Basic Design Features of Tall Structures
Any building having more than one floor is a multi-story building and a building to which provision of lift is essential is tall building. Various nomenclatures used for tall buildings are high rise buildings, sky scrapers, vertigoes, anthills, concrete castles etc. From architectural considerations any building higher than two floors, needing use of staircase or lift is called as tall building. From firefighting view, a tall building is one from where emergency escape is not practicable. From structural engineering aspect, a tall building would be one whose structural elements directly or indirectly are affected by wind forces. From physical planning, a tall building is one which by its height begins to affect its light, ventilation and climate of other surrounding buildings.
For all practical purposes, a building with more than one floor needing staircase is a multi-story building, while a tall building means a building which requires use of mechanical services like lifts. Buildings with more than 50 stories are now called ultra-high buildings. A new terminology that has been used for very tall buildings is ‘super tall’ (More than 100 m height), ‘mega tall’ (more than 600 M height), a new era entering into construction industry. It is not necessary to have symmetrical structure but it is essential to have balanced structural form/systems. Expansion joints if required should be provided and properly designed.
Hence, the design of tall buildings essentially involves a conceptual design, approximate analysis, preliminary design and optimization, to safely carry gravity and lateral loads. The design criteria are strength, serviceability, stability and human comfort.
Design Features of Tall Structures
- Strength and Stability
- Sequential Loading
- Drift Limitations and Stiffness
- Human comfort
- Foundation Settlement and Soil Structure
The structure must be designed to resist the gravitational and lateral forces, both permanent and transient that will be sustained during construction and during the expected useful life of the structure (from 60 to 100 years). These forces will depend on the size and shape of the building, and its location. Load combinations depend on the probable accuracy of estimating the dead and live loads, and the probability of the simultaneous occurrence of different combinations of gravity loading, both dead and live, with either wind or earthquake forces. The accuracy of these loads is included in limit states design through the use of prescribed factors.
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.
For dead loads, the construction sequence should be considered to be the worst case. It is usual to shore the freshly placed floor upon several previously cast floors. The construction loads on the supporting floors due to the weight of wet concrete and its formwork will greatly exceed the loads of normal service conditions. These loads must be calculated considering the sequence of construction and the rate of erection. However, the designer rarely knows who the contractor will be, nor his method of construction. If column axial deformations are calculated as though the dead loads are applied to the completed structure, bending moments in the horizontal components (for example, beams) will result from any differential column shortening. Because of the cumulative effects of column axial deformations over the height of the building, the effects are greater in the highest levels of the building. However, the effects of such differential movements could be greatly overestimated because in reality, during the construction sequence, a particular horizontal member is constructed on columns in which the initial axial deformations due to the dead weight of the structure up to that particular level have already taken place. The deformations of that particular floor will then be caused by the loads that are applied subsequent to its construction. Such sequential effects must be considered if an accurate assessment of the structural actions due to dead loads is to be achieved.
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.
Buildings subjected to both lateral and torsional deflections (plus vortex shedding and other usual effects) may induce in their human occupants from discomfort to acute nausea. These are major factors in the final design of the building. When a tall structure is subjected to lateral loads, the resulting oscillatory movements can induce a wide range of responses in the building’s occupants, ranging from mild discomfort to acute nausea. This may prove the structure undesirable or un-rentable. There are no codified standards for comfort criteria. A dynamic analysis is required to determine the response of the structure in order to determine its adequacy to the comfort criteria.
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.
Soil-structure interaction involves both static and dynamic behaviour. The former is generally treated by simplified models of subgrade behavior, and finite element methods of analysis are customary. When considering dynamic effects, both interactions between soil and structure, and any amplification caused by a coincidence of the natural frequencies of building and foundation must be included. Seismic forces may develop excessive hydrostatic pressures, causing liquefaction of the soil. These types of conditions must be considered and avoided.
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 labour. The longevity of reinforced concrete depends on the following factors in addition to the quality of material and construction are chemicals creating corrosion impacts, permeability or porosity of concrete, shrinkage, concrete cover to steel, curing of concrete, thermal influences, and acoustic as well as freezing and thawing impact.