There has been a debate on which material should be predominantly used in construction. Steel, on one hand, has been regarded to be susceptible to rust while reinforced concrete seems to resume its natural state of sand and gravel when overstressed. The two materials have been in competition but basically, some of the factors that determine which material will be used include the market conditions, the architecture of the building, span of the building and the local construction codes among others.
However, steel has proved very reliable for construction of buildings of more than 20 stories because of the many positive attributes associated with it (McCormac & Brown, 2014). For one, steel is very light and hence can be erected in sites where the soil conditions do not favor the use of other construction material like concrete. This is because steel tends to form a very light frame in comparison to the other materials. Secondly, the time factor is very inclined towards steel usage. In this, steel structures can be erected easily and very fast in comparison to other structural materials. The third advantage of steel in construction is the fact that it can be prefabricated which means it can save on the labor costs as well as time. Finally, steel is very predictable and therefore measures can be put in place for quality control. The main disadvantages of steel frames include: the cost of steel is basically higher than that of concrete or masonry, extra measures such as fire protection need to be implemented on the building structure and the building design need to consider separate elements for the floors and walls.
A typical building is made up of a framework which transmits the loads from the various structural elements to the foundation. It is from the foundation where the loads are dissipated into the ground. Moreover, the framework is made up of various structural elements such as beams, columns, bracing, trusses among others. These separate elements are linked in a precise way that enables the loads to be transferred from the topmost floors to the ground level. It is also very important to note that the foundation also plays a significant role in the stability of the building and as such, it is a very important structural element. Basically, the design depends on the bearing capacity of the soil. Soils with a lower bearing capacity are unsuitable for frames that exert too much weight while soils with higher bearing capacities are suitable for various loads.
In the design and construction of any steel building, there are various structural elements that need to be considered. All these ensure the smooth transfer of loads from the roofs, floors and the other load components to the foundation and eventually to the soil. The first element in any steel structure are the girders, which like beams carry the lateral loads that are imposed on the building by bending moments and shear forces. The second structural elements are the ties which are used to carry the axial loads that are usually imposed on the building during tension. The third bracket is composed of structural members whose basic function is to transmit axial loads. Axial loads are usually imposed when the building is in tension but the members may also be subjected to bending moments. Members serving the function include columns, struts, and stanchions.
The framed members of steel building basically include trusses and girders whose main function is to protect the building from lateral loads. Other members include purlins and sheeting rails. The former refers to beams that transmit the loads from the roof sheeting to the column while the later are beams that are used to effectively transmit the forces and loads obtained from the wall cladding. Finally, there is the bracing system basically designed for steel buildings. This bracing system is composed of ties and struts that are arranged in a horizontal format. In this, their basic function is to ensure that the building is in a stable state regardless of the wind loads that it is subjected to.
The types of frames for steel structures
There are various configurations that can be used for steel structures. Basically, the frame design is based on aspects such as structural support and load transfer.the most common types are: single story rigid portal, medium rise braced multi-story building and single story lattice roof building (McGinley & Ang, 1993).The three stated types can be used for different forms of buildings such as school, offices etc.
Fig 1: description of the different forms of frames used in steel buildings and the elements
However, the design and determination of an appropriate frame system is a complicated process that basically depends on the structural engineer’s specifications and design. Basically, the conceptual design stage in the design of such a building basically focuses on: the loading arrangement of the building, the loading at the critical points such as in the braces, girders, frames floors etc., the design of connections and structures and the arrangement and detailing from the engineer’s point of view. It is a very important stage in the design.
The floor systems that are designed for the steel frames fall into 3 categories: precast or prestressed concrete slabs, reinforced concrete slabs which are designed in situ and metal floors which are composite in nature (Arya, 2009). The precast floor is basically designed for spans which are about 6 to 8 meters and is made up of hollow planks which have been initially prestressed.The in-situ concrete slabs are designed when there is an increased need for stability. Therefore, they are designed to achieve composite action with the steel beams of the floors. Finally, the composite slabs are designed using a decking which is made of both steel and concrete
Stabilization of steel frames
The stability of any steel building depends on the type of supporting mechanism employed. As a matter of fact, a steel frame is not very strong against forces such as wind rain snow etc. and would buckle and sway if there was no supporting detail. There are four methods of stabilizing steel buildings namely: rigid core, diagonal bracing, beam to columns connections and shear walls. However, they are all designed per the unique site and building conditions and may be used in combination or individually.
To begin with, the rigid core may be centrally placed within the building structure and its main purpose is to resist torsion because of various lateral forces. The central core may be made of concrete or masonry and is designed in a vertical orientation in the form of stairs or elevator shaft. On the other hand, diagonal bracing is designed to prevent the building from lateral loads by using diagonal ‘X’ or ‘K’ bracing systems. However, this system may interfere with the external facade of the building and may interfere with the window system designed.
The moment to beam connection is very intensive in terms of labor. The system employs the bolt and connection system to members which may be vulnerable to moment forces. Therefore, the rigidity, as well as the stability of the building, is enhanced by putting in place extra connections. Finally, shear walls are designed so as to prevent the building from failing under the lateral loads. The system is basically made up of walls that use the cantilever properties to ensure the building is stable. However, they may be in the interior or the exterior parts of the building and as such, may affect the window system as well as the facade of the building
The design of the footing system of any building depends on the loading of the building as well as the bearing capacity of the soil. Soils with low bearing capacity such as clay tend to be used with foundations that go deeper into the earth’s crust (valley, 2009). On the other hand, soils that have a higher bearing capacity tend to be designed in a simpler manner.
The design methodology for any footing system begins with the issue of settlement. It has been widely noted that failure in most foundations is attributed to excessive settlements rather than the inability of the foundation to transmit the load into the ground (valley, 2009). However, a more critical scenario may occur when the settlement is not uniform over the whole length of the foundation (Schmertmann, 1991). Therefore, the service loads that are used in the preliminary design of the footing should limit settlement. Furthermore, the size of this footing is designed such that it meets the serviceability criteria as well as the bearing capacity. However, it is important to consider that shallow foundations are designed to meet the flexural demands.
Eccentricity in loading is a very important design consideration in foundations. Eccentricity has the effect of producing varying moments on the footing and if this is not considered in the preliminary stage, there is likely to be structural failure. However, the main concern should be the overturning stability of the foundation
There are various types of foundations basically designed to meet the ground conditions as well as the structural requirements of the building. Therefore the design has to consider several factors such as loads to be transferred from the superstructure to the soil, building codes of the locality in which the building is to be designed, the soil properties and the geological conditions. It is very important to consider that the soil properties have to take in place the stress and strain factors. Furthermore, the design has to consider the possibility of building pits for high rise buildings. Geotechnical knowledge is very important because the design of a foundation has to be based on a number of assumptions about the soil conditions as well as knowledge on the same. More to this is the fact that soils may not have uniform properties throughout the construction site.
There are two categories that any foundation may fall into a pad foundation or a deep foundation. Generally, the pad foundation is designed for soils which have a relatively higher bearing capacity while deep foundations are designed for soils with a relatively lower bearing capacity. As matter of fact, if the soil stratum at the upper parts of the ground is suitable to sustain a structure, the foundation of choice is usually pad and if a suitable soil stratum is located at a deeper depth, the type of foundation selected is usually the deep one. However, deep foundations and shallow foundations basically differ because of the difference in the ratio of the depth of foundation to the width of the base. The ratio is between 0.25 and 1 for shallow foundations while it is greater than 5 when it comes to deep foundations (Terzaghi, Peck, & Mesri, 1996).
Depending on the site conditions, there is an array of shallow as well as deep foundations that can be selected. The first type of spread foundation to be considered is the pad or footing foundation. The foundation is generally designed to meet the load requirements of columns. However, they can either be rectangular or circular in shape. As in our case of a steel frame, a steel grillage is necessary when the column is heavily loaded. The foundation may be individual or combined where the former refers to a footing holding a single column while the latter is a footing holding a number of columns. All this depends on the proximity of the columns to each other as well as the soil conditions.
The second type of spread foundation is the continuous beam. Generally, this is a modification of the footing foundation but it is specially designed for columns that are very close to each other. Furthermore, they serve the purpose of load transfer from bearing walls as well as the columns.
Thirdly, there is the wide strip foundation generally suitable for soils with very low bearing capacity. In essence, the base of the foundation is subjected to lateral loads and may be subject to cracking and therefore reinforcement is required. Finally, there is the raft foundation. This type of foundation is also necessary for soils with very low bearing capacity or where the columns, closely spaced, are arranged in all direction.
These are foundations designed for soils that do not have a suitable bearing capacity. Basically, the foundation goes deep into the earth’s crust, beyond the soil of low bearing capacity, into the bedrock where the soil is of suitable bearing capacity. It may be designed in areas where the immediate soils may be cotton soils. Deep foundations may be divided into three: pier, deep shaft and caisson foundations. In the design of pile foundation, the structural members used are piles which are basically made up of concrete or steel. However, this design and construction may cost more than that of shallow foundation but: may be necessary for structural safety in the following conditions: when the soil layers located on the upper surface can be easily compressed and therefore unable to offer structural support to building members, when the building is subjected to horizontal forces and therefore may be subjected to bending moments, and when the site conditions indicate the presence of expansive soils.. The second type of deep foundation is the deep strip which basically serves the same functions as the pile foundation and the only distinction arises from the methods employed in installation. In this, the deep strip is installed by excavating while the pile installed by driving it into the ground. Finally, there is the caisson foundation which is extensively used in areas where there is need to prevent water seepage into a working area. As a matter of fact, the caisson is a shell that descends to the base of the foundation basically becoming part of it.
The design of the foundation
The design of a foundation is a very intense method that involves soil conditions and the analysis of various moments and forces. However, the two methods used in the design of a foundation are the permissible state method and the limit state design method. Basically, the permissible state method is the oldest and most commonly used alongside the British standards. The method focuses on the most intense and unfavorable conditions that can be applied on the ground .in essence, the soil and ground conditions are safe when the permissible state is not exceeded. The downside of using this procedure for the design is the unaccounted settlement which may lead to structural collapse or failure. Therefore, the design has to consider a factor of safety to cater for the settlement that may arise. The factor of safety has to be considered alongside a number of factors such as the rates of loading, the variability in material properties, the ground conditions and the presence of groundwater etc.
The limit state design method focuses on ensuring the foundation does not reach the limit states and therefore unlikely structural failure. In this, there is the ultimate limit state and the serviceability limit state. The former caters for building collapse while the latter is centered on deformations and other building defects.
External walls and cladding
By definition, cladding is the attachments to the structure of the building, but on a non-structural aspect. The need for cladding as well as external walls cannot be overemphasized, especially in steel buildings. In the United Kingdom, panel walls are very common in buildings (Arya, 2009). It is worthwhile to note that panel walls are basically non-load bearing but are supported on a number of sides. These walls serve the purpose of resisting lateral loads as well as providing a desirable facade for the building. When the cladding is used on the roof system, the function is usually centered on wind loading as well as insulation. As a, matter of fact, the cladding provides a transfer mechanism of the wind loading to the beams and eventually to the columns. Therefore, the system used for cladding and external finishes should take the transfer mechanism into consideration.
In the analysis of the design loads for a building, the external walls and the cladding are classified as dead loads. However, the cladding system may be extended to the external walls. The external walls may include bricks or mortar and basically, serve the purpose of increasing the insulation properties of the building. In this regard, the external walls may be clad with tiles. Plastics and even timber boards. Other functions of cladding include; increasing the privacy of the building, increasing the security of the building, reduction on the effects caused by a fire break out, sound protection etc.
Types of cladding systems
Depending on the nature of the building, there are various types of cladding systems that can be employed. However, all this should be as per specifications such as the air leakage, the compatibility of the system, other attachments, the strength, and durability, detailing, condensation among others. Some of the cladding systems are explained below.
To begin with, there are the sandwich panels which basically provide an efficient insulation system. In this, there are two layers which have a central core. However, the sandwiching materials should be very rigid while the sandwiched material should be lightweight. The second type is the rain screen. The system is basically made up of two walls and serves the purpose of preventing water leakage into the building. However, the thermal insulation, as well as the stability of the building, is also enhanced when this system is used. Moreover, because of the price of timber and its availability, the timber cladding system can be regarded as the most popular. Basically, the system employs timbers that are softwood in nature but further checks such as defects should be conducted to ensure that the timber can be used. However, hardwoods, as well as thermowoods, can be used for this cladding system. Other cladding systems include brick slips, uPVC, patent glazing, curtain walling etc.
Building services and other design aspects
For the best outcomes from a building structure, other considerations need to be looked into. Some of the considerations include the lighting systems, heating systems, safety routes etc. All these serve the purpose of improving the comfort levels as well as the safety levels of the building. Besides all these amenities, the building layout has to consider all the aspects of movement and ease of escaping when a hazard arises.
The design consideration for the building structure
Wolver Hampton is an area where the predominant soils are loamy (Cranfield University, 2017l). However, more site properties can be conducted through boring tests on a number of locations that the site engineer specifies. As in our case, the borings need to be about 100-150’ apart as this is the standard for a building where the columns are to be closely spaced (Cranfield University, 2017l). The soils may be classified as fine-grained and as such, may require compaction prior to the design and installation of the foundation. Basically, the type of foundation in this scenario will depend on the bearing capacity as obtained from the engineer’s test.However, a raft foundation may be applicable because of the number of columns necessary and the proximity to each other.
A gambrel frame would be a necessity for this type of building structure. The frame is very useful in areas that have limited space or in buildings where the columns are closely spaced. Furthermore, the roof system associated with this type of frame is aesthetically pleasing. As a matter fact, the frame structure is very compatible with the in-situ composite floor system to be used for the structure. A composite floor system, as described, is made up of concrete floors and steel beams. However, this floor system will require finishes such as tiles while the topmost floor will require a ceiling dead load. Finally, the building may employ sandwich cladding because of the excellent insulation properties as well as the protection against winds and other loads.
The design of such a building basically requires extensive design for the structural as well as the supporting elements. All these have been detailed in codes of building and practice such as the British standards. The outline for the various designs to be used during construction has been widely covered and as such, the design process has been simplified.
In the above design, the foundation selected has been based on the soil properties alone. However, other factors such as the seismicity of the region as well as groundwater conditions need to be considered. However, Wolver Hampton is a region that does not experience seismic activities and as such, this factor may be overlooked. In essence, the site conditions, as well as the preliminary design, will determine the best and suitable size and form of the foundation.
Regardless of all the above factors, the design has also to consider the current building technologies and services. A major development in the recent past has been on the development of green building whereby technology focuses on the reduction in emissions. However, all this should be in line with the codes of practice and design as per the locality.
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