“This blog will acts as a precautionary tool for any structural engineer to allow customer/client to put any heavy point load on existing slab of structure or not “
Precaution is better than cure.
This blog will guide you whether you will permit additional point load on any building on which that much load was not considered during design. There are two vital points that will be discussed here.
One is failure due to punching due to gravity loading and second one is failure occurred due to change in mode of vibration of building during earthquake
1. PUNCHING FAILURE:
AS per clause -18.104.22.168 of Indian standard, IS:456-2000 (Indian Standard code for plain and reinforced concrete – code of practice, edition-2000), existing slab is not able to respond huge punching stress subjected by a point load of 1 ton or more which was earlier not considered in design.This happens due to inadequate thickness of slab. Actually during design of slab, that much unexpected point load was not supposed to occur throughout it’s design life & when unexpected heavy point load is put on slab, failure takes place. This kind of punching failure is brittle in nature and slab will fail without any alarm. Generally,dead load and imposed live load is applied as KN/m2 or Ton/m2 unless & until any provision of equipment or pointed object is there on slab.
Another reason for not permitting any point/concentrated load on existing slab at slab level is that concrete lose its strength when it becomes old due to its exposure to environment having deleterious chemicals which corrodes reinforcement.
Secondly, any building adjust on its own by prolonged creep and shrinkage action when structural members are subjected with load after few days or month of construction . But suddenly when building is subjected to huge load, it undergoes huge deformation exceeding limit limiting values of deformation.
2.CHANGE IN BEHAVIOR/FAILURE MECHANISM DURING EARTHQUAKE:
As per clause 7.3.1 of Indian standard IS:1893-2016(Part-1 for Criteria for earthquake resistant design of structure) , lump mass at floor level shall be dead load + some percentage of live load. Here in this case proposal of heavy point load at any floor will increase dead load. Consequently, lump mass will increase significantly. This will result in attracting more earthquake forces as earthquake forces are inertial forces. Also, greater super dead load at floor result in deviation in center of mass (C.M) & center of rigidity(C.R) .This will create torsion in building during earthquake. As it is well- known fact, column are very weak in torsion/twisting. Hence, columns are most likely to fail during earthquake as provision of torsion/twisting resistance system may have not considered in design.
Also, as per clause 7.1, table-6 of Indian standard IS:1893-2016(Part-1) , putting heavier load to upper floor will create soft storey at lower and upper floor and will distort failure mechanism in building. This kind of soft storey will change the mode shapes of vibration of building resulting in failure of column at soft storey level.
Note: This Blog is for those civil engineers who want to make career in Structural Designing of buildings
This is a high time to share with you a very vital information for a beautiful career in structural engineering. Essentially you need to be civil engineer. If you are M.Tech in structural engineering, then it is more preferable.However, it’s not a strict criteria to enter in the field of structural engineering and designing. You can Enter in this field with bachelor degree as well.
If you are very keen to work in this challenging field, then you need to know the inside story of structural engineer working in this field. Here i am going to tell you the entire procedure involved in the analysis, designing , co-ordination and issuing drawing to sites. If you know all these steps in advance, the it will be very easy for you to establish in this field.
Before writing steps involved from conceptual level to construction stage, you need to understand the team involved for the finalization good for construction drawings.
Following teams involved in the finalization of drawings at office & issuing it to site:
MEP (Mechanical, Electrical & Plumbing Engineers)
STEP: 1– Surveying
After receiving work order from a client, architect co-ordinate with a surveying agency to make contour plan for deciding buildings plinth level and site development and this contour plan is very essential to know the site condition whether it is above the existing road level or below the road level. Structural engineers substantially use reduced level mentioned in the contour plan to filling load if filling is required to achieve desired plinth level. Also, ramp and road are made based on this contour plan.
STEP: 2– Geo technical Investigation
Architect after receiving contour plan,they start planning building on it. They show road development and fit all building on this survey with orientation & based on this, master plan is made.
After putting all building desired by client, architect pass on this site plan having all blocks to the structural team to locate bore hole for soil investigation for foundation design. Structural engineer mar bore hole as BH-1, BH-2, Bh-3 & so on the site plan and issue it the Geo- technical investigation agency with certain specification to find out safe bearing capacity(for open foundation),pile load capacity if there is recommendation of pile & modulous of sub grade reaction if there is a recommendation of raft.
Note: No Geo technical report is assumed to be final until & unless it is approved by a structural engineer. A structural engineer thoroughly review the soil report. Structural engineer guides Geo technical engineer to provided additional data if needed giving a concrete reason. For example, if there is recommendation for pile foundation of 600 mm diameter, then it is our responsibility to ask Geo technical engineer to provide recommendation for 450 mm & 300 mm diameter pile as well for the small buildings. Also, in report, Geo technical engineer shall be reminded to provide recommendation sheet for isolated footing because for extremely small structures like guard room, porta cabins & boundary walls,this data will be useful. Also,in some Geo technical report,there may be only one recommendation of isolated footing but we should ask for the recommendation for sub grade reaction as in the same site , there may a provision of under ground tank or in some places most of individual footings for separate column may over lap each others, hence recommendation for sub grade modulus is essential for designing raft.
STEP: 3– Preliminary Framing & Column Placement
Once site plan is finalized, column is placed with different orientation in each building blocks and based on column placement, tentative framing is made. This framing with column is shared to the architectural team to get their approval as per the functionality of building. Also, tentative beam depth is shown in the drawing. Based on this, architect may change floor to floor height of the building if excessive depth of beam is required for the desired span. Apart from this,Architect coordinate with MEP team for the cut outs for plumbing & fire fighting work. Also, there may AC duct running inside the false ceiling. Based on these criteria, Architect finalize the floor to floor height.Once tentative framing is approved, architect again issue separate drawings to structural engineer of each building blocks with column matching with structural framing and all sections and elevation & after that preparation of tender or DPR(Detail Project Report) starts.
STEP: 4-PREPARATION OF TENDER DRAWINGS & SPECIFICATIONS
After approval of preliminary /conceptual drawings, tender drawings are made. For that,a structural engineer needs to start modeling & analyzing building in softwares like STAAD Pro, Etabs etc. Once analysis & designing is over,structural engineer gives final column sizes & beams to the structural draughtsman.Also,whatever type of foundation is mentioned in the approved soil report, structural engineer provide foundation and that is drafted by draughtsman.
Complete set of drawings having column marking plan, column schedule, foundation plans & sections,floor wise all framing plans having beams sizes and slab thickness, desired section in sunken,cantilever in framing along with staircase sections having waist slab thickness, landing beam sizes and if there is over & under ground tank , its detail shall be provided with section sizes.
Ins short, section sizes of all structural elements are required for quantity take off in making BOQ.
Also, specification for grade of concrete(Like M20,M25,M30…etc), grade of reinforcement(Like Fe500, Fe550 …etc.),grade of cement,type of cement(Like OPC, PPC or sulphate resisting cement) , type of sand, type water proofing materials & type of bricks /blocks shall be provided by structural engineer to the estimator to consider these specs in bill of quantity(BOQ).
STEP: 5-PREPARATION OF GOOD FOR CONSTRUCTION DRAWINGS(GFC)
After getting approval of tender drawings from department/client, department/client call contractor for bidding & this process take considerable time. Meanwhile, as a structural engineer, our main focus is to work on good for construction drawings. If there are some minor correction like missing dimensions, missing sections then these shall be resolved in the GFC drawings. Also, detailed reinforcement in beam as beam elevations and slab as reinforcement layout is made for construction. Also, if there is any instructions from department/client for drawing to be vetted from government institute or individual consultancy firm, then detail calculation report termed as DBR(design basis report) shall be made along with STAAD/ETABS file. These consolidated DBR & STAAD/ETABS file shall be submitted to vetting authority and time to time they shall be chase to get all submitted drawings approved so that there will not be any delay in construction.
I am a Structural Engineer. My main work frame is to analyse & design simple to complex structure. I encourage many young civil engineers who have just passed out from college and want to make a career in Structural Engineering.Also, my blogs will work as a tonic to some civil engineers who are already in this field but feeling very low due to lack of knowledge and skills. My blogs are certainly going to bring not only their professional growth but also they will start loving this challenging field.It will indulge them to be more innovative.
4’S&DE is my basic principle of analysing and designing any structure and same shall be followed by any Structural Engineers.I have seen many engineers who sometime think, if a structure is having sufficient strength against shear, moment and torsion instead of lacking in stiffness and durability, it looks fine to them. But i want to clarify one vital point, without having proper knowledge of deflection check(Based on Stiffness),Durability Check(Based on proper grade of material selection, life time exposure of structure to environment) and Economy (Based on client’s budget) no Structural Engineer is going to be very successful in their field.
My friends, if you want to make a career as a RCC Designer, then it is very important for you to understand deeply about concrete technology. Until & unless ,you are able to understand the chemical reaction tacking place in concrete and adherence it’s hydration product to different deleterious gases and chemicals in the environment,you will not be able to judge which materials like PPC or OPC or sulphate resisting cement is most suitable as far as durability is concern.You will be astonished by knowing the fact,sulphate reacts with the C-H-S gel(hydration product of cement) and concrete gets expanded by 300% by volume. You can feel now to what extent it can damage concrete by developing cracks and it further give path to ingress other deleterious chemicals like chloride to reach reinforcement. Also we should have knowledge about high performance concrete like high flow concrete & self compacting concrete where there is a congestion of reinforcement to avoid honey combing.
And last one is economy. It requires rigorous analysis or you can say this is the most tedious part. In order to design building under the specified budget, we require lot of reiteration process giving optimum member size fulfilling 4’SD.
My main area of blog writing is to cover 4’SDE. Time to time, i will share excel sheets that will help not only freshers but also practising engineers.Also, i will regularly update you with some hand calculation for the analysis and designing of simple frame and structural elements to bring you all to the basic. Please keep in mind, in fast pace of working environment, we are forgetting our basics and it is becoming very disastrous for all of us.Above all,i believe in sharing of knowledge and skills.What is the use if somebody is having huge knowledge and skills & carrying all these to grave. I think,noting at all. I have realised this now and understood the importance of sharing. So, hope somebody will get benefit from my sharing. Also believe, even if a single person is able to uplift his/her career,then my blog writing will be successful.
Most importantly, even a practising structural engineers will find something very new and innovative in my blog.
Last but not the least, i will request you all who reads my blog to give their valuable observations/suggestions to improve it’s content.