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Econference on Design and Construction of Tall Buildings in

 
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partha
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PostPosted: Wed Nov 21, 2012 8:17 am    Post subject: Econference on Design and Construction of Tall Buildings in Reply with quote

Pre-Stressing as a Green Building Issue:


- Saibal Saha, Sr. Manager (Civil Design), Usha Martin Ltd. (saibalsaha2@gmail.com (saibalsaha2@gmail.com))
[ Accredited Professional- Indian Green Building Council]
[Certified Energy Manager/Energy Auditor- Bureau of Energy Efficiency, Govt. Of India]

  • For any form of Green Building Concept, we are concerned of “Life-Cycle Consumption of Energy by a Building”

  • At the time of construction of a building, we use materials, machineries and labourers,

  • Every building-material & machinery has a tag of 'Embodied Energy', whereas machineries & laboures/operators have some operating energy consumptions also.

  • Now if we can reduce the consumption of building- materials as well as construction time substantially, those Embodied Energies as well as operational energies for the construction of that particular building would be much lowered, thus championing the concept of “Green Building”.

  • The above statement is true for any type of building irrespective of its height, use-group, location etc. However it is particularly more appropriate for “Tall Buildings”.

  • Because to build a “Tall Building”, we need a huge quantity of materails and a long period of time. By cutting a mere percentage of these resources could have a huge impact from Energy point of view as well as Environmental concerns. As production of conventional energy is nothing but generation of GHGs also.

  • But how can we reduce the consumption of materials and lowers the construction time:

    1. Firstly if we adopt Pre Stressed Concrete Slab Construction instead of conventional RCC slab with or without changing the framing plan as required by architectural design, dimensions of the members could be reduced at least by 15-20%, thus reducing the use of Concrete. Moreover Rebar consumption also has to be reduced in a significant amount. Hence use of PSC in place of RCC has a potential of reduction in consumption of Embodied Energy per unit of built up area, having sizable impact on Energy and Environment.

    2. Secondly we can reduce the construction time significantly by using the above stated technology. In conventional RCC we have to wait to get a 2/3rd strength of designed concrete before stripping of formwork or 7-10 days as per the size of span length. However in PSC we can start stripping on 4th day of casting, because it depends on a specific value of cube strength(25 MPa) of concrete and if it is reached within 3rd day of concreting which is very common in case of high strengths concrete (M40 and beyond), it is amply possible. Thus PSC has a capability to reduce the “Time-Cycle” of slabcasting in repititive manner which is very typical in Tall Buildings. For example in case of a 50 storied building it has potential of saving 250 or more days in casting of the entire building.

  • Last but not the least, since we are discussing the subject in terms of “Life Cycle Energy Consumption by a Building”, we definitely should focus of longivity of the building while servicable. Since PSC concrete is almost Crack-Free, it enhances the durability of the concrete and hence its life span. Therefore use of PSC, in long term delay the necessity of replacing the old building by a new one which ultimately consumes vigin resources (having a tag of Embodied Energy) from this earth again, to be constructed.


Idealisation in Software Tools and Structural Irregularities:
- Partha Pratim Roy, B. E. (Civil); M. E. (Structure); A. I. E.
Vice President (Technical), ADAPT International (pproy76@gmail.com)
When it comes to the analysis and design of Tall Building it is extremely important to idealise the correct mathematical model while using any software tool like STAAD, ETABS, ROBOT etc. It is often noticed engineers prepare a frame model consisting of Beams and Columns and perform a First Order Linear Analysis. These leads to following issues which becomes very critical and leads to inaccuracy.  

Modelling of Floor Slab:

  1. Residential administrative, office, educational and many other buildings and structures are made of continuous floor slabs without any opening. Industrial buildings, on the other hand, in many cases are having different sizes of cut-outs in the floor.  

  2. Presence of Slab not only contribute vertical loading but also impart a horizontal diaphragm action in the structure due to large in-plane stiffness of floor slabs.  

  3. Dynamic behaviour of a structure modelled taking into account the stiffness of the floor slab shows wide differences to that modelled without floor slab.  

  4. A floor slab can be modelled in two ways which are available in almost all software tools:

    1. By including plane stress elements

    2. By using master slave command


Soil-Structure Interaction:

  1. Generally engineers assume that the foundation is fixed in space. But, depending on the nature of soil, the base of the structure undergoes deformations. The soil can be seen as a spring with certain stiffness.  

  2. In almost all software tools, besides “Fixed”or “Pinned” options, user can also assign soil-springs by the User Defined or Fixed But option. Tools like Autodesk's Robot Structural Analysis even allow user to model soil strata at different levels with all parameters including its stiffness and damping ratio. Also utilities like Automatic soil-spring generation (i.e. MAT) can also be availed .

  3. In seismic analysis problems, research works has shown that a certain mass of soil vibrates along with the foundation. But no soil-mass is included in the model, as no specific guideline is yet available. Only stiffness and damping ratios of the soil-springs are generally included in a mathematical model.

  4. The spring constant of soil springs depends on factors like shear modulus of soil, Poisson's ratio of soil, foundation dimensions, and shear-wave velocity through the soil-strata.

  5. During dynamic analysis the damping ratios of the soil-spring may be obtained by summing two quantities: material damping and geometric damping of the soil-spring.

  6. Formulations of soil-spring stiffness and damping ratio are available in text books on machine foundations.  

  7. Once can analyse the same Building Frame using the same Software Tools with two different Spring Stiffness (say, 10E+10 kN/m^2/m run for Hard Soil and 10E+04 kN/m^2/m run for Soft Soil) it may be observed that the response is very much sensitive to soil-spring stiffness. Higher soil-spring stiffness increases overall stiffness of the structure, resulting in lowering of fundamental periods and attract more seismic force (increasing base shear).  

  8. The participation of soil in vibration is also altered which may be reflected by the values of composite modal damping.


Soft Storey and Modelling of Shear Walls

  1. According to IS 1893-2002 (Part 1), there are two types of stiffness irregularities as given below:

    1. A soft storey is one in which the lateral stiffness is less than 70% of that in the storey above or less than 80% of the average lateral stiffness of the three storeys above.

    2. An extreme soft storey is one in which the lateral stiffness is less than 60% of that in the storey above or less than 70% of the average lateral stiffness of the three storeys above.

  2. The lateral stiffness of a vertical element (column or wall) of a building frame is equal to (12*E*I/L^3). The lateral stiffness of a storey is the summation of the lateral stiffness of all columns and walls in that storey.  

  3. It is important to check Soft Storey and thus the modelling of Shear Wall is very much important as it will affect stiffness of the particular floor.  

  4. Some engineers even model compression only member or strut to resemble the presence of Brick Walls or Non-structural Partition Walls. However no specific guideline is available about formulation of equivalent strut members.


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