A tall framework is a structural system designed to resist lateral forces due to wind or earthquakes, considering the criteria for strength, drift, and the convenience of the occupants.
Tall frameworks are not novel to any territory in the world as this form of architecture has existed for centuries. Several historical monuments such as Pyramid of Khufu, Yongning temple, Hwangryongsa temple, etc. are the testament of such architectural marvel.
The field of civil engineering has witnessed numerous advancements and has been evolving continuously to adapt to the modern-day requirements. The developments in the domain of style and construction of tall structures is one such revolutionary leap perceptible to the masses.
Basic Design features of Tall Structures
When designing tall structures, basic features like load, strength, stability, durability, etc. should be given due consideration as discussed below:
The vertical loads, i.e. dead and live, do not pose any design issues as they are mostly deterministic in nature. However, the lateral forces caused due to winds or earthquakes make the design tedious. These lateral forces can generate vital tensions in the structure, set-up unwanted vibrations, or create excessive lateral sway of the structure, thus calling for unique considerations.
Developments in the design of multi-story structures have emphasized the value of restricting the side sway and also the activity of lateral loads. The provision of shear wall, as opposed to conventional inflexible frameworks, minimizes the lateral sway of the structure. A well-designed shear wall provides structural safety and security to the non-structural elements like false ceilings, wall panels, etc. from seismic disturbances.
The critical design aspect of a tall structure is its strength to withstand and remain steady under the worst possible combination of loads that might occur throughout the life of the structure, including the period of construction.
Additionally, the strains triggered by controlled differential activities such as creep, shrinking, or temperature should be inclusive in the strength criteria for the structure.
Tall structures must have inherent base stability due to the combination of vertical and lateral forces. Thus, they should be constructed to allow the application of the easy rule of resultant force passing within the one-third of the base, consequently making the resulting base stress compressive.
The equilibrium condition should be considered to establish that the designed lateral forces must not topple the entire structure due to rigid body movement about one edge of the base. Likewise, the resisting moment of the dead weight of the structure has to be greater than the overturning moment with an appropriate factor of safety.
Structures built with the right materials and methods will be stronger and resilient compared to a basic building of smaller size constructed without considering the codal provisions and with untrained labor.
The longevity of reinforced concrete depends on the following factors in addition to the quality of material and construction:
1) Chemicals creating corrosion impacts
2) Permeability or porosity of concrete
4) Concrete cover to steel
5) Curing of concrete
6) Thermal influences
7) Acoustic as well as freezing and thawing impact (for cold locations)
5. Stiffness and Drift constraints
The provision of appropriate rigidity, particularly lateral rigidity, is vital to be considered to avoid any feasible progressive failure. One straightforward specification that provides the estimation of the lateral rigidity of a structure is the drift index. Drift index is defined as the ratio of the maximum deflection at the top of the building to the total height.
For this reason, the establishment of a drift index restriction is an important design choice, which depends upon factors such as the building usage, the type of building, the material employed, the wind loads, etc. However, for conventional structures, the appropriate range is roughly 1/600 to 1/300, and enough stiffness should be given to ensure that the top deflection does not exceed this value under any possible load combination.
6. Soil-structure Interaction
Gravity and lateral forces acting on the building get transferred to the soil beneath the structure through the foundation. The primary concern of the structural design engineer is the influence of foundation collapse and settlement of the structure.
In the case of tall buildings, the loads transferred by the columns can be very heavy due to its height. When the good rock or stiff soil lies at a greater depth, foundations might be located at greater depth by the use of piles or caissons foundation. Issues are not usually experienced in this case because huge variances in column loading and spacing can be balanced with a minimal differential settlement.
In locations with poor soil, loads on foundation components should be restricted to avoid shearing failures or extreme differential settlement. Relief can be acquired by excavating the weight of soil equal to a significant portion of the gross structure weight.
Special attention needs to be given to the design of the foundation system for withstanding moments and shear. This is important, especially when the pre-compression due to the dead weight of the structure is not enough to overcome the tensile stress and stresses by wind moments, causing uplift on the structure.
7. Creep, Shrinkage and Temperature level results
In tall concrete structures, the vertical moments due to creep and shrinkage might be large enough to create distress in non-structural parts to cause significant structural actions in the horizontal components and the top area of the structure. The differential activities because of creep and shrinkage should be analyzed in the architectural details at the earlier stage of design.
In structures with partly or completely exposed exterior columns, considerable temperature level adjustments may occur between the exterior and interior columns. Any restriction to their relative deformation will generate stress and strain in the concerned members.
The evaluation of such activities requires the knowledge of differential temperature levels. This will enable the evaluation of cost-free thermal modifications that would occur in the absence of restriction. Hence, the resulting thermal stresses and deformation can be figured out using a conventional elastic analysis.
Fire should be considered as a key factor during the design process of tall structures. The temperature level and period can be estimated from the knowledge of the essential criteria involved, especially the quantity as and nature of the flammable product.
The mechanical properties of the materials, specifically the modulus of elasticity, rigidity, and strength, may abrade quickly with increasing temperature levels, and also, the resistance to loads reduces significantly. The temperature level at which the deflection or collapse takes place will depend on the materials used, the nature of the structure, and the loading conditions.
9. Human Comfort Requirements
If the lateral or torsional deflection occurs in a structure due to the fluctuating wind loads, then the resulting oscillatory activities can induce discomfort among the building’s occupants. Motions that have psychological effects on the occupants may cause an otherwise appropriate structure to remain unwanted and even unrentable.
It is a common observation that acceleration is a major factor in determining the human reaction to resonance, while elements such as duration, amplitude, body orientation, visual, and even past experiences can be influential. For this, limit contours can provide numerous limitations for human behavior in terms of acceleration and period.
Frequently Asked Questions:
The factors that contribute to the need for tall structures are:
(i) Shortage of land
(ii) Scarceness of funds
(iii) Architectural demands
(iv) Rise in the cost of land
(v) Drastically increasing population
With the increasing pattern in urbanisation, building of tall structures is taking place on a large scale. There are several aspects which restrict the elevation of tall buildings. In specific cities, the building regulations and also municipal by-laws suggest the maximum height up to which the structures might be constructed and also at particular places the foundation conditions might not be satisfying to support multi-storied buildings.
Steel is costly as compared to concrete, the cost proportion varying from 60 to 90, whereas the equivalent strength ratio is from 15 to 20. Almost, all tall buildings built are of reinforced concrete utilising high yield strength deformed bars. The strength of concrete utilised differs from M20 to M35 as well as the strength of support bars is 415 N/mm2. Besides the columns, in the lower floorings of the high buildings, the percent of reinforcement utilised in a lot of the aspects of the structures should be less than 2 percent. The portion of reinforcement in beams and slabs should be around one per cent.