Learn ETABS with Free Lessons & Tips

DESIGN OF STRUCTURAL ELEMENTS
Construction is the ultimate objective of the design. An engineer is a crucial person for successful completion of any project undertaken. Hence, he should adopt all means to reduce the cost of the project to a minimum, without decreasing serviceability aspect of the project as engineering structure is an assemblage of members for elements transferring the load and providing from space, on the enclosure and or a cover to serve the desired function. The objective of structural design is to plan a structure that meets the 'essential requirements such as serviceability, safety, durability, economy, aesthetic beauty, feasibility and acceptability.
Design philosophies:
The following design philosophies have been evolved for the design of R.C structures,
- Working stress method
- Ultimate load method
- Limit state design method
(a) Working stress method:
This method is based on classical elastic theory and developed for purely flexible materials. It assumes that the structure is made up, viz, concrete and steel both obey HOOK's law. In this approach, the margin of safety is provided for the available stress. IS 456-2000 code of practice specifies, different permissible stresses for different materials.
(b) Ultimate load method:
This method is based on the strength capacity of the member just before the collapse stage. In this method, safety has been specified concerning the behaviour at the ultimate stage of member are proportioned, so that the full strength of concrete and steel are utilised. When the final load occurs on it, the ultimate load is obtained by multiplying the working load by a factor known as the load factor.
(c) Limit state design method:
This method is used in practice in the design of reinforced and pre-stressed concrete structures. In this method, various components of a structure are proportional such that the structure does not attain the specified limiting conditions during its lifetime. It shall satisfy the serviceability requirements such as limitations on deflection and cracking. The acceptable limit for the safety and serviceability requirements before failure occurs is called a limit state method. The aim of the design is to acceptable probabilities that the structure will not reach a limit state.

Earthquake means ground movement and this movement induces an inertia in the building and this results in displacement. So the main factors that depend on the earthquake force is the acceleration of ground and the mass of the building. It is not really possible to exactly know the magnitude of the earthquake that would occur. ie; the acceleration of ground. The best thing we can do is to arrive at a possible acceleration based on earth strata and the history of occurrences. This makes it very difficult to conclusively say that we can design an earthquake proof building. Secondly, by saying that the building is earthquake proof means that the energy that the building absorbs during ground movement is dissipated by oscillation and deflection and with out any crack formation. It takes a lot of effort and cost to achieve this. Therefore the practice across the globe is to design for a part of this expected acceleration (generally 10 to 15%) and achieve earth quake resistance ie, no cracks during mild quakes and no sudden collapse during higher quakes so that people can move out of the building.

A simply supported or continuous CONCRETE beam shall be proportioned in such a way that it should be laterally restrained with in 60*b or 250*b*b/d which ever is less where b is width of compression face midway between the lateral restraints and d is effective depth

For a cantilever beam the clear distance from free edge to the lateral restraint shall not exceed 25b or 100b*b/d

If you take an example of a 200 x 450 continuous beam across a room of 20m, b = 200 d = 450  – 25 – 8 – 20/2 = 407 (considering 1 layer of 20 dia bar)
60*b = 12000mm
250b*b/d = 250*200*200/407 = 24570mm
This means that the beam should have a lateral restrain at 12m intervals.
Lateral restraint can be a secondary beam entering the beam which provides lateral stability.

Stiffness modification: Significance In IS 456,as per Cl.22.3 gross sectional properties are allowed to be used (if consistently used for all members) for force calculations. For deflections, 22.3.2 allows only the usage of cracked mom of inertia. What's wrong if we use uncracked mom of inertia? Concrete has inherent property of cracking what ever we do. So if we dont capture this, we are making the supports more stiff than real, which means support moments are more which means the span moments and deflections are lesser in the model than real. To take care of this, in tall buildings, generally cracked mom of inertia is recommended for both force calculations and deflection checks. However, it is not easy to formulate this for all members as crack widths varies. So, ACI makes it more generic based on what is seen actually and uses a flat value.

As per IS.456, the total deflection including the long term deflections need to be limited to Span/250,Span/350 or 20mm.

This includes elastic deflections(which can be retrieved from software or easily calculated) and the long term deflection which is not retrievable from most software. One need to use formulas available in annex of IS456 to calculate long term deflection which is a little challenging to use for many combinations.

Therefore code suggests span/depth ratios if you stick to can ensure that the total deflections are in limit.This Span/depth ratios are not accounting for the deflection control that the rebars provide. It is accounted as a modification factor for tension steel and another modification factor for compressive steel in code.

I have heard a lot about this subject from young aspiring Civil & Structural engineers in the recent time.

My own experience is that the best among the lott always find ways and succeed in the long run.

The bad period will end soon!How you manage and spend time in this period is what matters!

My suggestion to all young engineers is:

1. Find new things to learn and new ways to learn and please do learn! It could be a new technical topic listed in a website freely available.It could be a training programme.It could be a topic out of your field of study. This will keep you job ready more over confident.
2. Try to reduce the gap in your studies and practical application of what you study. Dont hesitate to talk to seniors.Many do not mind to share experience and knowledge.
3. Find out young firms that may need a helping hand.Offer them your free time and surely you will learn some thing new in the process and a certificate can be handy.

About us : We offer world class training in the area of civil and structural engineering.

We offer ETABS Training in Bangalore, ETABS Training in Kerala,ETABS Training in India, ETABS Training in Dubai, ETABS Training in Middle East, Online ETABS Training, Online ETABS video training.

Building Performance – Controlling Disproportionate Collapse
The importance of building structures has increased in the modern times due to the increasing complexity in the buildings due to increased demands in terms of functional requirement, aesthetics and height. This project requirement as well as many other external factors has increased the risk and chances of accidents.
This has also increased the importance of engaging structural consultants from start to end of the project and then evaluating and maintaining the building for its safe performance.
Unsafe practices and ignorance of structural design and good construction practices has resulted in many building collapses in the past as well as in recent times. Some of these are disproportionate collapses which resulted in large number of causalities and loss.

 
What is disproportionate collapse?
Collapse to an extent more than the cause of collapse is called a disproportion-ate collapse. The collapse of even a small, some times a non structural element like a wall may trigger larger collapses and this may end up in a progressive collapse in which a large or even the entire building comes down like a pack of cards.


Why should we attend to disproportionate collapse?
Disproportionate collapses results in severe causalities and economic losses which are much more in magnitude than the real reason of the accident which is small. Therefore it is very sensible and essential to control disproportionate collapses by appropriate structural design and detailing.
 
Is disproportionate collapse and its control Relevant in India?
Some of the recent failures of buildings should prompt us to look in to the history and provisions of disproportionate collapse in India and across the world.
IS-456-2000, the Indian standard for plain and reinforced concrete does not provide any guidelines. This does not mean that disproportionate collapse can be ignored. There have been many cases of disproportionate collapses in India in the near past.
Clause 0.3.3 of IS 875-Part II (Code of practice for design loads) states that “The buildings and structural systems shall  provide such structural integrity that the hazards associated with progressive collapse such as that due to local failure caused by severe overloads or abnormal loads not specifically covered therein are reduced to a level consistent with good engineering practice.’’ This clause is misinterpreted and it is often stated that good detailing practices takes care of the progressive collapse. This interpretation could be true for smaller less complicated buildings. However, as per this clause the designer has the responsibility to reduce accidental load to a lesser magnitude so that good detailing practice takes care of accidents. Reduction in load so that the detailing  takes care the effects of progressive collapse is possible only by a careful scheming of structure by providing adequate number and sizes for columns and beams. The main point is to ensure multiple load path in the structure incase of failure of a member so that a local failure (say of a beam) do not progress from the local area near the failed beam to areas away from the failure location thus avoiding the disproportionate collapse. Also the latest code of practice for General construction in steel IS 800â??2007â?? clâ??3.1.1 & 5.1 gives guidelines for progressive collapse. These guidelines are similar to that available in the British standards.
 
History of UK regulations for Disproportionate collapse.
The partial collapse of a building –Ronan Point in 1968 initiated the discussion in the UK for the provision of accidental load clauses. In 1968, an explosion in the kitchen on 18th floor of this apartment building blew out some walls. The walls fell on the slab and the 18th floor slab collapsed on to lower level slab and triggered a progressive collapse though the explosion was not major and was in 18th floor.
 
Provisions for disproportionate collapse in BS-8110
BS-8110-Part 1 & 2, UK Building regulations -Approved document A3 and National House Building Council Technical guidelines to A3 can be referred for guidelines for disproportionate collapse control.
 
BS categorizes buildings in to class 1, 2A, 2B and 3 (increasing order of importance)
For class 1 building (Houses up to 4 floors, agricultural shed etc.), following code of practice strictly and good detailing practices is recommended.
Logic: Less complicated buildings will resist a local failure if it is detailed properly since the forces due to a local collapse will be small.
This is true in Indian conditions as well provided even smaller buildings are designed for seismic and wind forces, detailed and execution inspected by a specialist structural engineer. Also the seismic design provides additional robustness for a local failure due to accident load.
 
For class 2A buildings (Houses up to 5 floors, 1 floor education building etc), Horizontal and Vertical ties are recommended. Peripheral and internal ties are recommended. Also the building has to be designed for a minimum/notional horizontal load. It shall be noted that even if the building is designed for a lateral wind load, notional horizontal load governs the designs if wind load is lesser.
Logic: The ties provide alternate load path should a local failure occur.
Minimum horizontal load is recommended since UK is not in a seismic zone. In Indian conditions, this minimum load case may not arise if we design buildings for seismic forces.
 
For class 2B buildings (residential up to 15 floors, buildings with area between 2000 to 5000m2 at each floor etc.), ties shall be provided similar to class 2A buildings. In addition, checks can be made that upon the notional removal of supporting elements the building remains stable and that the risk of collapse does not exceed 15% of the floor area of that storey or 70m2, whichever is smaller. If this exceeds the values, members has to be designed as a key element capable of sustaining an accidental design loading of 34 kN/m2 applied in the horizontal and vertical directions (in one direction at a time)

                       
Logic: For more important buildings, tying may not be sufficient to provide alternate load path should an accidental failure occur. Analyzing by removing certain elements is nothing but checking the bridging capacity of the structure if that particular member fails due to accident. If bridging also fails, then the additional design force of 34kN/m2 will absorb a certain amount of accident.
This should work for Indian conditions as well since seismic design provides certain robustness to absorb a gravity load failure. However it is important to remember that the figure of 34kN/m2 is derived from the Ronan Point apartment collapse in UK which was triggered by a cooking gas explosion. It may be wiser to derive a more realistic value for Indian conditions for various kinds of structures.
 
For class 3 buildings (Buildings more important that 2B) - A systematic risk assessment of the building should be undertaken taken into account all the normal hazards that may be foreseen together with any abnormal hazards.
The structural form and concept and protective measures should be chosen and the detailed design of the structure and its elements undertaken in accordance with the recommendations given in the Codes and Standards.
Logic: It may not be economic to design for all kind of risks. Therefore a risk analysis is recommended. Also some risks can be mitigated by a non structural solution mostly by avoiding the risk by taking measures to avoid the accident.
                                                                                                                                                                          
Conclusion: Buildings designed for disproportionate collapse has performed well during accidents and therefore it is sensible to cater for it during the conceptual stage itself. Since buildings in India are any ways to be designed for seismic forces, the additional cost for taking care of accidental loads may not be huge. Though it may add a little additional cost, it is sensible to mitigate risks. A careful analysis of risks and mitigating it first by non structural means and then catering for the non avoidable risks structurally shall be made mandatory for reducing accidents. Also this additional robustness, some times provides the builder/Architect/engineers flexibility to accommodate unforeseen changes.
Robustness also increases the blast resistance of the structure. When all this is possible with no much extra cost, it is sensible to look in to all these basic principles of risk mitigation.

ETABS is a powerful tool for Analysis and it wins hands down over many other analysis tools in tall building structural analysis and design.

The main advantages are

1) Its modelling and result interpretation interface is one of the easiest

2) It can perform P -Delta and response spectrum analysis/dynamic analysis.

3) Construction sequencing loading can be done.

4) Latest 2016 version of ETABS comes with many options available in SAFE

5) Compatible with REVIT 

Today,the structural design world is pressurized for economy and more economy!.There is a limit to economize if one want to abide by the codes of practice and also if one want the building to be robust and stable. Compromising should not be resorted to achieve economy,However improvisation can be.
At times,there are misunderstandings or no understanding that makes the design uneconomic.Torsion(Twisting of structural elements) in the structure is such an entity.
TORSION : Basically torsion can be divided in to 2.Equilibrium torsion and compatibility torsion.
Primary torsion/Equilibrium torsion/statically determinate torsion 
This exists when the external load has no alternative load path but must be supported by torsion. For such cases, the torsion required to maintain static equilibrium can be uniquely determined. This cannot be released/ignored since the structure will not be stable if released.
To understand it beter,consider a free cantelever slab from a beam.There is no back anchorage for the slab-Just a projection to 1 side from a beam.This is a case of equilibrium torsion.The slab can be in equilibrium or stable only if the beam absorbs this torsion and thereby supports the slab.This torsion from slab has only 1 load path and that is through beams.If the building has such type of structural element then it needs to be designed for torsion.
Secondary torsion/Compatibility torsion/statically non determinate torsion 
This arises from the requirements of continuity/compatibility of deformation between adjacent parts of a structure. An internal readjustment of forces is possible and an alternative equilibrium of forces can be found.That is torsion has more than 1 load path. Cantelevers with back anchorages/continuous slabs behind the cantelever etc are examples.
Code provisions 
IS-456 clause 41.1 says that compatibility torsion can be ignored in design if the torsion stiffness of member is completely ignored/released in the analysis model. Code further adds that nominal shear reinforcement provided as per clause 40, will be adequate to control any torsion cracking. 
Releasing compatible torsion in model has 2 parts. 1) Release of torsion in members when slabs are modeled as plates/shells and slabs designed as per the model forces. 2) Release of torsion in members when slabs are not modeled as plates/shells. 
• In case 1 there will be complete compatibility of deformation between adjacent parts and there is no need of designing for torsion. • In case 2 there will not be complete compatibility of deformation if the adjacent slab panels has different spans or loading.(Prudent if the span and/or load differences are considerable) In case 2 two design solutions are possible. A. Either the beam needs to be designed for that differential moment (as torsion) or B. Design the slabs as per clause 24.4.1 
If we adopt Solution A ie; designing for differential moment, we will have to provide additional steel for resisting torsion. Plain concrete will have a nominal capacity to resist torsion and if the design torsion increases the nominal value, the concrete needs to crack to transfer forces to additional steel provided. In that case, torsion stiffness shall be half the value calculated for plain concrete section. This means that torsional stiffness in analysis model shall be modified by using a factor of 0.5
SUMMARY 


Equilibrium torsion cannot be released. 


Compatibility torsion can be released. 


If we choose to design for compatibility torsion,the torsional stiffness needs to be modified by 0.5 
With out realising these points,certain designers considers torsion for all member design and results in uneconomic designs.Some do not consider even equilibrium torsion and this results in unsafe designs.
It is of high importance to understand these points for safe and economic designs.
Improvisation helps,compromise kills!

Ductility of Buildings is a key virtue. Ductility is the ability to prolong yielding or in simple words, prolonging deformation and failure.Why should we prolong yielding of steel?As we know, we design to resist a certain earth quake acceleration/ground movement (based on zone where the building is located) and not for unlimited ground acceleration.The zoning of places is done by looking at the history of seismic activity in the region and by studying the soil strata. This does not mean that an earth quake of more severity will never occur. Earth quake is a natural occurrence and we can only have an approximate estimation and hence ductility is as much important. What if in a location like Bangalore where it is zoned as Zone 2,experiences a larger seismic acceleration similar to that expected in Zone 3.The building would have been designed only for a seismic acceleration expected in Zone 2.However due to larger acceleration, chances of irreparable damages are more.

By plugging in a certain ductility will help in reducing the irreparable damages.

Also in case of a larger ground movement, it helps in increasing the evacuation time there by saving lives.

How do we achieve ductility and will this cost more?A structural engineer builds up a model and simulate the acceleration  and compute the deformation and the related forces developed in all members and designs. There are some design considerations and detailing principles  for achieving ductility and it is mandatory to do so in Zone 3 and above for all buildings irrespective of size or height.Basically, the overall amount of shear steel ie the links/stirrups increases and the overall amount of longitudinal steel slightly reduces when we do ductility design+ductile detailing.ie, Between, Just a seismic design vs seismic design + ductile consideration will keep the total steel quantity almost same. In zone 2 , seismic resistant design is mandatory irrespective of size or height of the building. Ductility design/detailing is not mandatory. However as mentioned before, zone 2 may experience larger scale acceleration and to prevent damages, a little addition of ductility is desirable. This can be achieved by using Fe500 D steel instead of Fe500 at a nominal additional cost. Also the stirrups in joints and laps shall be closely spaced.Conclusion : Safety is a virtue as much important as function and a nominal increase or sometimes no increase in cost  can ensure this. All required is a careful attention and consideration.

DUCTILE DETAILING