SELECTED EXAMPLES ON FINDING PLASTIC MODULUS, SECTION MODULUS AND EQUAL AREA AXIS

“In this blog, i am going to introduce with an example to deal with one of the basic parameters which are profoundly helpful to understand plastic analysis “

Here you will find very selective and interesting examples for finding equal-area axis, section modulus, plastic modulus to understand the whole concept of plastic analysis.

Above these parameters are said to be the backbone of plastic analysis. Also, in this blog, a very tricky method has been used to find out even section modulus or elastic modulus which is useful to do elastic modulus.

Download pdf for examples on plastic and section modulus

CO-ORDINATION FAILURE AMONG DIFFERENT TEAMS MAY RESULT SEVERE IMPACT ON FUNCTIONALITY ,STRENGTH & COST OF A BUILDING

“In this blog, i am going to share my experience gained in site visit as well as coordinating with different teams like architect, MEP engineers and contractor and few flaws during construction stage and post construction will help you to be cautious as far as co-ordination is concerned”

——- if chaos is created at site due to ambiguity in drawings then be cool, calm & audacious & behave like a spartan & don’t loose faith in yourself if you are correct “

CO-ORDINATION AT OFFICE:

PPR(Preliminary Project Report) Level:

When any project is assigned, an architect make tentative planning & costing i.e(PPR). In this stage , architect submit preliminary cost of the building with some percentage of deviation with few schematic drawings. Structural engineers hardly have any role in PPR stage except for the foundation type that is most likely to occur in the proposed building.

Architect determine preliminary cost by plinth area rate provided by CPWD or state PWD. In this plinth area rate for structure,foundation type shall be mentioned which is most likely to occur in the structure & structural engineer helps architect from soil type or foundation type in the vicinity of the existing building or kind of structure which is to be made. At PPR stage, there is no chances of co-ordination failure as far as cost and stability of structure is concerned.

DPR(Detailed Project Report) Level:

This is the most important stage in which drawings like architectural, structural, MEP and other services are prepared taking all specifications so that there may be little or no deviation at the construction level if tender is done on the basis of cost made on these drawings.

Proposed project become fragile and every flaws come in scanner of everybody’s eyes when things are exposed as a co-ordination failure among different bodies like architects,structural engineers ,PMC and site execution teams.

This can be understood from a simple example . let’s say, architect decides floor height of a building without consulting any HVAC engineer. Later on ,during preparation of HVAC drawings, if HVAC teams demand to raise the height of the building in order to carry the AC duct. If floor is raised in the DPR stage, then it will increase the cost of the building as height of the columns will increase. If architect does not increase the floor height, it may affect elegance and aesthetic of the building. To maintain the same proposed height,architect may consult structural engineers to make sleeves in the beams to carry the AC ducts. In this condition, structural engineer may deny the proposal of sleeves in the beam as the provision of duct might not have taken in the design of beams. From this, a hue and cry may occur between coordinating department & this is not healthy for any organisation. Also, it may tarnish the image of any company.

Furthermore, turbulence between client and design consultant & it’s impact on financial,functional and stability of any structure can better be understood from a another example. In this example, i will walk you through the failure at the master planning level in which architect may have not taken the set back and existing building properly in the master plan. Also, when the proper layout of existing structure is missed in the survey plan and level showing as a contour is not shown properly in the survey plan then these ambiguity creates turbulence at the site when foundation drawings made on the available data is not matching with the actual site. Due to ignorance of the proper layout of existing structure ,there may be a clash of foundation of proposed building with the foundation of existing structure. In that case, foundation shall be revised and most probably that shall be eccentric. In doing so, different provisions like foundation beam or tie beam shall be applied in the foundation system to ensure stability. Ultimately it will affect the cost of the project and will delay the project due to delay in approval from client.

Actually level in foundation drawing is shown as per the contour provided in the survey plan /master plan/site layout. Therefore, at actual site during construction,if the contour provided on the survey plan does not match with the level in the structure drawings then foundation system will be changed if the difference in proposed level and existing level is too high. To ensure stability and strength in the structure,additional tie beams shall be provided and additional filling load shall be applied in the design of the foundation. It will increase the size of the footing. Hence,incorporating these changes, there will be a huge cost implication of the project . Ultimately, it will delay the project.Client may hold the project and can impose penalty to the consultant if the deviation in the cost is too high. Also, contractor will not able to survive at site due to lack of work front due delay in delivery of approved drawings. Contractors can also pack up his team consisting labor and engineers/manager due to huge mobilization cost.

CO-ORDINATION AT SITE:

Before casting slab, electrical and plumbing pipe layout and certain cut out /sleeves for for fire fighting work shall be done . If any of the above is missed, then altering these will lead to vibration in the structural system and some cracks will induce in the brick walls,beams,columns etc.

I am going to share you one of the most frivolous act at site. You can say it a crime if i am not exaggerating it.Actually, in one of my project,ground floor roof slab was sagged . Because of that there was a hue and cry at site as well as at design office. Contractor started blaming design consultant stating that slab was sagged due to inadequacy or design flaws. On reviewing drawing and calculation at office,it was found slab design safe and stable as far as strength and serviceability was concerned. Later on investigating at site, it was found that slab was sagged due to placing prop for slab over sand at the plinth level. During pouring concrete over slab,there was slurry leakage through the shuttering. When these leaked slurry came in contact with the sand, sand started sinking & because of this, prop over this sand also started sinking. Hence, slab got sagged although design was safe. Hence, i will request all structural engineer not loose hope if this kind of things happen at site. Please be calm and cool and review all design calculation to defend faulty construction at site.

You can never say every failure of building is only due to design flaws. Failure can also occur due to poor workmanship and using low standard materials.

Also, i have seen many beams and columns over bulged at site. This bulging is due to use of poor shuttering and poor workmanship. If face of shuttering in beams and columns are not tied with tie bars or other arrangements, then it can lead to bulging of beams and columns.

Apart from this, excessive honey combing in structural elements may occur due to bleeding and segregation of concrete when excessively vibrated or poorly design of concrete in green state.

Note: It’s my humble request to all structural engineers.Please don’t feel low or discouraged if somebody blames you for any kind of ambiguity at site. Instead of wasting time to convince layman, you need to go back to your calculation and behave like a spartan & show audacity if you are correct and produce bravely your design documents and drawings of other coordinating bodies on the basis of that structural drawings were made.

Regards!

MSA

SEVERE CONSEQUENCE OF PROPOSAL OF POINT LOAD ON SLAB OF EXISTING BUILDING

“Feel the structure” MSA

“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 -24.3.2.1 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.

Thanks & regards!

MSA

                 

VERIFIED EXCEL SHEET FOR BAR BENDING SCHEDULE (BBS) FOR FOOTING & COLUMN

“Feel the structure” MSA

This blog is for all all structural engineers who generally need to estimate quantity of reinforcement to justify whether design is economical or not.It will also help quantity surveyor(QS Engineer)

I am very much delighted to share you an excel sheet for bar bending schedule(BBS) which is a very helpful and handy tools for any structural engineer to evaluate whether design of structural members are under the limit of industry norms as far as wight of reinforcement in terms of Kg/sqft or Kg/m3.

In order to convinced client, we as a structural engineer need to furnish BBS so that client may agree with our design. In this excel sheet of BBS, i have shown an example for calculation for footing & column.I will upload BBS for slab and beams in my next blog.

Although BBS is done by Quantity Surveyor(QS) but we should also know it so that we can check BBS made by QS Engineer.

Now please find excel sheet for BBS from the below link.

Download Excel Sheet For BBS for Footing & Column

Thanks & regards!

MSA

VERIFIED EXCEL SHEET FOR ESTIMATION OF CIVIL ITEMS UPTO PLINTH LEVEL

This blog is for all all structural engineers who generally need to estimate civil items particularly volume of RCC which further helps to find out reinforcement/m^3 and it helps designer to calculate reinforcement quantity based on certain thumb rule like quantity of reinforcement(in Kg)/m^3 & check whether design is economical or not.It will also help quantity surveyor(QS Engineer)

“Feel the structure” MSA

Here, i am uploading excel sheet for the calculation of civil items upto plinth beams. In this attached sheet, you need to put data in yellow highlighted area , automatically, earhwork,RCC, PCC and shuttering will come in the same sheet.

Please download attached sheet from the below link

Download Excel Sheet for the estimation of civil items upto plinth level.

This is my first excel sheet for the estimation of civil items except reinforcement. Later on i would upload civil items estimation for a whole building with an example with working drawing.Also, will upload excel sheet for BBS(Bar Bending Schedule). In my later blogs, you will find some thumb rule to evaluate tentative quantity for estimation of reinforcement with the help of concrete quantity. It will help you to check whether your design is under present industry limits of using reinforcement in on the basis of built-up area in square feet or in meter or concrete volume. It will help you evaluate the cost of the building.

It is my suggestion to all structural engineers. You need to know BBS and estimation RCC and certain industry thumb rules as it will help you in cost optimization of building.

Please download attached sheet from the below link

Download Excel Sheet for the estimation of civil items upto plinth level.

Thanks & regards!

MSA

EXAMPLE ON ANALYSIS OF 3-SPAN CONTINUOUS BEAM BY MOMENT DISTRIBUTION METHOD(MDM) – SERIES(III)

“Feel The structure” MSA

Dear reader,

Here i have uploaded a solved example on moment distribution method. This is the third series on moment distribution.I request all readers to read series-(I) & series-(II) on same subject to clarify basic concept as i have started series-1 &2 with two span only. In this example, i have solved a 3-span continuous beam in which first span is uniformly loaded, second span is point loaded and third span is loaded with triangular loading. By going through this example, you will be able to solve 3-span continuous indeterminate beam effortlessly.

Download the complete solved example with detailed explanation on moment distribution from this below link.

Download PDF of attached example on moment distribution method

Thanks & regards!

MSA

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

SPREAD SHEET FOR THE DESIGN OF DOG LEGGED RCC STAIRCASE

“Feel the structure” MSA

Attached here is the ultimate excel sheet for the design of dog legged staircase in reinforced cement concrete(RCC).

Download excel sheet for the design of staircase from the below mentioned link.

DOWNLOAD EXCEL SHEET FOR THE DESIGN OF DOG LEGGED STAIRCASE

Thanks & regards!

MSA

EXCEL SHEET FOR THE DESIGN OF REINFORCED CEMENT CONCRETE (RCC) SLAB

“Feel the structure” MSA

Attached here is ultimate excel sheet for the design of slab as per IS-456-2000. Here, you will find input as well as output data in the same sheet and same has been clearly stated inside the this incredible sheet.

Download spread sheet/excel sheet for the design of slab from the below mentioned link.

DOWNLOAD EXCEL SHEET FOR THE DESIGN OF SLAB

Thanks & regards!

MSA

EXAMPLE ON ANALYSIS OF 2-SPAN CONTINUOUS BEAM BY MOMENT DISTRIBUTION METHOD(MDM) – EXAMPLE, SERIES(II)

Feel the structure” MSA

Download here an attached example solved for two span continuous beam by moment distribution method. In this example, a two span continuous beam in which one span is with uniformly distributed load and another span is loaded by a point load at center. In this example, far end is fixed.

Download pdf of solved example on MDM fromB

DOWNLOAD PDF FOR EXAMPLE SOLVED BY MOMENT DISTRIBUTION FOR CONTINUOUS BEAM WITH FAR END FIXED, EXAMPLE,SERIES-II

In this example, i have tried to detailed out analysis of a continuous beam with moment distribution calculating BMD and SFD and i have cleared concept on carry over moment and balancing moment with giving some special notes.

Thanks & regards!

MSA