Structural Principles To Medium Rise Buildings: Footing Systems And Design

Site Specific Footing Systems

Discuss about the Structural Principles to Medium Rise Buildings.

In any kind of construction work, footing and foundation of the structure under construction is inarguably the most vital constituent of the whole design. The reason behind such importance is obviously because of the fact that the whole structure will be constructed on the foundation which will bear its total weight and grant stability to it. Several factors and conditions can lead to the creation of a poor footing system with disastrous consequences. Soil properties of different sites vary greatly irrespective of the soil profile. Site and soil investigation through geotechnical reports can be very advantageous both in large scale construction projects and in small scale or residential construction cases. In case of medium rise buildings, the total construction budget may not allow for a holistic analysis of soil properties and creation of a complete geotechnical report, however, a minimum amount of investigation should be done into understanding the type of soil, soil movement, soil profile and other key elements since the characteristics of soil must be completely and properly understood in order to create the perfect and most stable footing and foundation system with appropriate reinforcement systems (International Building Code, 2006).

Sites are classified according to a number of properties; one of the most commonly used and very important aspects of the soil is the “expected ground surface movement” and “depth to which the movement takes place”.

The soil on which the construction project is undertaken consists of very deep expansive clays which are subjected to high amounts of wetting and drying and as a result experiences surface movements which are in excess of130 mm. Thus, this site has been classified as “Extremely Reactive Sites” which are subjected to extreme ground movement of above 75 mm. due to deep seated moisture changes. Expansive soils pose a number of problems. A proper construction of a footing system actually seals the surface of the ground and prevents any kind of natural changes in moisture content of the soil from occurring, due to rail or evaporation of moisture. Now in case of expansive soils, the moisture content of the soil changes all around the perimeter of the building under construction due to either increased rainfall or drainage, a particular dishing effect will take place where as the ground under the foundation expands more than the ground around it. And in case, the soil perimeter around the foundation becomes drier than the soil underneath it, footings will gradually begin to depress as a doming effect is created due to soil contraction. This makes the whole foundation of the building comprehensively unstable (Douglas Partners, 2015).

Adjustable Stump Footing System

According to the soil geotechnical report, as stated before, the ground is subjected to extreme lateral movement due to high content of expansive clay. The report itself has been created by registered design professionals. The report has reinforced the initial assumptions that the oil is expansive in nature and soil tests are to be performed to determine the level of expansion and ground surface movement. Observations and investigations were done by exploratory soil boring and creating test pits.

Exploratory boring were done by registered personnel to assess soil constituents and accordingly classify the soil with respect to them. The report thus puts forth certain factors & provisions that clearly classify the soil to be expansive in nature and thus subjected to excessive ground surface movement. The four provisions fulfilled are:

Plasticity Index (PI) of around 17, where an index of 15 or greater proves that the soil is expansive in nature.

According to the geotechnical report, more than ten percent of the soil particles had passed through a Number 200 sieve of pored diameter of around seventy five micrometre.

Again more than ten percent of the soil particles have been found to have a diameter of less than five micrometers.

Expansion index observed has been found to be more than twenty.

The construction types selected for the development of the medium rise building was selected in accordance to the properties, characteristics and subsequent classification of the soil. Clad frame construction with construction of metal frame which is clad on the exterior by sheet materials which are not at all sensitive whatsoever to any minor lateral movement. The maximum amount of differential deflection which this particular framed construction can stand is about forty millimetres (Mishra G., 2014).

And, coming to the subject of discussion, the footing system utilized in medium rise building construction project are built of “Adjustable Stumps”. These stumps are designed in accordance to those principles of civil engineering which primarily and specifically deal with the coping mechanism of expansive soils. This particular type of footing system consists of steel stumps which are directly cast into bored piers made of concrete. On the top of the metallic stump, an adjustable top is fixed and this top can be easily adjusted after completion of all construction work and modifications, in order to make sure that the building remains at the desired level and height. The stumps can support suspended and lightweight framed flooring systems and even normal & conventional flooring systems. The concrete bored pier is typically of a diameter of around 300 to 450 millimetres and extends down to a length that surpasses the depth of seasonal moisture changes, making the footing system verifiably stable.  Reinforcement measures such as sleeves and belled bases can also be added to enhance and improve performance of the stump footing system (Mishra, 2014).

Concrete Slab Footing System

Fig 1. Adjustable Pier Footing System

Source: (Votano and Sunindijo, 2014)

Another important element of the adjustable stump footing system is the usage of bored or screwed in piers. Their usage is necessary in order to reinforce footings systems such as suspended slabs or adjustable stumps. These piers are added to improve the stability and performance of these footing systems on expansive soils. The spires are able to provide support to the footing system from a depth which is lower than that of the soil depths which are affected seasonal moisture variations (Designing Structures In Expansive Clay: A Guide For Architects And Engineer, 2005). The top portions of such piers are designed to be sleeved by a smooth pier liner, thereby allowing the soil to expand upwards around the pier with a minimum amount of influence. And in case of the bottom of the footing system, the piers provide support & anchoring in the ground from a depth lower than the influencing zone. For acquiring the best performance, the pier reinforcing system is arranged and distributed around all the footings of the building and not just along the building perimeter.

Another type of footing system utilized in the construction of the medium rise building is the concrete slab footing type. This particular footing system type has the added advantage of acting as both the flooring and the footing of the building. The flooring system is suspended and has been framed with piers and stumps. The concrete slabs are laid on the ground and forma continuous footing.

The floors of those sections of the building, where the concrete slab footing system has been utilized, are connected to the ground or the foundation using the raft slab design.  Waterproofing of the slab has been done in the form of a plastic membrane covering so as to prevent moisture absorption. The slab on ground footing system is the most commonly used footing system used in the country, as it combines all of the elements which form the base or foundation of a building. Appropriate precautions have been taken with regards to prevention of moisture absorption or damage by termites (Rajagopalan and Elkadi, 2014).

Raft slabs concrete system used in the construction project have been designed to have large beams which are moulded into a grid beneath the slab and are tied to it by reinforcement bars, so that they act as a cohesive unit. Grid beams are poured simultaneously with the pouring of the slab panels.

Fig 2. Concrete Slab Footing

Source: (Rajagopalan and Elkadi, 2014).

Wide varieties of construction materials are involved in any kind of construction project, large of small.

For earthwork purposes of the involved construction project, aggregates such as gravels are used as supports beneath slabs of concrete foundations and for drainage system beneath the ground. Coarse aggregates used are known as chips of stone or gravel whereas the finer variety of aggregates used are sand. In the project, several different kinds of coarse aggregates have been used such as “pea gravel”, “river gravel”, ”crushed stone chips” etc.

The concrete used in the construction project consist of Type 1 Normal Portland Cement and are mixed with different aggregates and admixtures such as gravel, sand to produce a fast setting and extremely hard & stable concrete mixture. The mix used is 10 percent cement, 15 percent water, 25 percent fine aggregates and 45 percent coarse aggregates with a small amount of air that gets automatically entrapped within the concrete mixture. The admixtures used in the concrete mixture consist of accelerators and water reducers.

It is an obvious fact that the  whole construction project is to be built using reinforced concrete, that is, the concrete mixture used is cast around reinforced steel bars with provide a high tensile strength to the structure in addition to high compressive strength. Number 14 bars are used in the construction project (County Of Los Angeles Department Of Public Works, 2011)t.

The brick masonry utilized in the project is kiln burned bricks along with concrete bricks.

The metals utilized in the project are mainly consisted of ferrous metals such as iron and steel (Bowles, 1998).

Lumbers and plywood have been used to construct doors & windows along with their frames, to construct panelling & moulding etc.  Surfaced lumber has been used throughout the project. The plywood utilized in the project is constituted of the same surface finished lumber and has a modular & uniform size, usable in different situations.

Finishing building materials used in the project are gypsum wallboard and plaster; ceramic tiles of 4 ¼” x 6 “; plate float glass and thermal insulation is provided using flexible & reflective insulation.

A soil excavation work plan typically consists of several sub plans & guidelines.

A sampling & analysis Plan which involves provisions of sample collection procedures and citing the requirements of field screening & also assurance & control procedures require for removal is one of the first and most important constituent of the excavation work plan and has been accordingly included.

A health and safety plan in relation to the construction work must be drawn before the excavation and removal activities are initiated. These plans are formulated in accordance to the laid down rules and guidelines of the relevant authorities.

Approved schedule plans and routine charts are another requirement of a successful excavation plan and thus added (Excavation Work Plan, 2007).

Acquirement of necessary permits from the municipality and other concerned authorities is another essential requirement for any kind of excavation and subsequent construction activity or work, which forms a part of the excavation work plan.

Profiling of waste materials and proper management of water dumping is to be properly done before execution of the excavation work plan.

Detection of any kind of underground utility such as water supply, gas supply etc. is another important constituent factor and appropriate steps are to be defined in the excavation work plan to take those utilities into consideration during execution of eth work.

All activities related to mobilization of all equipments, vehicles, personnel are described in their appropriate section in the excavation work plan.

The guidelines regarding the dumping & proper disposal of any kind of hazardous and contaminated materials obtained during excavation are described in detail in the work plan.

A provision to screen the constituents of the excavated soil has been added to the excavation report in order to detect the presence of any harmful or contagious elements.

Any kind of backfilling and compaction if necessary are done using the instruction in the work plan. Clean fill is to be done using soil from an offsite source (Environmental Resources Management, 2011).

Fig 3. Excavation of Site

Source: (Votano and Sunindijo, 2014)

The minimum amount of reinforcement in footing slab constructed in the project has been found to be around 0.12% and the maximum spacing which has been specified is around 3 times the effective depth or 450mm, the lower value been chosen.

Typically, only tensile reinforcement is provided. Furthermore, the total reinforcement shall be provided uniformly for the square footings. In case of rectangular footings, a central band is to be constructed which has a breadth equal to the width of the footing. The reinforcement in the central band which is to be provided is in accordance with the following equation.

Where, B= footing’s long side / footing’s short side (Nathaniel S. and Michael J., 1998).

Any columns foundation needs adequate amounts of strengthening as a varied number and type of additional loads are applied on it. Widening and strengthening of existing foundations are to be carried out through the construction of a pile system which will reinforce the existing footings.  The foundation and the footing are supported using piles driven into the ground.  Typically, a group of piles are used and they are topped by a pile cap, which is a large block of concrete into which all of the piles are embedded. (Dot.state.fl.us, 2016).  Grade beams are used in conjunction with the piles which help in transferring the loads from the load bearing footings onto the pile caps and into the pile support system. Grade beams also help to withstand large moments from lateral loads and help to stabilize the footing system through effective reinforcement (Foundation-repair-guide.com, 2007).

Pile support systems are able to bear higher amounts of loads than any other kind of footing support system.

Fig. 4 Pile Support System

Source: (Votano and Sunindijo, 2014)

The isolated footing system utilized in the construction project by increasing the size of the footing and the reinforcement steel bars as follows:

Excavation of the area around the footing

Proper cleaning and subsequent roughening of the concrete surface.

Installation of dowels at 25-30cm spacing in both the directions of the footing using a proper and suitable epoxy material.

Fastening all new steel bars along with the dowels by the usage of steel wires. The number and diameter of the steel bars which are to be utilized must be in accordance to the design and dimensions of the footings.

The surface of the footing is to be coated with a suitable bonding agent so that proper and sufficient amount of bonding is achieved between the old concrete and the newly added concrete.

The new concrete mixture is poured onto the new concrete before the crying of the bonding agent. Moreover, the new concrete is to contain a non shrinking admixture material (Mishra, 2014).

The advantages of using a pile system for footing reinforcements and support are numerous. Pile support system boasts several advantages such as:

• High Strength
• Fast Setting
• Long Lasting
• Increased stability
• No disruptive weight added

In some cases, the bearing area of footing is to be increased along with strengthening of the column, in such cases the additional support from the reinforcement system helps in transferring soil pressure on the extended foundation area appropriately along the foundation. Transfer of soil pressure to the existing footings which are without reinforcements is done by excavation which is properly done below the existing footing. 

Thus, reinforcement addition should make sure that the building is properly supported and any kind of settlement of the foundation has been successfully avoided (Page, 2012).

In cases where a tendency of the new concrete to split from the old concrete is observed, sufficient numbers of well anchored/welded hoops are added to avoid and completely eliminate this particular problem (Mishra, 2013).

Fig 5. Concrete Slab Footing with foundation

Source: (Page, 2012)

A number of concrete placement techniques were utilized in order to place concrete at different areas of the construction site.

Boom pump have been used in some cases to place concrete mixtures in certain area. They offer a varied number of benefits and advantages regarding the placement of concrete in walls. The reach of the boom tube helps in minimizing and in some cases outright eliminating the need & necessity for relocation of the concrete hose or pump at the site. The hose is every easy to manoeuvre all around the perimeter of the construction project and the associated automatic controls helps and assists greatly to properly manage the concrete mixture flow rate throughout the pour(American PolySteel, LLC, 2005).

Concrete conveyors were used in some cases and in other cases, concrete has been placed directly from the truck chutes of the elephant concrete mixer trucks. Concrete which has placed in its final resting or placing position is placed there before the cement reaches its initial setting state and the concrete which is thus compacted in that final position is made to do so within a time frame of around 30 minutes of leaving the mixture intact. Once the placed concrete is compacted, it is not disturbed and allowed to settle & harden accordingly (Mishra, 2015).

Concrete is placed in every section of the construction project in successive horizontal layers which are of uniform thickness that range from 150 mm to 900 mm. The concrete is to be placed as rapidly as possible and practicable; this is to prevent any kind of formation of any cold joints or planes of any weakness between each of the successive layers within the poured concrete mixture.

Fig 6. Concrete Placement using boom tubes

Source: (Love et al. 2001)

References

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