Project proposal
Introduction
Background
Different structures respond differently to the earthquake in a dynamic phenomenon that depends on the intensity, dynamics of structural properties, frequency, and duration of the earthquake. Earthquakes one of the most dangerous natural disaster that leads to devastating effect in buildings and structures. The magnitude of the effects on structures are related to either shoddy construction procedures and deliberately ignoring the possibility of an earthquake on structures during construction. Aa a precautionary measure, it is of importance to analyze and understand the possible seismic effects buildings and structures thus enabling architects and designers to develop structural plans that have detailed consideration to seismic forces on buildings, therefore, enabling prevention mechanisms in case of an earthquake or earth tremor.
When an earthquake occurs, seismic force hit structures, it generates inertia forces that lead to deformation and vertical and horizontal shaking off the building weakening the pillar joints within the government. The study aims to present the static and dynamic analysis of the steel structures in buildings concerning their behavior and strength when subjected to seismic forces. The mode and scope of analysis involve the incremental dynamic analysis and pushover analysis on the resisting steel frames. When steel frames are exposed to inelastic nonlinear time history assessment in different scaled ground motion both in the far-field and near fields. Earthquake actions are dynamic in nature and occurrence there are set building codes and standards that are recommended for use in establishing static loads that offer an earthquake –resistance buildings.
The earthquake-resistance building designs are developed with the focus on the predominant response mode thus facilitating the development of the equivalent static force that produces an equivalent corresponding mode shape with the same empirical adjustments on the higher mode effects. Static load analysis in determining seismic effects on the seismic designs is justified due to difficulties and complexities in the modes of dynamic analysis of steel structures. In the case when non-linearity in geometry and material is considered in the design of systems, the dynamic analysis of such structures becomes more complex and demanding. Nonlinearity in geometry and materials structures is essential in the design of structural elements; therefore more advanced analytical tools have been developed in the past few years with the intention of simplifying the static and dynamic analysis of the steel structures in buildings.
Steels in multi-story buildings
The use of steel frames in multistory buildings has been the trend in the construction of high rise structures. In the last few years, the use steel in building construction has had an increasing application in building and construction industry, this has been due to its numerous advantages such as compatibility and flexibility with different structural designs, safe solutions in constructions using steels and the fact that steel are4 relatively cheap as compared to other alternative metals hence more economical. Generally, steel structures are quick to assemble, slim, and efficient in construction processes. Safety is very essential in high rise buildings therefore detailed structural analysis and of slim steel structures are so fundamental since it enables adequate safety analysis of structures and efficient buildings. In some cases, structural steel frames are subjected to dynamic loads of higher magnitude that often causes deformations and permanent damage on the on these steel structure resulting in either partial or permanent damage. However, there have been some elastic mechanisms used in correcting these defects that have been so uneconomical since they consume a lot of funds and time to correct such defects. Due to high-cost expense in correcting defects, the construction standards and codes allow damage to a certain amount as long as the structural safety is not compromised.
The random baes motion occurs whenever the building is subjected to seismic forces caused by earthquake occurrence; the seismic forces are always accompanied by dynamic actions thus affecting the structural strength. Therefore, the structural response of buildings to earthquakes is a transient phenomenon. Although there is low seismicity in other countries, this action cannot always be ignored, even for small buildings. In the events of extreme loading, building structures exhibits nonlinearity in their behaviors due to the start of charging. In the analysis of steel structures used in multistory buildings, second-order effects of nonlinearity in geometry, the flexibility of the joints and connection, and most importantly the material nonlinearity in inelastic behavior. The ductility nature of steel metal is the main reason being that steel can absorb a considerable amount of energy and withstand a considerable large amount of deformation stresses before failure or rapture.
Ductility always contributes to the capability of absorbing more energy and allowing force redistribution from structures that have reached the ultimate limit to other adjacent structures. This capability of steel becomes more vital in the event of seismic loadings due to earthquakes. The plastic analysis of steel frame structures enhances several benefits in terms of strength assessment compared to elastic analysis since plasticity considers the steals ductility in a substantive range. Many researches have been conducted in relation to inelastic nonlinear analysis of frames with rigid and semi-rigid connections for space and plane frames. Though few structural works are found in these kinds analysis is directly related to the dynamic analysis of steel under earthquake conditions. the inelasticity assessment of frame structures, the methodology of evaluating the plastification is conducted by two robust approaches, that is the plastic zone with its variations approach known as distributed plasticity approach and the plastic hinge with its variations approach referred to as concentrated plasticity approach. The only disparity in the two inelasticity approaches lies in the level of refinement applied to represent the plastification of each structural member.
There other second-order models that are more precise and accurate since they consider distributed explicitly and plasticity of the effects and residual stresses thus enable extraction of solutions that almost the same the actual value. However the limitation of the distributed plasticity analysis requires additional supplementary systems that involve higher computational efforts. The distributed plasticity is often used in small structures for validation and calibration of numerical computations and other logistics.
The period determination
Period of the time interval is a crucial parameter of design processes that playa an important role in the design computations of the design shears. The estimation of the period is outlined in the structural and construction codes and standards and the codes provide more accurate methods of mechanics are also elaborated in the structural building standards. The rule of permitted mechanics as a mode of estimating time interval should not be more than 1.5 times more than the empirical value. The limit is justifiable by the point of empirical estimations, such as the existence of uncertainties involved in the application of nonstructural elements whose impacts may not have been considered during the period computations and determination of seismic response, possibilities of inaccuracies from model analysis while applying more complex modeling technics such as methods of mechanics and lastly the possibility of potential disparity between as-built and design condition, more especially on terms of stiffness and mass.
Effects of earthquake on structures
Earthquake causes seismic forces to alter the structural strength through various effects. The following are the summarized effects of earthquakes on building structures.
Induction of inertia forces in structures
The generation of inertia forces in a structure is one of the seismic influences that detrimentally affect the structure. When an earthquake causes ground shaking, the base of the building would move but the roof would be at rest. However, since the walls and columns are attached to it, the roof is dragged with the base of the building. It is the induced inertia that keeps the roof of structures to resist motion and tends to remain in their static positions thus causing shearing of the structural elements which eventually translate the accumulated stresses to the weak joint or walls thus resulting to failure and even worse causing the collapse of the structure depending on the magnitude of the seismic forces released by an earthquake.
The direction of induced inertia force on structures
The induced inertia in buildings depends on the weight and mass of structures and frames used in the construction of buildings, induced inertia is proportional to the overall weight of the building. Thus the heavier or larger the structure the more it is vulnerable to seismic forces. For this reason, is why small and lighter buildings often sustain ground shakings caused by earthquakes better than huge buildings. Therefore it is of the essence to have a construction with materials with higher seismic resistance.
Inertial force development in a multistory building
Deformation effect of structures
In the event of an earthquake, the ground shaking also occurs at the same frequency of the seismic waves, making the foundation base of the structure to shake as well, when this occurs to the structure, it creates a difference in motion or stability of the structure since the uppermost parts of the structure might not experience the same motion of the seismic frequency. The difference in the base movement with the top of the structure causes the development if internal resistance forces that tend to force columns to their initial positions. The prolonged disparity in movement dynamics develops to stiffness forces. The stiffness forces would be higher as the size of columns gets higher. The stiffness force in a column is the column stiffness times the relative displacement between its ends.
Problem statement
Earthquake is a natural disaster that no man has control over or could dictate when and how it occurs. But upon its occurrence its impacts are devastating as it comes along with numerous negativities such as loss of lives, destructions of properties, disconnection on communication networks, and mostly it results in other calamities such a fire outbreaks in the surrounding, causing the double tragedy. More often, large magnitude earthquakes cause a fire on buildings within metropolitan areas due to interference on electric cables. The fire escalates depending on the materials used in the construction of the building. Research has revealed that the effects of fire on structures can be significantly controlled by using fire-resistance materials such as steel in construction.
Most importantly is to ensure that any construction project within regions considered as earthquake-prone areas need to be carried out using earthquake resistance materials and techniques to avoid all these effects caused by earthquake such as the collapse of the structure. Since steel structures have gained favor in such construction activities, there is dire need to comprehensively understand the dynamics of structural steel members and their behavior when subjected to seismic forces.
Research justification
Due to the rapid increase in infrastructural developments all over the world, the construction of these infrastructures needs to be established with all the relevant safety measures against any kind of natural disaster. The earthquake has caused havoc and fear among many as it is associated with dreadful outcomes upon its occurrence. For decades now, the construction industry has been on the look to find ways and measures that would ensure that buildings in earthquake-prone areas have improved resilience for earthquakes and other natural calamities. Thanks to the introduction of metals, most specifical steel in construction processes. Steel in the last 20 years has gained favor in structural construction in high rise constructions due to its relatively reduced cost and most importantly its improved strength and cushion against seismic forces caused by earthquakes.
Steel as a metal has various properties that make it the darling of most recent constructions; however these properties are dictated by various underlying factors. Therefore there is a need to have an exclusive and intensive dynamic analysis of these steel buildings when subjected to different earthquake loadings to establish the most suitable properties of steel for application on a varied range of earthquake magnitudes.
Research objectives
This research study aims to evaluate the performance of steel structural members and systems of buildings under the cascading hazard of seismic forces and earthquake using both probabilistic and deterministic approaches. The major focus of the study is to evaluate the stability of steel structures and columns when subjected to lateral sway as a result of earthquake effects in a probabilistic and deterministic manner.
This research analysis also aims at providing the most suitable recommendation on the most appropriate structural strength of materials that should be incorporated in civil constructions on areas that are prone to earthquake and frequent ground shaking
Research scope and limitation
Literature review
Due to the rapid increase in population that resulted to an equivalent rise in demand for structures, steel building construction emerges to as a supplement to the then-dominant concrete construction, at this time it was not utilized in bulk since concrete and time frames were still stuffiest to cater for the much-needed structures. With the evolution in construction technology, man discovered the need to conserve and make good use of the scares available land to settle its fast-rising population, and therefore the construction of multistory buildings became the order of the day. But still the strength in these multistory structures became a challenge for a long period until steels were incorporated in the construction of more stringer high rise multistory buildings.
Expansion of settlement as a result of the rapidly growing human population pushed people to settle even to areas and regions that were kept off due to the fact that they were prone to seismic effects, earthquake, and sliding of the plate tectonics within the earth’s crust. Structures in these red zone areas were constantly faced with damages or collapse of the entire building even when they subecte4d to low-intensity earthquake or ground shaking. For this reason, various techniques and methods were devised to help overcome the challenge of structural deformation when subjected to seismic or tectonic forces. Steel buildings became handy in these regions as it provided the much needed structural strength both lateral stiffness and vertical strength of the buildings erected in seismic zones. Steel structure becomes the darling in a construction project in areas that were considered earthquake-prone areas since it provided higher strength than concrete structures, it had improved safety factor that any other construction mode, improved strength boosted the durability and service life of steel buildings and most importantly was on its economy and sustainability. Steel buildings are considerably less expensive to erect as compared to the masonry of concrete structures.
Apart from favorable earthquake-resistant properties of steel structures, steel buildings are preferred for their good wind resistance properties that make its use more rampant in the construction of superstructures and skyscrapers in towns and urban centers. The dynamics of wind movement are that the velocity of wind increases with height therefor at higher heights the velocity of wind blowing is huge to the extent of demolishing structures. When the disparity in wind speed not properly factored in the structural design might lead to devastating effects such as the collapse of the structure. For this reason high rise buildings are made using steel since it has high lateral stiffness and improved ductility that can withstand up to huge values of wind velocity hence improved safety factor of such buildings. To other reasons, steel structures were fully embraced in the construction industry since it was in line with the sustainable goals in the construction industry. It provided an alternative solution for structural concretes that were considered as contributors to the greenhouse emission through Portland cement used in concrete preparation.
Therefore this chapter elaborates on the chronological development in the application of steels in the construction industry, with its numerous advantages in earthquake-resistant buildings and wind-resistant structures. It outlines the progress in the dynamic assessment of these steel structures in the development of improved earthquake resistant structures around the globe. this chapter draws information from already published articles, journals, and book reviews relating to seismic effects on structures, structural resilience in the event of an earthquake, and the dynamic analysis of these structures in relation to earthquake resistance properties. The chapter is segmented into subsections relevant to the subject of the research study.
Resilient structures
The earthquake has caused havoc and damages to properties mostly structural assets. Since time immemorial, it has been the responsibility of the government, semi-government, and non-government agencies to organize and deal with the management of the earthquake aftermath. These undertakings relegated engineers responsibility to retrofitting of structural elements strengthening of the weaken structural elements by ground shaking. The rate of earthquake disaster has been in continuous rise for decades now with areas that were initially considered as stable grounds now experiencing landslides. The research conducted in the 1980s showed that these regions traditionally marked as disaster zones areas are ever-expanding into a new domain. These trends led to the institution of international codes and standards that guide and control building construction in these areas regarded as the earthquake zones as a measure of reducing the very many cases of deaths and lost properties as a result of the earthquake. The increased cases of damages registered annually caused by the earth-shaking prompte4d the call to architects and engineers together with non-engineer residents to help in the online and physical campaign to create awareness and collectively work with one another to find an alternative prevention and mitigation effects. Available publications give various aspects of individual analysis methods of earthquake disaster on structures as a whole. Through the study of international publications on the prevention and mitigation techniques for natural disasters such as earthquakes, they give set methodologies that are for use on structural design and structural analysis for earthquake resistance on buildings. However there are no common or universal methodologies or standardized procedures for other standardized protocols for disaster control and management. Therefore in the past, before the advanced innovation on technology methods and tools of analysis, the process of assessing complex dynamics and real-time forces of the earthquake on structures was quite challenging. Then with the introduction of assessment software, there was an improvement of structural assessment techniques, though software analysis gave little scope of flexibility on the specific structural needs and special aspects that would be disastrous to the structure. In the plight of software development, there came the development of more advanced modeling software that would analyze the seismic effects on the structure before it is even constructed, allowing both static and dynamic analysis of structural elements such as steel used in frames, column and beans of the building. This modeling software improved the efforts and assurance of a robust model that depict the real situation upon the occurrence of any given natural disaster such as an earthquake.
Therefore through literature and developed software-enabled engineers and architects to develop a mechanism of finding solutions to the problems that have not incurred yet through the use of past information extracted from literature, previous findings, and recommendation in the line of good governance. Both literature covered and technological methods through software have necessitated engineers work towards finding the most appropriate mechanism of finding solutions that are capable of customizing the behavior of buildings when exposed to seismic forces thus allowing the presentation of obtained data in 2D and 3D dimensional graphical outputs.
Considering the studies and research conducted in the past that were focused on the analysis of structural behavior subjected to various aspects of natural disasters. The study shall consider the review of journals and experimental outcomes based on the following aspects of structural strength.
- Pushover technique bases on performance assessment of buildings
- Retrofit options and damages due to seismic forces
- Virtual reality analysis in structural steel engineering
- Cyclonic wind mitigation on structural buildings
- Fire loads on steel structures
- Flood effects mitigation of flood effects
- Damage and repair tsunami and blast
Structural designs have set codes and standards outlining basic procedures and guidelines for dynamic and static analysis of structures when exposed to seismic forces. Though in other engineering disciplines, performance analysis has been preferred for giving detailed design analysis of system structures. Following the continuous occurrence of earthquakes leads to an increased need for structural seismic analysis, hence pushover analysis tool emerged as a technique for seismic analysis of non-linear static performance based on methods of examining the response any given structure when exposed to seismic forces.
Pyle and Habibullah in their 1998 research established a simple procedural step for performing pushover analysis using simple software known as SAP2000. The software basically could do static pushover analysis with the integration of the findings into a program that allows easy and quick interpretation and implementation of the pushover analysis procedures as outlined in the FEMA-273 and ATC-40 standards for both 2D and 3D structural analysis.
In 2000 Tso and Moghadam, improved the pushover analysis to widen the scope of methodology to cover eccentric buildings and accommodate the three-dimensional torsional effects on structures. Tso and Moghadam argued that due to floor displacement and torsional deformation, and then the structure or building obviously will consist of both rotational and torsional components. Therefore they added that torsional effects could be damaging factors to members located at or near the flexible edges of buildings where the rotational and torsional elements of the floor displacement are additive. Moghadam and Tso based their findings on the observations made on the damages on many eccentric structures from the pat ground shaking and seismic effects from the earthquake.
Chopra in 2001 made a publication of the PEER report, where he mentioned the of the standard response spectrum (RSA) analysis for elastic buildings through the formulation of modal pushover analysis (MPA). In her study she mentioned that the peak response of elastic buildings is due to their nth vibration mode and the mode of structural vibration could be determined by pushover analysis on the building subjected to lateral forces distribute over the building’s height according to expression Sn = mœn. Where œn. is the nth mode and m is the mass matrix.
Research methodology
The study aims to incorporate various methodologies that would help accomplish the objective of the research topic. Among methods to be used in developing this analysis is through having a critical review of the already published journals and books about the analysis of seismic forces and earthquakes on buildings and structures. Through this, background understanding of the dynamics of the steel structural behavior when subjected to a large magnitude earthquake will be established to shape the research analysis. it will also help in emulation the work of previous scholars on the structural analysis when exposed to earthquake forces thus appreciating their contributions.
The deterministic analysis of the topic will be attained through the use of analysis tools such as finite analysis of structures under different earthquake loadings to determine their behaviors. These findings will help in formulating recommendations on the most desirable structural strength needed to overcome specific earthquake forces thus improving structural resilience. The probabilistic aspect of the study will focus on model simulations to establish a structural response to different magnitudes of earthquakes.
The results and findings of the study analysis will be computed and documented for further discussion and scrutiny to derive the conclusion of the study and recommendations for future analysis related to the topic of study.
Project outline