arc-seconds. Measurement of distance is accomplished with a modulated microwave or infrared carrier signal, generated by a small solid-state emitter within the instrument's optical path, and bounced off of the object to be measured.
 The modulation pattern in the returning signal is read and interpreted by the onboard computer in the total-station.
 The distance is determined by emitting and receiving multiple frequencies, and determining the integer number of wavelengths to the target for each frequency.
Most total-stations use a purpose-built glass Porro prism as the reflector for the EDM signal, and can measure distances out to a few kilometers, but some instruments are reflectorless, and can measure distances to any object that is reasonably light in color, out to a few hundred meters.
 The  typical total-station EDM can measure distances accurate to about 3 millimeters or 1/100th of a foot. Moreover, some modern total-stations are 'robotic' allowing the operator to control tne instrument from a distance via remote control. This eliminates the need for an assistant staff member to hold the reflector prism over the point to be measured. 
The operalor holds the reflector him-herself and controls the total-station instrument from the Observed points. Though a number of companies are manufacturing total-stations, to acquaint the reader, Leica TCA 1800 and Nikon C-100 total-stations. 

Materials for Formwork, Types of Formwork , Advantages of steel for over timber form-work

Materials for Formwork
formwork can be made out timber, plywood,steel precast concrete or fibre, glass, used separately or in combinations.
the type of material to be used for formwork depends upon the nature of construction as well as the availability and cost of material.
for small works involving less number of re-uses, timber formwork proves economical.
fibre-glass is used mainly for making moulds for repetitive castings of precast concrete products.
moulds made up of precast concrete, fibre glass and aluminium are used in cast-in-situ construction such as waffle slabs or members involving curved surfaces.

Types of Formwork
1.Timber Formwork
it is used for the formwork should satisfy the following requirements.
(i) it should be well seasoned.
(ii) it should be light in weight .
(iii) It should be easily workable with nails without splitting.
(iv) It should be free from knots.

• the sizes of timber sections for different components of form-work depend upon the span of the slab or beam, floor to floor height and the centre spacing of centering supports.
• The size of timber planks or joists that can be adopted for different components of formwork for shuttering of 4.5 m span and 3.5 m height.
• for normal construction work where repetitive use of shuttering is possible , the quantity of timber shuttering can be worked out on the assumption that one set of shuttering can be used 10 to 12 times.

                         Table.1 Sizes of members for timber formwork

S.No.     Components of formwork              Size                                                                                                                (Varying according to the spacing of            centering props from 1 metre to 1.2 metres.)

(i.) Sheeting for slabs, beam and           25 mm to 40 mm--- thick

column side and beam bottom

(ii.)Joists, ledges                                50 x 75 mm to---cross section 50 x 150 mm

(iii.) Posts                                             75 x 100 mm. to 100 x 100 mm. .....

(iv.) Ballies                 Not less than 100 mm at mid length and 80 mm at thin end

2. Plywood Formwork

Use of plywood instead of timber planks is getting popular these days.

In this case resin bonded plywood sheets are attached to timber frames to make up panels of required sizes.

the panels thus formed can be easily assembled by bolting in the form of shuttering.

this types of shuttering ensures quality surface finish and is specially recommended in works where large exposed areas of concrete are to be constructed such as floor slabs faces of retaining walls etc.

it may prove to be cheaper in certain cases .

(i) By use of large size panel it is possible to effect saving in the labour cost of fixing and dismantling.

(ii) No of re-uses are more as compared with timber shuttering. for estimation purposes, number of re-uses can be assumed as 20 to 25 .

(iii) it is possible to have perfectly plain and smooth surface (without joint marks) by use of plywood shuttering. thus expenditure on surface finishing can be saved.

3.STEEL FORMWORK

this consists of panels fabricated out of thin steel plates stiffened along the edges by small steel angles.

• The panel units can be held together by two or more clamps pr bolts provided along each edge and the shuttering can be assembled and kept in alignment by use of horizontal or vertical centring of timber or steel.
• The panels can be fabricated in large number in any desired modular shape or size.
• The usual size for wall or slab panel varies from 60 cm x 60 cm to 60 cm x 120 cm. 
• this types of shuttering is considered most suitable for circular or curved shaped structures such as tanks, Columns , chimney etc.and for structures like large sewer, tunnels,and retaining walls.

Advantages of steel for over timer form-works 

 (i) Steel forms are stronger, more durable and have longer life as compared with timber forms. 

(ii) They can be put to sufficiently larger number of te-uses. For estimation purposes the number of reuses can be assumed to vary from 100 to 120. 

(ii) Steel forms can be installed and dismantled with greater ease and speed which results in saving in labour cost for this item of work. 

(iv) The quality of exposed concrete surface obtained by use of steel forms is excellent and it needs no further treatment. 

On the other hand construction carried out by use of timber formwork invariably requires plastering to obtain desired finish of the concrete surface. Thus there is saving in the cost of finishing the surface by use of steel forms.

 (v) There is no danger of the formwork absorbing water from the concrete and hence the chances of honeycombing are minimised.

 (6) They are not liable to shrink or distort and hence it is possible to achieve better workmanship and higher degree of accuracy by use of steel forms.   

Economy in Form-work, Economy of shuttering , Economy of Centering, Effect economy of in the cost of form-work

Economy in Form-work

it may be noted that the total cost of concrete construction includes the cost of form-work as well.
Construction of form-work involves considerable expenditure in terms of cost of material, cost of labour for fabrication, erection and removal of form-work time element.
in case of buildings, the cost of form-work may vary range between 30 to 40% of the cost of concrete work.
in case of special structures like bridges, tall chimney, dams etc. the cost of form-work may range between 50 to 100% of the cost concrete work, or even more.

Following steps should be followed to effect economy in the cost of form-work .
(i) The Building should be planned in such a way that there are minimum number of variations in the size of rooms, floor area etc. so as to permit re-use of the form-work moulds repeatedly.
(ii) The Scheme of the form-work should be efficiently planned and suitably designed to determine the most economical but safe sizes of different components including the supports or props.
(iii) the Form-work should be constructed in such a way that timber (where used) is cut to the minimum and it can be struck off with ease an in such  works  re-use with least damage.

(iv) In case the form-work is made out of rough timber and is not constructed properly, the resultant concrete surface will be irregular and full of defects.     

  The expenditure involved in rectification of defects in such works is invariably more than the saving made in the cost of form-work.

Form work & A good requirements of form work, Satisfy Condition of Form work

Form work - Centering, Shuttering or form-work is a sort of temporary construction provided for laying cast-in-situ concrete to required shape.


A good form-work should satisfy the following requirements.

(i.) It should be strong enough to withstand all types of dead and live loads such as self weight,weight of reinforcement , weight of wet concrete,loads due to workmen, construction equipment, other incidental loads and forces caused by placement and consolidation of concrete, imposed upon it during and after casting of concrete. 
(ii)  It should be rigidly constructed and efficiently propped and braced(both horizontally and vertically) so as to retain its shape without undue deflection.
(iii) The joints in the form-work should be tight against leakage of cement grout.
(iv) The form-work should be constructed in such a manner that it may permit the removal of various parts is desired sequence without jarring of damaging the concrete.
(v) The material of the form-work should be cheap, easily available and should be suitable for re-use several times.
(vi) The form-work should be set accurately to the desired line and levels and should plain surfaces.

(vii) The form-work should be as light as possible.
(viii) The material of form-work should not wrap of get distorted when exposed to sun, rain or water during concreting.
(ix) The form-work should rest on firm base.

Embedment length and Development of Length of Rienforcement

Embedment length
The length of embedded steel reireinforcement, 1 provided beyond a critical section.
The fibre length also defines the embedment length. This is especially important for thick, short hooked end steel fibre and low strength concrete.
A slippage of the reinforcement at the beam-column interface is observed because of the accumulation of the rebar strain along the embedment length in the beam-column connection.
Therefore, it is important to understand the proper embedment length of the longitudinal reinforcement considering the interface behavior between the infilled concrete and the steel casing in the Cast In Steel Shell pile.
Development Length
A development length can be defined as the amount of reinforcement(bar) length needed to be embedded or projected into the column to establish the desired bond strength between the concrete and steel (or any other two types of material)

Fig 1: Development length in Footing
Reason for providing Development length

• To develop a safe bond between the bar surface & the concrete so that no failure due to slippage of bar occurs during the ultimate load conditions.
• Also, the extra length of the bar provided as development length is responsible for transferring the stresses developed in any section to the adjoining sections (such as at column beam junction the extra length of bars provided from beam to column).
• where less development length against the required is provided the structures will be prone to encounter failure due to slippage of joints, bonds, anchors & Laps, in such cases the bars will not yield first but the failure will happen at joints & laps prior to yielding of reinforcement bars.

Fig 2: Development length as per IS 1786
Calculation of Development Length

Where
Ø = nominal dia of reinforcement bar
s= Stress in bar at the section considered at design load
bd= Design bond stress
The above given formula is used to calculate the required development length in mm for any given dia of bar, same formula is used for limit state method as well as working stress method.
The only change in calculation in both methods is due to the different value of design bond stress; the values of design bond for Limit State & working stress are as follows;
Table No 1: Design Bond Stress in Limit State Method
Design Bond Stress in Limit State Method
Concrete Grade
M20- 1.2
M25-1.4
M30-1.5
M35-1.7
M40 and above-1.9
(For Plain Bars in Tension
Design Bond Stress )
bd,N/mm2)
M20- 1.92
M25-2.24
M30-2.4
M35-2.72
M40 and above-3.04
(For deformed bars in tension)
(Design Bond Stress bd,N/mm2)
Footnotes-
Development Length of Reinforcement Bars