Structure - Types of Structures - Definition & Classification

Definition:

A combination of members connected together in such a way to serve a useful purpose is called structure.

Types of Structure Rigid Frame Its is that type of structure in which the members are joined together by rigid joints e.g. welded joints.

Truss (Pin connected joints) A type of structure formed by members in triangular form, the resulting figure is called a truss. In truss joints are pin connected and loads are applied at joints. No shear force & bending moment are produced. Only axial compression and axial tension is to be determined while analyzing a truss Structural Members Those members that are interconnected in such a way so as to constitute a structure are called structural members. Beam Beam is a flexure member of the structure. It is subjected to transverse loading such as vertical loads, and gravity loads. These loads create shear and bending within the beam. Columns A long vertical member mostly subjected to compressive loads is called column Strut A compressive member of a structure is called strut. Beam-Column A structural member subjected to compression as well as flexure is called beam column Grid A network of beam intersecting each other at right angles and subjected to vertical loads is called grid. Cables and Arches Cables are usually suspended at their ends and are allowed to sag. The forces are then pure tension and are directed along the axis of the cable. Arches are similar to cables except hath they are inverted. They carry compressive loads that are directed along the axis of the arch. Plates and Slabs Plates are three dimensional flat structural components usually made of metal that are often found in floors and roofs of structures. Slabs are similar to plates except that they are usually made of concrete.

CONTAMINATED CONSTRUCTION SITE INVESTIGATION

Contaminated construction sites are those which possess risk to human health and environment. With the increase in demand for infrastructure development around the world and shortage of land available for the same, contaminated sites may be used for new construction projects.

The Contaminated construction site can be the result of:

Any industry in the past on current site

Wastes being dumped at site

Contamination through chemicals used for agricultural purposes

Contamination in site filling materials

Soil contamination through demolition of existing structure.

The presence of chemical contamination of soil or ground about to subside creates risk of health hazards for construction workmen.

Following are the objectives for contaminated construction site investigation:

1. To identify the types of hazards, their extent and importance for assessment of potential risks to human and environment.

2. To identify suitable remedial measures for the existing contamination hazards.

The hazards that may occur at contaminated sites are:

1. Settlement problems of ground such as ground subsidence due to decomposition, weathering and natural compaction soil, leaching and sudden collapse.

2. Obstructions from existing remains of old foundation, buried walls, pile foundations etc.

3. Radioactive substances, biological contaminations, toxic powders, asbestos, fibres, liquids, explosives etc. which can attack construction workers.

4. Fire, smoke, gases, volcanic areas, microbial reaction of organic matter, explosions from combustible materials etc. posses greater risks for construction personnel.

5. Contamination affecting the construction materials with chemical reactions, contaminated ground and ground water can affect the health of humans.

6. Polluted streams of water, aquifers, wind action on contaminated dusts etc. which may affect the health.

The site investigation for contaminated soil is not sufficient just by observation of ground surface conditions and the investigation needs to be carried out for more details on the ground conditions below the site. The extent and intensity of this investigation depends on the type and project and its magnitude, conditions of construction site and its variations.

The code of practices for contaminated soil investigation for construction should be followed as per the local applicable standards. The investigation should be properly planned and executed sufficiently to get all the data pertaining to ground conditions to minimize the risk of health hazards to workmen and damage to the environment.

Too little investigation of contaminated sites may not reveal potential hazards of construction site and extra expenditure will be required for safety, while in-depth site investigation more than required may prove to be uneconomical for construction project.

REPAIR OF REINFORCEMENT IN CONCRETE

Repair of Reinforcement in Concrete

The reinforcement repair techniques are different for mild steel and prestressing steel.

1. Mild reinforcing steel

The damaged bars may either be replaced or supplemented by additional reinforcement based on engineering judgment, the purpose of the reinforcement and the required structural strength of the member.

a) Replacement: In case it is decided to replace the bars, splicing of reinforcement with the remaining steel must be done. The lap length must be according to the provision of ACI 318 and the welding (if used) must satisfy ACI 318 and American Welding Society (AWS) D1.4 (or the codal provisions of the respective country). Butt welding is usually avoided due to the high degree of skill required to perform a full penetration weld because the back side of a bar is not usually accessible. Welding of bars larger than 25 mm may cause problems because the embedded bars may get hot enough to expand and crack the surrounding concrete. Mechanical connectors may also be used according to the code requirements.

b) Supplemental reinforcement: This alternative is selected when the reinforcement has lost cross section, the original reinforcement was inadequate, or the existing member needs to be strengthened. The allowable loss of cross-sectional area of the existing reinforcing steel and the decision to add supplemental reinforcement must be evaluated on a case-by-case basis and is the responsibility of the engineer. The damaged reinforcing bar must be cleaned and extra space is to be created by removing concrete to allow placement of the supplemental bar beside the old bar. The length of the supplemental bar must be equal to the length of the deteriorated segment of the existing bar plus a lap-splice length for smaller diameter bar on each end.

Reinforcing bars, having corrosion of their original deformations, give less bond and this factor must be considered while designing the repair of the reinforcement.

c) Coating of reinforcement: New and existing bars that have been cleaned may be coated with epoxy, polymer cement slurry, or a zinc-rich coating for protection against corrosion. The coating must have a thickness less than 0.3 mm to minimize loss of bond development at the deformations.

2. Prestressing steel

Deterioration or damage to the strands or bars can result from impact, design error, overload, corrosion, or fire. Fire may anneal cold-worked, high-strength prestressing steel. The unbonded high-strength strands may need to be detensioned before repair and retensioned after repair to restore the initial structural integrity of the member.

a) Bonded strands: Because the prestressed strand is bonded, only the exposed and damaged section is restressed following repairs. The repair procedure requires replacing the damaged section with the new section of strand connected to the existing ends of the undamaged strands. The new strand section and the exposed lengths of the existing strand must be post-tensioned to match the stress level of the bonded strand.

b) Unbonded tendons: The strands are protected against corrosion by the sheathing, corrosion-inhibiting material (commonly grease), or both. Corrosion of the end connections and the strand has been the primary cause of failure of unbonded tendons. A deteriorated portion of a strand can be exposed by excavating the concrete and cutting the sheathing. Unbonded tendons can be tested to verify their ability to carry the design load. This can be done by attaching a chuck and coupler to the exposed end of the strand and performing a lift-off test. This usually requires at least 20 mm of free strand beyond the bulkhead. If there is excessive corrosion in the strand, failure occurs and the strand must be replaced or spliced. Shoring of the span being repaired and adjacent spans up to several bays away may be required before removing or retensioning unbonded prestressed strands.

The strand is cut on both sides of the deterioration and the removed portion of the strand is replaced with a new section. The new strand is spliced to the existing strand at the location of the cuts. The repaired strand is then prestressed. Carbon fiber or equivalent systems are available to supplement the reinforcement in prestressed, post-tensioned, and mild steel reinforced structures. This system is normally glued onto the exterior surface. Unless the component being reinforced is unloaded, the strengthening system only provides reinforcement for future loadings. Fiber wrapping is commonly used for reinforcing columns, especially in earthquake zones. There are systems available that recover the dried and damaged protective barrier within the sheathing.

POOR CONSTRUCTION METHODS AND WORKMANSHIP TO AVOID

Poor construction methods and workmanship is responsible for the failure of buildings and structure. The poor construction methods and workmanship is caused due to negligence and inadequate quality control at construction site. The effects of some of the poor construction methods are discussed below:

(a) Incorrect placement of steel

Incorrect placement of steel can result in insufficient cover, leading to corrosion of the reinforcement. If the bars are placed grossly out of position or in  the wrong position, collapse can occur when the element is fully loaded.

(b) Inadequate cover to reinforcement

Inadequate cover to reinforcement permits ingress of moisture, gases and other substances and leads to corrosion of the reinforcement and cracking and spalling of the concrete.

(c) Incorrectly made construction joints

The main faults in construction joints are lack of preparation and  poor compaction. The old concrete should be washed and a layer of rich concrete laid before pouring is continued. Poor joints allow ingress of moisture and staining of the concrete face.

(d) Grout leakage

Grout leakage occurs where formwork joints do not fit together properly. The result is a porous area of concrete that has little or no cement and fine aggregate. All formwork joints should be properly sealed.

(e) Poor compaction

If concrete is not properly compacted by ramming or vibration the result is a  portion of porous honeycomb concrete. This part must be hacked out and recast. Complete compaction is essential to give a dense, impermeable concrete.

(f) Segregation

Segregation occurs when the mix ingredients become separated. It is the result of

1. dropping the mix through too great a height in placing (chutes or pipes should be used in such cases)

2. using a harsh mix with high coarse aggregate content

3. large aggregate sinking due to over-vibration or use of too much plasticizer

Fig: Seggregation of concrete

Segregation results in uneven concrete texture, or porous concrete in some cases.

(g) Poor curing

A poor curing procedure can result in loss of water through evaporation. This can cause a reduction in strength if there is not sufficient water for complete hydration of the cement. Loss of water can cause shrinkage cracking. During curing the  concrete should be kept damp and covered.

(h) Too high a water content

Excess water increases workability but decreases the strength and increases the porosity and permeability of the hardened concrete,which can lead to corrosion of the reinforcement. The correct water-to-cement ratio for the mix should be strictly enforced.

PLASTIC SHRINKAGE CRACKS & ITS PREVENTION IN CONCRETE

Plastic Shrinkage Cracks and Its Prevention in Concrete

Cracking caused by plastic shrinkage in concrete occurs most commonly on the exposed surfaces of freshly placed floors and slabs or other elements with large surface areas when they are subjected to a very rapid loss of moisture caused by low humidity and wind or high temperature or both.

Plastic shrinkage usually occurs prior to final finishing, before curing starts. When moisture evaporates from the surface of freshly placed concrete faster than it is placed by curing water, the surface concrete shrinks. Due to the restraint provided by the concrete on the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks that are usually not short and run in all directions. In most cases, these cracks are wide at the surface. They range from a few millimeters to many meters in length and are spaced from a few centimeters to as much as 3 m apart.

Preventing Plastic Shrinkage Cracks in Concrete

Plastic shrinkage cracks may extend the full depth of elevated structural slabs. Since cracking because of plastic shrinkage is due to a differential volume change in the plastic concrete, successful control measures require a reduction in the relative volume change between the surface and other portions of the concrete. There are many methods and techniques to prevent this type of crack in case of rapid loss of moisture due to hot weather and dry winds. These methods include the use of fog nozzles to saturate the air above the surface and using plastic sheeting to cover the surface between the final finishing operations. In many cases, during construction it is preferable to use wind breakers to reduce the wind velocity; sunshades to reduce the surface temperature are also helpful. Additionally, it is good practice to schedule flat work after the walls have been erected.

FACTORS AFFECTING CONCRETE MIX DESIGN STRENGTH

Factors that affects the concrete mix design strengths are:

Variables in Mix Design

A. Water/cement ratio

B. Cement content

C. Relative proportion of fine & coarse aggregates

D. Use of admixtures

A. Water/cement ratio

Water to cement ratio (W/C ratio) is the single most important factor governing the strength and durability of concrete. Strength of concretedepends upon W/C ratio rather than the cement content. Abram’s law states that higher the water/cement ratio, lower is the strength of concrete. As a thumb rule every 1% increase in quantity of water added, reduces the strength of concrete by 5%. A water/cement ratio of only 0.38 is required for complete hydration of cement. (Although this is the theoretical limit, water cement ratio lower than 0.38 will also increase the strength, since all the cement that is added, does not hydrate) Water added for workability over and above this water/cement ratio of 0.38, evaporates leaving cavities in the concrete. These cavities are in the form of thin capillaries. They reduce the strength and durability of concrete. Hence, it is very important to control the water/cement ratio on site. Every extra liter of water will approx. reduce the strength of concrete by 2 to 3 N/mm2and increase the workability by 25 mm. As stated earlier, the water/cement ratio strongly influences the permeability of concrete and durability of concrete. Revised IS 456-2000 has restricted the maximum water/cement ratios for durability considerations by clause 8.2.4.1, table 5.

B. Cement content

Cement is the core material in concrete, which acts as a binding agent and imparts strength to the concrete. From durability considerations cement content should not be reduced below 300Kg/m3 for RCC. IS 456 –2000 recommends higher cement contents for more severe conditions of exposure of weathering agents to the concrete. It is not necessary that higher cement content would result in higher strength. In fact latest findings show that for the same water/cement ratio, a leaner mix will give better strength. However, this does not mean that we can achieve higher grades of concrete by just lowering the water/cement ratio. This is because lower water/cement ratios will mean lower water contents and result in lower workability. In fact for achieving a given workability, a certain quantity of water will be required. If lower water/cement ratio is to be achieved without disturbing the workability,cement content will have to be increased. Higher cement content helps us in getting the desired workability at a lower water/cement ratio. In most of the mix design methods, the water contents to achieve different workability levels are given in form of empirical relations.

Water/cement ratios required to achieve target mean strengths are interpolated from graphs given in IS 10262 Clause 3.1 and 3.2 fig 2. The cement content is found as follows: –

Thus, we see that higher the workability of concrete, greater is cement content required and vice versa. Also, greater the water/cement ratio, lower is the cement content required and vice versa.

C. Relative proportion of fine, coarse aggregates gradation of aggregates

Aggregates are of two types as below:

a. Coarse aggregate (Metal): These are particles retained on standard IS 4.75mm sieve.

b. Fine aggregate(Sand): These are particles passing standard IS 4.75mm sieve.

Proportion of fine aggregates to coarse aggregate depends on following:

i. Fineness of sand: Generally, when the sand is fine, smaller proportion of it is enough to get a cohesive mix; while coarser the sand, greater has to be its proportion with respect to coarse aggregate.

ii. Size& shape of coarse aggregates: Greater the size of coarse aggregate lesser is the surface area and lesser is the proportion of fine aggregate required and vice versa. Flaky aggregates have more surface area and require greater proportion of fine aggregates to get cohesive mix. Similarly, rounded aggregate have lesser surface area and require lesser proportion of fine aggregate to get a cohesive mix.

iii. Cement content: Leaner mixes require more proportion of fine aggregates than richer mixes. This is because cement particles also contribute to the fines in concrete.

CONSTRUCTION MATERIALS MANAGEMENT

Construction Materials management can be defined as "the function responsible for the coordination of planning, sourcing, purchasing, moving, storing and controlling materials in an optimum manner so as a pre-decided service can be provided at a minimum cost". By another definition, "materials management can be said to be that process of management which coordinates, supervises and executes the tasks associated with the flow of materials to, through, and out of an organization in an integrated fashion".

Lee and Dobler define materials management as, "a confederacy of traditional materials activities bound by common idea – the idea of an integrated management approach to planning, acquisition, conversion, flow and distribution of production materials from the raw material state to the finished product state."

From the above definitions, it is clear that the scope of materials management is vast. It has, directly or indirectly, impact on the activities of many related departments in the organization. Broadly, following can be identified as its main functions:

Based on the sales forecast and production plans, the materials planning and control is done. This involves estimating the individual requirements of parts, preparing materials budget, forecasting the levels of inventories, scheduling the orders and monitoring the performance in relation to production and sales.

Purchasing

This includes selection of sources of supply, finalization of terms of purchase, placement of purchase orders, follow-up maintenance of smooth relations with suppliers, approval of payments to suppliers, evaluating and rating suppliers.

Stores and Inventory Control 

This involves physical control of materials, preservation of stores, minimization of obsolescence and damage through timely disposal and efficient handling, maintenance of stores records, proper location and stocking. Stores is also responsible for the physical verification of stocks and reconciling them with book figures. The inventory control covers aspects such as setting inventory levels, ABC analysis, fixing economical ordering quantities, setting safety stock levels, lead time analysis and reporting.

                                    Fig: Construction Materials Management

IMPORTANCE OF CONSTRUCTION MATERIALS MANAGEMENT

The fast developing Indian economy has placed before the materials manager a tremendous challenge and responsibility. In many organizations, materials form the largest single expenditure item. An analysis of the financial statements of a large number of private and public sector organizations indicate that materials account for nearly 60% of the total expenditure. The information on the average materials expenditure for different industry groups is shown in Table 1.

                 Table 1 : Average Material Cost as Percent of Total Cost

Percentage of Total Cost	Industry Groups
Above 75	Construction, fabrication, electrodes, tea etc.
65 – 75	Wool, sugar, jute, cotton, yarn, commercial vehicles, earth moving equipment, scooters, furniture etc.
55 – 65	Cotton textile, bread, ship building, cables, electricity generators, refrigerators, heavy machinery etc.
45 – 55	Chemicals, cement, pharmaceuticals, electronics, paper, engineering, non-ferrous type machine tools, explosives etc.
35 – 45	Fertiliser, steel, cigarettes, transportation, asbestos, news print, newspapers, ferrow alloys, aircraft manufacturing.

Thus, the importance of materials management lies in the fact that any significant contribution made by the materials manager in reducing materials cost will go a long way in improving the profitability and the rate of return on investment. Such increase in profitability, no doubt, can be affected by increasing sales. But with the increased competition in the market, this alternative is not very easy to achieve.

Besides, some increase in the profitability can be achieved by concentrating on the materials cost which is typically a major rupee item for most organizations. In fact, as market pressure intensifies, organizations will be forced to cut down the costs and here, the materials management steps in to play its role.

Since materials form major part of total cost, these offer a very good scope for reduction of total cost. A small percent in materials cost can result in large percent increase in profitability.

Consider, for example, a small company has total sales of Rs. 1000. Total cost is Rs. 900. Thus, the profit is Rs. 100 which amount to 10% of the sales. Suppose, out of total cost of Rs. 900, materials cost is Rs. 600. Now if one percent saving in materials cost can be achieved, then the resultant saving is Rs. 6 (1 percent of 600) which directly adds to the profit, thus, profit becomes Rs. 106.

Therefore, in this case, we can see that 1 % saving in materials cost results into 6% increase in profit.