Types of Foundations
The foundation is the lowest part of a structure that is in contact with the ground. It defines the resilience and strength of a structure as well as its durability. A lot of consideration and attention is required in choosing and setting up the most suitable type of foundation based on the nature of the ground and water levels. Foundations are broadly categorized depending on the load to be carried and the depth of the soil on which a structure is to be constructed. These are shallow foundations and deep foundations.
Shallow Foundations
It is the most common and basic form of foundation. Shallow foundations are preferred because they are highly economical to erect. It is most suitable in locations with a low ground level. Shallow foundations are further subdivided into four different classes depending on the shape of the base that is used to support the building’s construction. These are isolated spread footing, combined footing, wall footing, and raft footing.
Isolated Spread Footing
It is a straightforward and widely recognizable type of foundation. The isolated spread footing foundation is common in simple establishments to transmit and dissipate concentrated pressures exerted by columns or pillars. It is very economical and mostly applicable to a building of up to five stories. In general, the foundation lies at the base of the segment and every section has its footing. The foundation directly transfers the load from the column to the soil in a square, rectangular, or circular form. The material can either be reinforced or non-reinforced; however, if reinforced, the status of the ground floor has to be firm enough to allow for the stronger spread of the load. They cannot be used in heavier loads and the size of the base is estimated by dividing the total load at the column base by the permissible carriage capacity of the soil.
Combined Footing
The combined footing foundation resembles the isolated footing type in that it has two or more columns placed close to each other, or the individual isolated footings overlay each other. It has a rectangular footing and the loads from the building are carried by the columns. Basically, it involves a combination of several footings with different types, such as beam type, slab, slab type, raft, rectangular, and strap beam type. They may be tee-shaped, squared, or trapezoidal. The main goal is to allow for a uniform distribution of loads under the whole area of the footing. This helps to overlap the center of gravity of the total loads with the center of gravity of the footing area.
Wall Footing
It runs along the orientation of the wall, but its width is two to three times the width of the wall. The wider base helps to provide more stability to the structure. It is also called a continuous, spread, or strip footing foundation that comprises a fixed slab strip along the length of the embankment. The foundation is erected using bricks, stones, or reinforced concrete. It is preferred when the ground is laced with dense sand and rocks and the load to be conveyed is of a small volume. It is not suitable on floors with a ground flow of water above the bearing layer of soil as it may cause liquefaction or scour.
Raft Foundation
Also called mat foundation, it is spread across the whole ground area of a structure to support large loads from walls and columns. It is used in places where the carriage capacity of the soil is weak, but the load has to be cast over a vast area, or the structure is exposed to continuous jerks or shocks. It comprises a tee-beam slab or reinforced concrete slab positioned over the entire expanse of the building such that the basement floor slab acts as the foundation. It helps to spread the total load of the structure evenly over the whole building area. The structure appears to be floating on soil, that’s why it fits areas with weak soil where other foundations are not feasible. However, it is also not suitable on floors with a ground flow of water above the bearing layer of soil as it may cause liquefaction or scour.
Deep Foundations
Deep foundations entail sinking a structure’s foundation deep into the earth to enable the building to sit directly on the rocky soil. The foundations are driven into the hard stratum and are most reliable in watery areas. The depth of the foundations makes it suitable to support heavy loads above the ground level. There are two main variants of deep foundation: pile foundation and caisson foundation.
Pile Foundation
Pile foundation is best suited for soils where the soil formation near the ground surface is not desirable for hefty loads. The weight is transferred by piles deep down to the hard rock strata with a depth of up to fifty meters. The foundation withstands the loads from the building by end bearing and skin friction. It also curbs differential settlement of foundations, which enables the piles to support a heavier load with greater stability. The piles are majorly constructed using concrete and sand but can also be made of wood.
Caisson Foundation
Also called drilled shafts, this type of deep foundation functions with the use of piles of high-carrying capacity cast-in-situ foundations. It withstands loads from a structure through toe resistance, shaft resistance and/or a mixture. They are constructed using an auger and are made up of watertight hollow cylindrical columns. It is best suited for very deep depths of hard strata located between ten meters to 100 meters below ground level. It is recommended in the construction of deepwater operations, such as ship repairs, concrete dams, or bridge piers in rivers. However, they are not suitable when deep residues of soft clays and detached coarse soil exist.
Recommended Foundation
The case study involves a structure spanning 4,500 square meters on the ground with a high water level and bedrock of up to three meters from the ground. The presence of water eliminates the use of shallow foundations. Therefore, the pile foundation is best suited to the structure as it is economical when soil with considerable carriage capacity is at a greater depth. It is also preferable to use rafts despite their wide area coverage because of the marshy ground. The inclusion of the roadway also makes it suitable for piles to support the heavyweight.
Alternative Types of Frame Solutions
Frame solutions refer to an assortment of columns, beams, and slabs to withstand gravity and lateral loads. In building construction, there are a variety of frame structures that can be used to support the edifice. The framing solutions are broadly categorized into two main types. These are rigid frame solutions and brace frame solutions, which are as well further subdivided into other types. The varying types of frame solutions can be shaped using varied materials such as steel, reinforced concrete, and wood. The framing solutions help to subdue the large movements developing as a result of pressure exerted by the applied load.
Rigid Frame Solution
It is also termed as a moment frame solution that comprises linear elements like columns and beams. The name rigid is derived from the frame’s capacity to resist any deformation through the use of reinforced concrete and steel beams and columns. The solution is characterized by the absence of buckled joints within the frame and generally is statically indeterminate. The solution can support a structure to resist lateral and vertical loads by the bending of columns and beams. Its strength is earned primarily by the bending rigidity of columns and beams that are made up of rigid connections. The joints are formulated in such a fashion that they have ample stability and stiffness and are not easily deformed.
The solution is a product of detailed structural analysis using a number of techniques, such as the force method, the portal method (approximate), Castigliano’s theorem, the technique of virtual work, the stiffness method, the slope-displacement approach, and matrix analysis. The tools help to solve for moments, internal forces, and support effects. The solution is further cascaded to fix fix-ended rigid frame solution and pin-ended rigid frame solution.
The rigid frame solution is preferred for gigantic structures spanning a height of up to 300 feet above ground level, with an entirely column-free middle. This means it is the only solution of its kind with the required versatility for multi-story buildings, which practically cannot be constructed using the arched style. It is highly stable and can withstand the shear, torsion, and moment more effectively than any other kind of framing solution.
Additionally, rigid framing provides longevity to a building. It allows a structure to resist heavy loads and strong winds manifest in times of stormy weather conditions. The rigid solution is favored for creating ample storage space as opposed to the arched style that restricts a building’s available square footage, both in general space and headspace. Should anything be placed directly against the walls in an arched space, it substantially restricts the surface area.
On the contrary, the rigid solution is drawn back by the hefty amount of money required to set it up. The price per square foot is considerably on the higher side compared to arched styles largely. It is also time-consuming, considering the effort that has to be put in planning, designing, and construction. However, the pressure felt in paying the high amount and the time sacrificed is rewarded by the strength, versatility, and durability of the rigid framing solution.
Braced Frame Solution
The braced frame is made up of columns and beams that are “pin” attached through bracing to support lateral loads. The loads are supported through vertical and horizontal bracing. By design, bracing is considerably simple to put up and analyze. Type of frame is simple to analyze and simple to construct. Bracing is normally attained by positioning diagonal members between the columns and beams. A number of ways can be used in bracing, such as diagonal bracing, knee-bracing, chevron or k bracing, X bracing, and shear walls that can withstand high lateral pressures in the plane of the embankment.
The gabled frames are structured with a peak at the apex, which enables them to resist extreme weather by snow and heavy rains. Portal frames are styled like a door and are a common feature in structures, including multi-story residential, industrial and commercial buildings. Chevron bracing supports two braces framing a beam from different paths. This can either be a right brace-to-gusset connection, a left brace-to-gusset connection, or a gusset-to-beam connection. Single diagonal bracing is most suitable for vertical tension and condensation on different sides of the frames due to the bending effect.
There are several strengths for which bracing systems are used in building construction. It offers a more efficient resistance against extreme stormy weather and earthquakes. It is more effective than the rigid frame solution in resisting the buckling of the main beam in the building. Bracing helps to distribute the lateral loads as well as vertical loads between the main beams equally and reduces lateral displacements. It is incredibly easy to install, which makes it cost-effective. Bracing is flexible to design to enable the building to get the mandatory strength and stiffness.
However, the bracing solution has several weakness that make it not desirable for particular types of structures. The major drawback is that it is limited in scope as it can only be applied in structures not exceeding a vertical height of up to forty feet when reinforced. This necessitates altitude-dependent adjustments to a structure in an area characterized by seismic tensions. The system also requires keenness that can only be exercised by using skilled craftsmen to construct. Skilled labor may not be readily available and commands higher pay, which increases the construction costs.
Recommended Frame Solution
Frame structures are preferred in high-rise buildings because they afford the required versatility, durability, and stability to support a building. Hence, they are preferred as opposed to the traditional load-bearing structures. Of the two main frame solutions discussed above, the bracing solution would be the best answer to the building in the case study. Compared to the rigid framing solution, bracing is highly economical in terms of costs and time to be expended in planning and construction. Essentially, it does not require a lot of effort in designing and erecting the building. The bracing solution provides flexibility that can be applied in integrating the structure to a new roadway and modifications to the existing carriageway since it allows for future rehabilitation. Furthermore, the structure spans three floors, which is a height that can be managed under the bracing solution to enable the structure to resist the load.
Requirements of the Highway Act 1980
The Highway Act 1980 governs aspects of highways in England and Wales. Principally, the act designates bodies that qualify to be highway authorities along with their duties and powers. The majority of the matters involving buildings and highways must have a license of an agreement from the highway authority. The matters are broadly classified into three categories:
- Section 38 agreements that involve the adoption of new highways constructed as a component of development,
- Section 278 agreements that involve modifications to existing highways at the cost of the developer,
- Section 116 agreements that involve upgrading or diversion of highways (including bridleways, footways, or limited byways).
The Highway Act 1980 requires that all highway agreements should be negotiated with the highway authority for each site. The deliberations must be structured to meet the specific case at hand; hence, are likely to require a lot of time to plan and conclude. Additionally, the act stipulates different fees depending on the site and may take in the cumulative sum to cater for future maintenance expenses. The case study proposal pertains to works covering the construction of a new roadway and changes to the existing public highway. Therefore, it encompasses both section 38 agreements and section 278 agreements, respectively.
Legal Obligations and Procedures for the New Roadway
Section 38 agreements stipulate that once planning approval passes for new development, the developer is allowed to request the highway authority to ‘adopt’ the new road that will be built as part of the project. The request should also cover allied infrastructure such as lighting, drainage, and supporting structures. The act defines adoption as the consent by the highway authority to undertake maintenance work on the road from an agreed date with financing from public funds. In this case, the highway authority can either be a local highway authority or the Secretary of State for Transport.
The developer is required to have obtained planning permission as well as approval of any reserved matters before agreeing. In general, the permission will outline an indicative layout of the roads to be adopted. The Section 38 agreement is expected to contain the following elements:
- Provision for land use.
- Drawings sketching the scope of the area to be adopted.
- Details of the relevant planning permission.
- Technical drawings of the works.
- Provision for inspection and certification of the works.
- A program for the works and the adoption.
- Agreement about the adoption, or not, of allied structures.
Section 38 agreements attract a fee that is expected to cater for expenses incurred in preparing the agreement, checking designs, inspection of the works, and ongoing maintenance of items not critical for highway purposes (commuted sums). The developer may further be required to deposit a bond to shield the highway authority against the likelihood that the developer possibly will fail to fail the works as required due to challenges such as insolvency.
Furthermore, the Highway Act 1980 requires that works must be performed to a standard and design approved by the highway authority. The coat of the works is borne by the developer who will also be in charge of maintaining the road before it is adopted. The act provides for a 12-month maintenance period between the date of completion of the works and the adoption of the roadway. If the construction is undertaken in phases, adoption will transpire after the last phase is finished given that the roads may continue to be used as transport channels by the construction traffic. In general, the process of arriving at the ultimate agreement can be prolonged and consume a lot of time. Therefore, the aspiring developer needs to kick off the negotiations with the highway authority as early on in the project planning or initiation stage.
Potential Costs for the Roadway Alterations
The Highway Act 1980 provides applicable guidance to alterations to an existing highway under section 278. In general, a developer is permitted to carry out works that may require modifications to the public highway. However, the individual or organization must do so with approval from a highway authority, which may be a local highway authority or the Highways England for the strategic road network in England.
Section 278 of the agreement delineates that the developer may have to pay money for the design, construction, and other expenses required for the works to meet the requirements of the local highway authority. Alternatively, the highway authority may be required to carry out the works. The act requires that the developer may cater for all or part of the costs incurred by the highway authority if they are conducted in a fashion, at a time, or are custom-built to the developer’s wishes. Other costs that the developer has to contend with include:
- Crafting the agreement.
- Permissions linked to the works.
- Consenting to the scheme for the works.
- Land acquisition for the works.
- Constant maintenance expenses of the works (commuted sums).
- Administrative costs are borne by the highway authority.
Summary
Foundation type and framing solution are two key considerations to be keenly evaluated before setting up a structure. The foundation is chosen depending on the soil type, ground level of water, and the structure load. Out of the different types of foundations covered in the paper, the pile foundation would be the most suitable for the case. The solution suits the high ground water level and depth of three meters to the rock which will provide a firm base for the structure. On the framing solution, bracing is preferred, given that the building will span three floors. The framing solution is highly economical in time and money, it is highly flexible and easy to design. An additional factor to be considered pertains to the authority to construct a new roadway and modify the existing carriageway. The permission to undertake these activities is captured in the Highways Act 1980. Two main sections of the act apply to this case; section 38 that covers the procedure to be followed in the construction of a new roadway and section 237 that outlines the probable costs that may be incurred in modifying the existing carriageway.