Causes Of Design And Detailing Errors In Construction Industry

Design And Detailing Errors In Construction Industry

Common design and detailing errors in construction arises due to either inadequate structural design or due to lack of attention to relatively minor design details. These types of design errors are discussed below:

Inadequate Structural Design

Due to inadequate structural design the concrete is exposed to greater stress than it can handle or strain in concrete increases more than its strain capacity and fails.

The symptoms of such kind of failures due to inadequate structural design shows either spalling of concrete or cracking of concrete. Excessively high compressive stress due to inadequate structural design results in spalling of concrete. Also, high torsion or shear stresses results in spalling or cracking of concrete. High tensile stresses also results in cracking of concrete.

To identify the inadequate design as cause of the structural damage, the structure shall be inspected and locations of the damage should be compared to the types of stresses that should be present in the concrete. For rehabilitation projects, thorough petrographic analysis and strength testing of concrete from elements to be reused will be necessary.

Prevention: Inadequate structural design can be prevented by thorough and careful review of all design calculations. Any rehabilitation method that makes use of existing concrete structural members must be carefully reviewed.

Design-Detailing-Errors-In-Concrete-Construction

Poor Design Details:

Poor design details can cause localised concentration of high stresses in structural members even if the design is adequate to meet the requirements. These high stresses may lead to cracking of concrete that allows water or chemicals to pass through the concrete. Thus poor design detail may lead to seepage through the structural members.

Poor design detail may not lead to structural failure, but it can become the cause of deterioration of concrete. These problems can be prevented by a thorough and careful review of plans and specifications for the construction work.

Types of poor design detailing and their possible effects on structures are discussed below:

Abrupt Changes In Section: 

Abrupt changes in section may cause stress concentrations that may result in cracking. Typical examples would include the use of relatively thin sections rigidly tied into massive sections or patches and replacement concrete that are not uniform in plan dimensions.

Insufficient Reinforcement At Corners And Openings: 

Corners and openings also tend to cause stress concentrations that may cause cracking. In this case, the best prevention is to provide additional reinforcement in areas where stress concentrations are expected to occur.

Inadequate Provision For Deflection: 

Deflections in excess of those anticipated may result in loading of members or sections beyond the capacities for which they were designed. Typically, these loadings will be induced in walls or partitions, resulting in cracking.

Inadequate Provision For Drainage: 

Poor attention to the details of draining a structure may result in the ponding of water. This ponding may result in leakage or saturation of concrete. Leakage may result in damage to the interior of the structure or in staining and encrustations on the structure. Saturation may result in severely damaged concrete if the structure is in an area that is subjected to freezing and thawing.

Insufficient Travel In Expansion Joints: 

Inadequately designed expansion joints may result in spalling of concrete adjacent to the joints. The full range of possible temperature differentials that a concrete may be expected to experience should be taken into account in the specification for expansion joints. There is no single expansion joint that will work for all cases of temperature differential.

Incompatibility Of Materials: 

The use of materials with different properties (modulus of elasticity or coefficient of thermal expansion) adjacent to one another may result in cracking or spalling as the structure is loaded or as it is subjected to daily or annual temperature variations.

Neglect Of Creep Effect: 

Neglect of creep may have similar effects as described for inadequate provision for deflections. Additionally, neglect of creep in prestressed concrete members may lead to excessive prestress loss that in turn results in cracking as loads are applied.

Rigid Joints Between Precast Units: 

Designs utilizing precast elements must provide for movement between adjacent precast elements or between the precast elements and the supporting frame. Failure to provide for this movement can result in cracking or spalling.

Unanticipated Shear Stresses In Piers, Columns, or Abutments: 

If, through lack of maintenance, expansion bearing assembles are allowed to become frozen, horizontal loading may be transferred to the concrete elements supporting the bearings. The result will be cracking in the concrete, usually compounded by other problems which will be caused by the entry of water into the concrete.

Inadequate Joint Spacing In Slabs: 

This is one of the most frequent causes of cracking of slabs-on-grade.

Guide for RCC Slab Design and Detailing in Building Construction

Guide for RCC Slab Design and Detailing in Building Construction

RCC Slab design and detailing guidelines for depth of slab, loads on slab, reinforcement guide for one-way and two-way slabs have been tried to present here. Following are the RCC Slab Design and Detailing guidelines:

RCC Slab Design Guidelines:

a) Effective span of slab:

Effective span of slab shall be lesser of the two

1.  L = clear span + d (effective depth )

2.  L = Center to center distance between the support

b) Depth of slab:

The depth of slab depends on bending moment  and deflection  criterion.  the trail depth can be obtained using:

• Effective depth d= Span /((L/d)_Basic x modification factor)
• For obtaining modification factor, the percentage of steel for slab can be assumed from 0.2 to 0.5%.
• The effective depth d of two way slabs can also be  assumed using cl.24.1,IS 456 provided short span is 3.5 m and loading class is < 3 KN / m²

Type of support	Fe-250	Fe-415
Simply supported	L/35	L/28
Continuous support	L/40	L/32

Or, the following thumb rules can be used:

• One way slab, d = (L/22) to (L/28).
• Two way simply supported slab, d = (L/20) to (L/30)
• Two way restrained slab, d = (L/30) to (L/32)

c) Load on slab:

The load on slab comprises of Dead load, floor finish and live load. The loads are calculated per unit area (load/m²).

Dead load = D x 25 kN/m² ( Where D is thickness of slab in m)

Floor finish (Assumed as)= 1 to 2 kN/m²

Live load (Assumed as) = 3 to 5 kN/m² (depending on the occupancy of the building)

Concrete Slab

Detailing Requirements of RCC Slab as per IS456: 2000

a) Nominal Cover:

For Mild exposure – 20 mm

For Moderate exposure – 30 mm

However, if the diameter of bar do not exceed 12 mm, or cover may be reduced by 5 mm. Thus for main reinforcement up to 12 mm diameter bar and for mild exposure, the nominal cover is 15 mm.

b) Minimum reinforcement:

The reinforcement in either direction in slab shall not be less than

• 0.15% of the total cross sectional area for Fe-250 steel
• 0.12% of the total cross-sectional area for Fe-415 & Fe-500 steel.

c) Spacing of bars:

The maximum spacing of bars shall not exceed

• Main Steel – 3d or 300 mm whichever is smaller
• Distribution steel –5d or 450 mm whichever is smaller Where, ‘d’ is the effective depth of slab. Note: The minimum clear spacing of bars is not kept less than 75 mm (Preferably 100 mm) though code do not recommend any value.

d) Maximum diameter of bar:

The maximum diameter of bar in slab, shall not exceed D/8, where D is the total thickness of slab.

How to design Reinforced Concrete Foundations - RCC Foundation Design

Reinforced concrete foundations are designed based on column loads and moments at base and the soil data. Following are the types of foundations in order of preference with a view to economy:

(i) Individual footings (isolated footing)

(ii) Combined footings (combination of individual footings

(iii) Strip footings with retaining wall acting as strip beam wherever applicable.

(iv) Raft foundations of the types (a) slab (b) beam-slab.

The brick wall footings can also be designed. Often plinth beams are provided to support brick walls and also to act as earthquake ties in each principal direction.

Important considerations in design of foundations:

Foundations are the structural elements which transfer loads from the building or individual columns to the earth. If these loads are to be properly transmitted, foundations must be designed to prevent excessive settlement or rotation, to minimize differential settlement and to provide adequate safety against sliding and overturning.

Depth of foundation:

Depth of foundation below ground level can be obtained by using Rankine’s formula:

Where, h = minimum depth of foundation

p = gross bearing capacity

= density of soil

 = angle of repose or internal friction of soil.

Recommendations of IS456: 2000, Limit state design, bending, shear, cracking and development length:

To determine the area of foundation required for proper transfer of total load on the soil, the total load (combination of dead load, live load and any other load without multiplying it with any load factor) are considered.

Thickness of the edge of footing:

As per clause 34.1.3 of IS456: 2000, the thickness at the edge shall not be less than 15 cm on soils.

Dimension of pedestal:

In the case of plain cement concrete pedestals, the angle between the plane passing through the bottom edge of the pedestal and the corresponding junction edge of the column with pedestal and the horizontal plane shall be governed by the expression.

Dimensioning of Foundation

Where qo = calculated maximum bearing pressure at the base of the pedestal/footing in N/mm2

Fck = characteristic strength of concrete at 28 days in N/mm2

Dimensioning of Pedestal

Fig: Dimensioning of pedestal

Maximum Bending moment in footings:

The bending moment will be considered at the face of column, pedestal or wall and shall be determined by passing through the section a vertical plane which extends completely across the footing, and over the entire area of the footing or one side of the said plane. The reference clause is 34.2.3.1 and 34.2.3.2 of IS456: 2000.

Shear capacity checks for footings:

The shear strength of footing is governed by the following two factors:

a) The footing acting essentially as a wide beam, with a potential diagonal crack intending in a plane across the entire width, the critical section for this condition shall be assumed as a vertical section located from the face of the column, pedestal or wall at a distance equal to the effective depth of the footing in case of footings on soils.

For one way bending action of footing (one way shear)

For one way shear action, the nominal shear stress in calculated as:

Where,

= shear stress

Vu = factored vertical shear force

b = breadth of critical section

d = effective depth

(shear stress = design shear strength of concrete based on % longitudinal tensile reinforcement. Refer table 61 of SP -16)

Critical section for one-way shear in foundation

Two way shear (or two way bending action or punching shear) of foundation:

For two way bending action, the following should be checked in punching shear. Punching shear shall be around the perimeter 0.5 times the effective depth away from the face of the column or pedestal.

For two way shear action, the nominal shear stress is calculated in accordance with clause 31.6.2 of IS456: 2000 as follows:

Where = shear stress

bo = periphery of the critical section

d = effective depth

Vu = factored vertical shear force

When shear reinforcement is not provided, the nominal shear stress at the critical section should not exceed

Where, Ks = 0.5 + Bc (but not greater than 1)

Bc = (short dimension of column or pedestal / long dimension of column or pedestal)

 N/mm²

Note: It is general practice to make the base deep enough so that shear reinforcement is not required.

Development length of reinforcement bars in foundation:

The critical section for checking the development length in a footing shall be assumed at the same planes as those prescribed for bending moment in clause 34.2.3 of code and also at all other vertical planes where abrupt changes in section occur. Refer clause 34.2.4.3 of IS456: 2000.

Reinforcement in foundations:

The minimum reinforcement in footing slab specified by the code is 0.12% and maximum spacing specified is 3 times the effective depth or 450mm whichever is less. (clause 34.3).

Only tensile reinforcement is normally provided. The total reinforcement shall be laid down uniformly in case of square footings. For rectangular footings, there shall be a central band, equal to the width of the footing. The reinforcement in the central band shall be provided in accordance with the following equation.

Where,

Transfer of load at the base of column:

Clause: 34.4 of IS456: 2000.

The compressive stress in concrete at the base of column or pedestal shall be transferred by bearing to the top of supporting pedestal or footing.

The bearing pressure on the loaded area shall not exceed the permissible bearing stress in direct compression multiplied by a value equal to

Transfer of load at the base of column

 but not greater than 2.

Where,

A1 = supporting are for bearing of footing, which is sloped or stepped footing may be taken as the area of the lower base of the largest frustum of a pyramid or cone contained wholly within the footing and having its upper base, the area actually loaded and having side slope of one vertical to two horizontal.

A2 = loaded area at the column base.

For limit state design, the permissible bearing stress specified is 45 fck.

If the permissible bearing stress is exceeded either in the column concrete or in footing concrete, reinforcement must be provided for developing the excess force. The reinforcement may be provided either extending the longitudinal bars into the footing or by providing dowels in accordance with the code as given by the following:

1. Minimum area of extended longitudinal bars or dowels must be 0.5% of cross-sectional area of the supported column or pedestal.

2. A minimum of four bars must be provided.

3. If dowels are used their diameter should not exceed the diameter of the column bars by more than 3mm.

4. Enough development length should be provided to transfer the compression or tension to the supporting member.

5. Column bars of diameter larger than 36 mm, in compression only can be dowelled at the footing with bars of smaller diameters. The dowel must extend into the column a distance equal to the development length of the column bar. At the same time, the dowels must extend vertically into the footing a distance equal to the development length of the dowel.

Rigid and Spread Footings

How to design an isolated individual footing - Step by step procedure

Isolated footing design example with step by step procedure and isolated footing design excel sheet (spreadsheet) is also provided for easy and fast calculation.

Learning design with examples is always the best method of learning. Step by step procedure for structural design of isolated footing is discussed below:

Isolated Footing Design Example:

Let us consider an isolated footing for an RCC column of size 450mm x 450mm. Loads from this column to the foundation are:

Vertical Load: 1000 kN

Uniaxial Moment: 100 kNm

The safe bearing capacity (SBC) of soil is 300 kN/m2. The grade of concrete to be used is M30 and grade of steel is Fe415.

Step by Step Procedure of Footing Design:

Step -1: Determining size of footing:

Loads on footing consists of load from column, self weight of footing and weight of soil above footing. For simplicity, self weight of footing and weight of soil on footing is considered as 10 to 15% of the vertical load.

Load on column = 1000 kN

Extra load at 10% of load due to self weight of soil = 1000 x 10% = 100kN

Therefore, total load P = 1100 kN.

Size of footing to be designed can be square, rectangular or circular in plan. Here we will consider square isolated footing.

Therefore, length of footing (L) = Width of footing (B)

Therefore area of footing required = P/SBC

= 1100/300 = 3.67 m2

Provide Length and width of footing = 2m

Area of footing = 2 x 2 = 4m2

Now the pressure on isolated footing is calculated as

Pressure on Isolated Footing

When calculated, pmax = 325 kN/m2

pmin = 175 kN/m2

But pmax is greater than SBC of soil, so we need to revise the size of footing so that Pmax is below 300 kN/m2.

Consider width and length of footing = L =B =2.25m

Now, pmax = 250.21 kN/m2 (< 300 kN/m2 = OK)

and pmin = 144.86 kN/m2 > 0 (OK)

Hence, factored upward pressure of soil = pumax = 375.315 kN/m2

pumin = 217.29 kN/m2

Further, average pressure at the center of the footing is given by Pu,avg= 296.3 kN/m2

and, factored load, Pu= 1500 kN, factored uniaxial moment, Mu= 150 kN-m.

Step 2: Two way shear

Assume an uniform overall thickness of footing, D =500 mm

Assuming 16 mm diameter bars for main steel, effective depth of footing ‘d’ is

d = 500 – 50 – 8 = 452 mm

The critical section for the two way shear or punching shear occurs at a distance of d/2 from the face of the column (Fig. 1), where a and b are the dimensions of the column.

Isolated Footing Design

Fig 1: Critical section for Two Way Shear (Punching Shear)

Hence, punching area of footing = (a + d)2 = (0.45 + 0.442)2 = 0.796 m2

where a = b = side of column

Punching shear force = Factored load – (Factored average pressure x punching area of footing)

= 1500 – (296.3 x 0.0.796)

= 1264.245 kN

Perimeter along the critical section = 4 (a+d) = 4 (450+ 442) = 3568 mm

Therefore, nominal shear stress in punching or punching shear stress is calculated as below:

= 1264.245 x 1000/(3568×442) = 0.802 N/mm2

Allowable shear stress =

where allowable shear stress,   = 1.369 N/mm2

= = 1

therefore, allowable shear stress = 1×1.369 = 1.369 N/mm2

Since the punching shear stress (0.802 N/mm2) is less than the allowable shear stress (1.369 N/mm2), the assumed thickness is sufficient to resist the punching shear force. Hence, the assumed thickness of footing D = 500 mm is sufficient. Please note, there is much difference between allowable and actual values of shear stress, so depth of footing can be revised and reduced. For our example, we will continue to use D = 500mm.

Step 3: Design for flexure:

The critical section for flexure occurs at the face of the column (Fig. 2).

Design of isolated footing - Design for Flexure

Fig. 2 Critical section for flexure

The projection of footing beyond the column face is treated as a cantilever slab subjected to factored upward pressure of soil.

Factored maximum upward pressure of soil, pu,max= 375.315 kN/m2

Factored upward pressure of soil at critical section, pu= 312.1 kN/m2

Projection of footing beyond the column face, l = (2250 – 450)/2 = 900 mm

Bending moment at the critical section in the footing is given by:

Mu = Total force x Distance from the critical section

Considering uniform soil pressure of 375.315, Mu = 180 kN/m2

0.92

from SP 16, percentage of reinforcement can be found for M30 concrete, fe415 steel for above pt = 0.265%

Ast = pt x bxd

considering 1m wide footing, Ast required = 1171.1 mm2/ m width

Provide 16 dia bar @ 140mm c/c

Repeat this exercise for other direction as well. Since, uniform base pressure is assumed, and it is a square footing, Mu and Ast for other direction will be same.

Step 4: Check for One-Way Shear:

The critical section for one way shear occurs at a distance of ‘d’ from the face of the column.

Factored maximum upward pressure of soil, pu,max= 375.315 kN/m2

Factored upward pressure of soil at critical section, pu= 375.315 kN/m2

For the cantilever slab, total Shear Force along critical section considering the entire width B is

Vu = Total Force X (l – d) X B

= 375.315 X (0.9 – 0.442) X 2 = 343.8 kN

Nominal shear stress = Vu/(Bxd) = 0.346 N/mm2

For, pt = 0.265, and M30, allowable shear force from Table – 19, IS456 is greater than 0.346 N/mm2

Therefore, the foundation is safe in one-way shear.

Step 5: Check for development length

Sufficient development length should be available for the reinforcement from the critical section.

Here, the critical section considered for Ld is that of flexure.

The development length for 16 mm diameter bars is given by

Ld= 47 x diameter of bar = 47 x 16 = 752 mm.

Providing 60 mm side cover, the total length available from the critical section is

0.5 x ( L – a) – 60 = 0.5 x (2250 – 450) – 60 = 840 > Ld, Hence O.K.

Reinforcement detailing of RCC Slab Openings and Cutouts in Design

Many times openings (cutouts) are required to be provided in reinforced concrete slab in buildings to provide way for lifts, or cables, ducts or other instrument to pass through one floor to other floors, mainly in the case of industrial buildings. But in that case, special care need to be taken while detailing of reinforcement for such openings in slabs.

Detailing of opening in slabs is based on factors like size of opening, loads on slabs, floor vibrations etc. In the case of large openings it is desirable to place beams under the opening area so that no additional detailing of slab reinforcement is needed and load is directly transferred to main beams through secondary beams.

Small opening is slabs:

Where openings are small and are less than 150 mm in larger dimension, with no special loading or vibration condition of slabs, following type of detailing should be done:

Detailing of small slab openings

Detail of Small Slab Openings - Cutouts

For Holes less than 150mm, reinforcement bars may be displaced, additional reinforcement is not required.

Slab openings of 150 mm to 450 mm

For slabs with openings more than 150 mm and less 450 mm, at least one half the quantity of main steel reinforcement intersected by the opening is provided parallel to the main reinforcement bars on each side of the opening extending a length of development length Ld beyond edges of the opening as shown in figure below. This reinforcement is provided on both top and bottom face of the slab.

Detailing of medium slab openings

Detail of Medium Slab Openings - Cutouts

Slab openings of 450 mm to 900 mm

For slabs with openings more than 450 mm and less 900 mm, at least one half the quantity of main steel reinforcement intersected by the opening is provided parallel to the main reinforcement bars on each side of the opening extending a length of development length Ld beyond edges of the opening on both top and bottom slab face as shown in figure below. In addition to this, diagonal bars are provided on both top and bottom face of the slab as shown.Detailing of large slab openings

Detail of Large Slab Openings - Cutouts

Larger Slab Openings:

For larger slab opening larger than 900 mm, it should be specifically designed and detailed. It is recommended to use beams below larger slab openings.