Fundamentals of RCC Design

8/10/2019 Fundamentals of RCC Design

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FUNDAMENTALS OF REINFORCED CONCRETE DESIGN

By

ENGR. VICTOR O. OYENUGA

(HND, BSc(Hons), MSc, DIC, PGD(Comp. Sc.), FNSE, FNIStructE, FNICE, MNIOB

Managing Director: Vasons Concept Consultants Ltd(Consulting Engineers and Town Planners)

Engr. V. O. Oyenuga became aPartner of M/S Vasons Concept

Group in 1991 and currently the

MD/CEO. He worked briefly with

Yaba College of Technology and

Lagos State Polytechnic, Isolo,

Lagos, where he resigned hisappointment in 1989 as a Senior

Lecturer and Acting Head of

Department of Civil

Engineering. His design work

include: Teslim Balogun

Stadium, Surulere, Lagos,

Reconstruction of Petroleum

Products Jetties Apapa, Ikeja

Plaza and the various projects of

Babcock University, Ilishan

Remo, his town of birth. He is a

Fellow of the Nigerian Society

of Engineers and the Nigerian

Institution of Structural

Engineers.

Engr. Oyenuga is the author ofthe following publications: 1).

Todays Fortran 77

Programming 2). Simplified

Reinforced Concrete Design, 3).

Concise Reinforced Concrete

Design 4). RCD2000

Reinforced Concrete Design

Programs and 5) Design and

Construction of Foundations.

Engr. Oyenuga is married with

children and they are members

of the Seventh-Day Adventist

Church in Nigeria.

ABSTRACTStructural design is an art and the artist must be convinced of the implications of the final product. The objective of

this paper is to highlight the basic load and design fundamentals that must be observed for the economic and safe

design of the structure. Various load forms are highlighted and practical examples given. Wind load and itsapplication on the structure are briefly discussed.

Ability to trace the load path up to foundation level is discussed. The various design philosophies are enunciated. To

assist in the design, values of some important parameters are given in tabular form.

1.0 INTRODUCTIONStructural and Civil Engineers deal with forces of nature, which can only be predicted to a reasonable extent. For

example, a dam was designed for a 50year rain and one month after its completion a 100year rain fell causing a total

damage to the dam structure. Who is to be blamed? Thus, no engineer could say with all certainly that he has got a

perfect solution to any design problem.

However, as a result of intensive research, experimental and observational data, a level of confidence has beenachieved in virtually all aspects of civil/structural engineering to such a level that a near certainty can be achieved.

The objective of this paper is to discuss the various loads and load forms that must be thoroughly looked into as well as

their application in the design of building structures. In most cases, poor load estimation as well as poor load tracing

lead to collapse of building structures aside poor materials and workmanship. In addition, some basic design

fundamentals are discussed.

Structural loads must be properly assessed and successfully transferred to the founding member. The receiving soil

must also be of such composition and texture so as to receive the imposed load without undue stress. It is a common

believe that all buildings on poor marshy soil be founded on raft foundation. It should be clearly stated here that raft

foundation is NOTa solution to all foundation problems. For example, a soil with 20kN/m2bearing capacity imposed

with 50kN/m2building on raft foundation will definitely sink, the foundation type notwithstanding The building may,

however, not crack, that is, it may tilt in one direction because of the structural rigidity of the foundation and the

superstructure frame. Such a building in question may require short pile footings. On the other extreme, building a

bungalow on raft foundation may be highly uneconomical since simple wide strip foundation may have been verysuitable. The summary of the foregoing is that soil tests and their correct interpretation are necessary even for the most

simple structure especially where the soil structure is very doubtful.


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2As a guide the structural form and possible foundation type is shown in Table 1. The information are for

guidance only and the designer is advised to seek tests information.

Table 1: Suggested Appropriate Foundation Type for Building

Soil Type

Type of Buildings

Bungalow 2-storey 3-5 storey Medium Rise High Rise

Good soil >100kN/m2 Strip Strip Pad Pad Pile

Average soil 75100kN/m2 Strip Wide Strip Pad Pile Pile

Poor Soil 4075kN/m2 Wide Strip Wide Strip Raft Pile Pile

Bad Soil


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3750mm and 900mm are reasonable but may consume over 30% of the total reinforcements for the slab. A span of

1200mm or more may be difficult to manage in terms of deflection. Buildings tend to be disfigured when large span

cantilevers are used. Double cantilever leads to building instability and must be avoided as much as possible.

In most construction sites, it is amazing to notice that all the corners of bungalows and two storey buildings have their

corners blocks replaced with columns. These columns are rarely linked with beams, at most with mass concrete along

the line of external lintels. Some even go to the extent of lining corners of septic tanks and soak-away pits with

reinforced concrete beam and columns. Concrete beam and columns are vertical load bearing members while loads

exerted by septic tank and soak-away pit soil are purely horizontal. The construction of reinforced concrete columns atcorners of non-framed buildings may be counter productive.

3.0 TYPE OF LOADINGThe Advanced Learners Dictionarydefines load as that which is to be carried or supported and Chambers Mini

Dictionarysays a heavy weight. Hence, from these two basic definitions, we can summary load as the weight of a

material that is to be structurally supported. Thus, all loads whether permanent (dead) or transient (live) are weight of

the materials in question and at times their impact on the structure e.g. wind load, train impact load and spectators

impact load when a goal is scored in a football match.

Structurally, the following loads are common.

1. Dead load - weight of the material of construction, permanently present.

2.

Permanently superimposed load, that is, a live load that can be considered as permanent on the structure e.g.machine base or plinth.

3. Live load - transient load, that is, a load that can be moved in and out of the structure.

4. Wind load, that is, effect of wind forces or pressure (force/unit area) on the structure. This is a lateral load.

5. Impact load that is, due to impact of the live load. This may be taken as 10-20% of the live load.

Load type 1,2, 3 and 5 act vertically while load type 4 acts horizontally. Another horizontal load, though not common

in building structure, is breaking load, that is, when a brake is suddenly applied to a vehicle on a bridge. Each of these

loads is briefly discussed.

3.1 Dead LoadThe dead load is the weight of the structure itself, and the structural elements such as the ceiling, cladding and

permanent partitions. When machines and equipment are permanently located they can be assumed as dead loads. In

case of equipment and machines, the manufacturer would be in a position to give the details. To arrive at a dead load,the member is preliminarily sized. The obtained load which is the product of the member size and its specific weight

can be adjusted (or rounded) up so that any little difference in size during actual design will not significantly affect the

analysis. For example a 450 x 225mm beam can be assumed to be 5.0kN/m run which includes own weight and

finishes rounded up. Table 2 shows some values that could be useful during design.

Table 2: Materials Basic Weight

S/N Materials Basic Weight Unit

1. Concrete - dense (normal)

- light weight

24.0

7.018.0

kN/m3

-ditto-

2. Block -225mm hollow

- 150mm hollow

2.87

2.15

kN/m2

-ditto-

3. Wall finishes-both sides 0.60 kN/m2

4. Screeding - 37mm thick 0.80 kN/m2

5. Terrazzo Paving 0.022 kN/m2

6. Roofing felt and screed 2.00 kN/m

7. Asbestos rooting sheet etc. 0.40 kN/m2

8. Amanitas and nails 0.30 kN/m2

The dead weight must be assessed as much as possible. However, an ultimate partial factor of safety of 1.4 is oftenapplied. The application of dead load in design of structure is discussed in this paper.

3.2 Superimposed Permanent Load

This can be treated as dead loads.


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43.3 Live LoadsThese are transient loads to be carried by the structure and because of their nature are more difficult to determine

precisely. Hence, a more generous partial factor of safety of 1.6 is used. B.S. 6399: Part 1: 1984, deals with the

design loading for buildings. Some values from this Code are as stated in Table 3.

Table 3: Imposed Load for Slabs

S/N Description Values kN/m2

1. Dwelling units 1.52. Class rooms 3.0

3. Place of assembly - with fixed seating

- with no fixed seating

4.0

5.0

4. Offices - General use

- Filling room

2.5

5.0

5. Library 5.0

6. Motor Rooms 7.5

7. Car ParkLight 2.5

8. Pedestrian foot path 4.0

3.4 Wind LoadA wind load or wind pressure is a lateral load and it is mandatory when a structure is more than five storeys in height.

A building with high aspect ratio (ratio of height/width) must also be considered for wind. Unlike pressure due towater or grain which is linear and the magnitude of which depends on height, wind pressure is uniform and not

dependent on height. It depends mainly on locality and the isopleths of basic wind speed that is always available show

the values of various basic wind speed across the country.

The basic wind speed is converted to wind force as follows.

Let V be the local basic wind speed.

Vs= V S1S2S3 m/s and Wk= 0.613 Vs2N/m

2.

Where: Vs= Design wind speed in m/s

S1= multiplying factor relating to topology which can be taken as 1.0

S2 = multiplying factor relating to height above ground and wind braking, obtainable from literature andranges between 0.55 and 1.27.

S3 = multiplying factor related to the life of the structure which again can be taken as 1.0 that corresponds to an

excessive speed occurring once in fifty years.Wk= the wind load in N per square metre. Normally, these are multiplied by the projected area to determine the wind

force on the structure and the wind pressure (Wk) is assumed uniform over the entire surface.

For purposes of an example, assume a 20-storey building is to be located in Lagos, assuming each storey to be 2.85m,

the wind forces calculated per m face of the structure are as shown below.

Basic wind speed = 36 m/s

S1= 1.0, S3= 1.0 and for the value of S2we have the following conditions:

Topographical factor - Open countryBuilding width - Less than 50m

Therefore,

5m, Wk = (0.83 x 36) x 0.613 = 547N/m10m Wk = (0.95 x 36)

2 x 0.613 = 717

15m, Wk = (0.99 x 36)2x 0.613 = 779

20m, Wk = (1.01 x 36)2x 0.613 = 810

30m, Wk = (1.05 x 36)2 x 0.613 = 876

40m, Wk = (1.08 x 36)2 x 0.613 = 927

50m, Wk = (1.10 x 36)2 x 0.613 = 961

60m Wk = (1.12 x 36)x 0.613 = 997

Note: The values of S2are taken from Table 13 of Reinforced Concrete Designers Handbook by C. E. Reynolds and

J. C. Steedman, 10th

Edition. Thus the higher the level of consideration of the forces, the higher the pressure. The

building should be broken down to storeys corresponding to the heights above for purpose of application of these loads.

In this case we have:

Grd to 2nd

floor slab = 0.55kN/m2

2n

to 3 floor slab = 0.72kN/m


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53

rdto 5 floor slab = 0.78kN/m

2

5t

to 7 floor slab = 0.81kN/m

7th

to 10th


10th

to 14th


14t

to 17t

floor slab = 0.96kN/m

17th

to roof level =1.00kN/m2

Each is multiplied by the projected width of the building to obtain the force per m run. These inturn can be calculated as point loads and applied at the floor level as illustrated in Figure 1.

Figure 1: 5th

7.8kN/m. 2850 22.23kN

2850 21.38kN

7.2kN/m

2850 19.67kN

6.6kN/m 2850

Please note: 22.33 = 7.8 x 2.85kN; 21.38 = (7.8 x 7.2) x 0.5 x 2.85kN., and so on.

3.4 Load Combination

Every structure must be able to carry the loads imposed and it is always a combination of loads. The commonest are

dead plus live loads and dead plus live plus wind loads. Each of the combinations must be accompanied with theappropriate partial load factor as enunciated in the codes of practice. For residential buildings of not more than five

storey the load combination is limited to dead plus live loads only. Table 2.1 of B.S. 8110: Part 1: 1997, reproduced

here as Table 4 gives the various values of the partial factor of safety.

Table 4: Load Partial factor of safety for various load combinations

Load Combination

Load Type

Dead Imposed Earth Water &

PressureWind

Adverse Beneficial Adverse Beneficial

1. Dead and imposed(and earth and water

pressure

1.4 1.0 1.6 1.0 1.4 -

2. Dead and wind (and

earth and water

pressure)

1.4 1.0 - - 1.4 1.4

3. Dead and wind (and

earth and waterpressure)

1.2 1.2 1.2 1.2 1.2 1.2

4.0 DESIGN OF STRUCTURES

4.1 Design Objective

A reinforced concrete design must satisfy the following functional objectives:

Under the worst system of loading, the structure must be safe.

Under the working load, the deformation of the structure must not impair the appearance, durability and/or

performance of the structure and

The structure must be economical, that is, the factor of safety should not be too large to the extent that the

cost of the structure becomes prohibitive with no additional major advantage except for robustness.These requirements call for good assessment of the intending loads, right choice of materials and sound workmanship.

To ensure these, the various components forming the reinforced concrete and the concrete itself must pass the various

tests as detailed in the controlling code of practice.

4th

3rd

2nd

1st


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6The determination of the size of the structural member and the amount of reinforcement required to enable it

withstand the forces or other effects to which it will be subjected is the object of design or detailed design. Detailed

design is, however, only one of the two main parts of structural design, the other being the primary design. This is theinitial planning or arranging of the members so that the external forces or loads on the structure are transmitted to the

foundation in the most economical manner consistent with the purpose of the structure. This is borne out of

experience, from a study of existing structures and from comparison of alternative designs.

4.2 Shearing Force and Moment Envelopes - Slab and Beam Design

Most designers assume uniform loading of full dead and live loads on the structure. The implication of this is to

produce maximum bending moments and shearing forces at the supports. Alternate loading of maximum and

minimum loads on the other hand will produce higher span moments especially at the end support. This could be

beneficial.

Section 3.2.1.2.2 of B.S.8110:Part 1: 1997 states that it will be sufficient to consider two loading cases as follows:

a) All spans loaded with 1.4Gk+ 1.6Q andb) The spans loaded alternatively with (1.4Gk+ 1.6Qk) and 1.0Gk

Where: Gk = Characteristic dead load, QkCharacteristic live load.

Hence, if Gk= 5.8 and Q = 1.5 we have the following loading regime on the slab in Figure 2.1.4G

k+ 1.6Q = 1.4(5.80) + 1.6(1.5) = 10.52kN/m

2

1.0Gk = 1.0 x 5.80 = 5.80kN/m2

Figure 2:

6000 5000 6000

6000 5000 6000

These should be analyzed and the maximum results picked for the purposes of design. The single case loading can beused with moment re-distribution.

4.1 Load AssessmentLoad on superstructures must be assessed starting from the roof to the walls (or roof beams) to the slab, beam, columns

and foundations.

Column and foundation loads may be determined from the static loads, that is, floor area supported by the column

multiplied by the floor load per square metre. To this is added the beam and wall loads and the column own weight.

Column Design

Design of beams and slabs do not pose much problem to most designers. However, column design does. Structurally

column can be categorized into axially loaded, uniaxially loaded and biaxially loaded. Most designers, due to either

laziness or ignorance, assume all columns to be purely axial. This is generally not in the best interest of the job.Should the designer, nevertheless, insist, the values in the following table could be used to convert the loads to axial

and the columns designed as such. The values in the Table 5 are quite conservative

Table 5: Column Axial Load Multiplier

Column/Storey Top Next to Top Lower

Axial 1.0 1.0 1.0

Uniaxial 4.5 2.0 1.4

Biaxial 6.0 2.3 1.8

4.4 Foundation DesignThe major objective of foundation design is to prevent settlement of the structure. It should be noted that raft

foundation is not a solution to all foundation problems and not an antidote to settlement. A poorly designed raftfoundation can still settle but may settle uniformly or by tilting avoiding cracks in the structure.

10.52kN/m

10.52kN/m

5.80kN/m10.52kN/m


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7Soil/geotechnical investigations must be carried out prior to any foundation and where possible, the building could

be broken down into several sections and different types of foundations used. This is illustrated in the example shown

as Figure 3.

In this figure, a proposed Church building at Ikate, Surulere, Lagos, the congregation area is lightly loaded and the side

columns could be supported on single pad, while the rear columns could be joined together on a continuous reinforced

concrete footing. In real life, the soil permissible bearing capacity is 45kN/m2.

In view of the heavy loading towards the front (Ground floor, First floor and Second floor), a raft foundation would bethe most suitable. Efforts should be made to ensure that the resulting bearing pressure under each type of foundation is

the same. The foundations could be linked up with ground beams.

6000 6000 6000 6000 3600

5400

4200

4200

Altar

5400

4200

Figure 3: Church Project in LagosPlan and Section.

Entrance

Porch

2ndFloor - Offices

1stFloorChurch Gall ery

Altar Congregational Sitting


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85.0 CONCLUSIONTo conclude, here is a quotation from Man on the Job leaflets, published by Cement and Concrete Association, United

Kingdom, titled I T DEPENDS ON YOU. It says:

A good concrete job i s only good, strong, long-lasting, good-looking and economical to buil d, if every man on the

job shares in making it so.

A good concrete bui ldi ng or road or bri dge does not onl y depend on a good designer or a clever engineer : i t dependson good materi als, accurate batching, the ri ght amount of water and thorough mix ing: i t depends on well -placed

reinf orcement, well -made formwork, careful compacting: i t depends on good finish. No stage is unimportant . One

mans carelessness can let down the whole job: every mans care can make it a job to be proud of.

SO IT REAL LY DOES DEPEND ON YOU

Good structural design must be backed up by good construction materials, good workmanship, and good supervision.

Structures are to be designed to provide safe accommodation and not a coffin for mass burial.

References:

1. Simplified Reinforced Concrete Design, by Victor O. Oyenuga, 2nd

Edition, Asros Ltd., Lagos, 2005.

2. Design and Construction of Foundations, by Victor O. Oyenuga, Asros Ltd., Lagos, 2004.

3. BS 8110: Parts 1 and 2, Structural Use of Concrete, BSI, United Kingdom.

4. Reinforced Concrete DesignersHandbookby C. E. Reynolds and J. C. Steedman, 10th

Edition.

Documents

Fundamentals of RCC Design