GENERAL PROVISIONS FOR THE DESIGN OF REINFORCED CONCRETE MEMBERS

“Feel the Structure” MSA

This blog is dedicated to those who is a beginner in the field of reinforced cement concrete.It will help them to gather key note for RCC design,detailing & site execution.

Before designing reinforced cement concrete(RCC) structural members, it is essential to know the environmental factors in which designed member will be exposed through out it’s life & second aspect for RCC design is it’s strength against shear, bending and torsion which depends upon the characteristics & quantity of its ingredient/components.

How environmental condition will affect proposed grade of concrete, it is mandatory to know the characteristics of different components ,water cement ratio & their proportion in mix design of concrete and reaction of C-H-S gel of concrete in green state & in hardened state with certain deleterious chemicals like chloride & sulphate.

Basically, concrete is made up of cement, sand , aggregate & water. When ingredient like cement, sand & aggregates are mixed with water in proper proportion , plain cement concrete is formed.This is usually called as PCC (Plain Cement Concrete).When reinforcement or rebar is cast along with PCC, then it is called as reinforced cement concrete (RCC) as reinforcement (rebar) reinforces flexural strength of concrte. It is well known fact and experimentally proved that PCC is weak in bending/flexure and strong in compression.Let say, if we are using M25 grade of concrete then this grade of concrete will have characteristic strength as 25N/mm2 which is compressive in nature. As per IS456-2000,flexural strength of M25 is 0.7x (sqaure root of grade of concrete) i.e 0.7X (25)^0.5= 3.5N/mm2. Now you can see, how flexural strength of concrete is much lower than compressive strength .

Hence, when you are asked to design RCC members, then your first priority shall be to calculate shear force and bending moment and accordingly you have to find out which fibre of concrete member is under tension and which one is under compression.On the basis of that, you need to place calculated reinforcement in terms of numbers or spacing to improve tensile strength of concrete. For example, a cantilever slab & beam will have top surface undergoing tension whereas bottom of same will be under compression.As we know that flexural/bending tensile strength of concrete is weak,hence,main reinforcement shall be placed at top whereas nominal reinforcement shall be at bottom of this slab as prescribed by code to control cracks. You can better understand from from the following images.

From above three images like,Fig-1,Fig-2 & Fig-3, i have walked you through the concept of tensile bending & how designed reinforcements are placed on tension face of RCC structural members. If you see Fig-1, which is a typical section for cantilever slab where main reinforcement has been placed at top face of concrete slab as top of this concrete slab is under tension/tensile bending whereas bottom slab is under compression.Hence, main reinforcement has placed at top face of slab whereas distribution bar has been placed at bottom face of slab.As a structural engineer, our main task is to calculate bending moment and accordingly reinforcements in terms of numbers or spacing are placed on tension face. Rest of distribution bars or binder bars(which are minimum) are placed as prescribed in standard code for concrete design to control cracks.

let us see Fig-2 which is a typical section for footing. In this case, when a column is subjected to load(Axial Load with uni- axial or bi-axial moment), a base pressure from soil will be generated in upward direction, hence bending of footing slab will start taking place in the same direction.Bottom face of this footing will be like parabolic curve in upward direction.Here, you can analog a footing as a inverted cantilever slab. Being bottom of footing in tension, main reinforcement has been placed at bottom. It is not mandatory to provide compression reinforcement at top of footing as it is in compression. However, we sometime provide nominal or minimum reinforcement prescribed in code to control cracks.

Now to understand better about tensile bending and placement of main reinforcement in tension area,i would refer you to see fig-3.In Fig-3, i have shown a typical detail of cantilever retaining wall in which two faces of wall have been shown.Left face is towards earth whereas right face is towards road side. As this retaining wall is a vertical cantilever, hence, left face will be in tension whereas right face will be in compression.As earth has retained in left side of this wall, force exerted by earth will try to bend wall in right direction creating tension in left face.Hence, main reinforcement has been placed in left face of the wall.Main reinforcement can be placed in both faces of wall only when free end (top of wall) is hinged or fixed.

Now to deal same concept in more detail, i have come up with Fig-4 ,which is a liquid retaining structure in which reinforcement in RCC wall has been shown in the both faces of wall as main reinforcement. If you see closely W-2 & W-3, same face of wall may be sometime in tension & sometime in compression depending upon whether tank is full or tank is empty. Hence, main reinforcement has been placed in both faces of wall.

Above discussion is all about the detailing part only as far as bending strength is concerned.But this is not enough.In order to have a safe,durable & aesthetically appealing structure we need to check certain codal provisions and criteria at design as well as at construction stage. Point wise these are explained below.

  • Grade of Concrete: As per the exposure of proposed structure throughout it’s life and to reduce section size in high rise buildings, grade of concrete is decided.For example, if you are going to design a marine structure then minimum grade of concrete shall be M30 as per IS:456-2000 whereas for any other structure, we can start our design with a minimum garde of M20. Apart from grade of concrete, water cement ratio shall be different in different component of same building like foundation, column,beams & slabs.Sometimes, soil having sulphate, sulphate resisting cement(SRC) is used instead of normal PPC( Portland Pozzolana Cement ) & OPC ( Ordinary Portland Cement ) for durability.As sulphate voluminize concrete up 300% after reacting with C-H-S gel(Complex hydration product of cement) & hence huge cracks are formed which are further source of deterioration of reinforcement & concrete.Furthermore,if there is high congestion of reinforcement and column sizes are big like (1.5m x 1.5m, 2m x 2m and so one),self compacting concrete is used to avoid mechanical vibrator as it will be very difficult to compact such a mass concrete. Also, self compacting or high flow concrete (which are also known as special concrete) is used when there is high congestion at junction of column & beams. In this case if normal concrete is used , then concrete will not be compacted properly even after using mechanical vibrator. This happens because it is very difficult to insert needle of mechanical vibrator inside heavily congested reinforcement and therefore, honey comb is formed.Due to formation of honey comb , strength of RCC member is reduced. If honey comb is formed due to slurry leakage, lack of compaction, improper mixing & placement,this shall be immediately repaired by injection grouting or any other approved method.

Above explanation on concrete was only on selection of concrete in design stage. Until & unless, proper mixing, placing and curing of concrete is done at site, we can not ascertain durability & strength.As bad quality of shuttering (leading to slurry leakage),improper placing & compaction will lead to the formation of honey comb. On the other hand, improper curing will lead to formation of weaker concrete in hardened state as hydration process will be completed because of lack of water.If we say,water is blood of concrete then it will not be an exaggerated statement.

Thanks & regards!

MSA

HOW TO MAKE CAREER IN STRUCTURAL ENGINEERING

“Feel the structure” MSA

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:

  1. Architects
  2. Structural Engineers
  3. Structural draughtsman
  4. MEP (Mechanical, Electrical & Plumbing Engineers)

STEPS INVOLVED:

STEP: 1Surveying

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: 2Geo 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: 3Preliminary 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.

Thanks & regards!

MSA