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CHAPTER THREE DESIGN REQUIREMENTS 21

1 CHAPTER 3: DESIGN REQUIREMENTS

3.1 Structural Concrete Design At first, the general planning is carried out by the architect to set out the

layout of the building floors based on customer's needs. Only then, the

structural engineer determines the most appropriate structural system to

ensure strength, serviceability and economy of the building. This is done

through the following steps.

1. Setting out the building structural system/systems.

2. Evaluating the external loads on the members. These loads include own

weights of the members, which are estimated at the start, in addition to other

loads that the members are intended to support. Own weights of the members

are to be checked later once the design process is done.

3. Carrying out the structural analysis using computer or manual calculations

to determine the internal forces. The analysis is done using manually or using

computer software.

4. Determination of member dimensions and required reinforcement.


5. Preparation of structural drawings.

3.2 Types of Concrete Design

Concrete design can be classified into three main categories; plain concrete design, reinforced concrete design, and prestressed concrete design.

3.2.1 Plain Concrete Design

With the advent of reinforced concrete, plain concrete is hardly used as a structural material. It is mainly used for nonstructural members. This is due to the low strength of concrete in tension which results in large sections, especially, when required to resist tensile stresses resulting from direct tension or bending.

3.2.2 Reinforced Concrete Design

The compressive strength of concrete is high while its tensile strength is low. To alleviate the situation, high tensile strength reinforcement in the form of steel bars is added in the tension regions to enhance the capacity of concrete members as shown in Figure 1.1. The reinforcement is usually placed in the forms before casting the concrete. Once hardened, the resulting composite material is called reinforced concrete.


Figure 1.1: Mechanics of reinforced concrete: (a) beam and loads; (b) a plain concrete beam; (c) a reinforced concrete beam

1.1.1 Prestressed Concrete Design

Since the strength of reinforced concrete can be enhanced by the elimination of cracking, prestressing is used to produce compressive stresses in tension regions. Prestress is applied to a concrete member by high-strength steel tendons in the forms of bars, wires, or cables that are first tensioned and then anchored to the member. When the tendons are tensioned before the concrete is cast around them, the concrete member is called pre-tensioned. When the tendons are passed through ducts and tensioned after the concrete has hardened and gained enough strength, the concrete member is called post-tensioned.


When compared to classical reinforced concrete design, prestressed concrete design produces lighter sections, thus allowing the economic use of much longer spans.

1.2 Design Versus Analysis

It involves the determination of the type of structural system to be used, the cross sectional dimensions, and the required reinforcement. The designed structure should be able to resist all forces expected to act during the life span of the structure safely and without excessive deformation or cracking.

1.2.1 Analysis

It involves the determination of the capacity of a section of known dimensions, material properties and steel reinforcement, if any to external forces and moments.

1.3 Limit States of Reinforced Concrete Design

When a structural element becomes unfit for its intended use, it is said to have reached a limit state. The limit states are classified into three groups:

1.3.1 Ultimate Limit States

These involve structural collapse of some structural elements or the structure altogether. These limit states should be prevented as they tend to cause loss of life and property. Elastic instability, rupture, progressive collapse, and fatigue are forms of these limit states.

1.3.2 Service Limit States

These involve the disruption of the functional use of the structure, not its collapse. A higher probability of occurrence can be tolerated than in case of an ultimate limit state since there is less danger of loss of life. Excessive deflections, immoderate crack widths, and annoying vibrations are forms of these limit states.


1.3.3 Special Limit States

These involve damage or failure due to abnormal conditions such as collapse in severe earthquakes, damage due to explosions, fires, or deterioration of the structure and its main structural elements.

Generally, for buildings, a limit state design is carried out first in order to proportion the elements, and second a serviceability limit state is conducted to check whether these elements satisfy those serviceability limit states.

1.4 Design and Building Codes

A code is a set of technical specifications that control the design and construction of a certain type of structures. Theoretical research, experiments, and past experience help in the process of setting these specifications. The purpose of such code is to set minimum requirements necessary for designing safe and sound structures. It also helps to provide protection for the public from dangers resulting from the use of inadequate design and construction techniques.

There are two types of codes; the first is called structural code, and the second is called building code. A structural code is a code that involves the design of a certain type of structures (reinforced concrete, structural steel, etc.). The structural code that will be used extensively throughout this textbook is The American Concrete Institute (ACI 318-08), which is one of the most solid codes due to its continuing modification, improvement, and revision to incorporate the latest advancements in the field of reinforced concrete design and construction. Supplements containing such revisions are made on yearly basis. Every three or six years, a comprehensive code edition is made, combining all revisions made since the last comprehensive edition.

A building code, on the other side, is a code that reflects local conditions such as earthquakes, winds, snow, and tornadoes in the specifications. Usually the building code which describes the prevailing conditions in a


certain city or state, is used in addition to the main structural or national code. Prior to the year 2000, there were three model codes: the Uniform Building Code (UBC), the Standard Building Code (SBC) and the Basic Building Code (BBC). In 2000, these three codes were replaced by the International Building Code (IBC) , which is updated every three years.

1.5 Design Methods

Two methods of design for reinforced concrete have been dominant. The Working Stress method was the principal method used from the early 1900s until the early 1960s. Since the publication of the 1963 edition of the ACI Code, there has been a rapid transition to Ultimate Strength Design. Ultimate Strength Design is identified in the code as the Strength Design Method. The 1956 ACI Code (ACI 318-56) was the first code edition which officially recognized and permitted the Ultimate Strength Design method and included it in an appendix. The 1963 ACI Code (ACI 318-63) dealt with both methods equally. The 1971 ACI Code (ACI 318-71) was based fully on the strength approach for proportioning reinforced concrete members, except for a small section dedicated to what is called the Alternate Design Method. In the 1977 ACI Code (ACI 318-77) the Alternate Design Method was demoted to Appendix “B”. It has been preserved in all editions of the code since 1977, including the 1999 edition mentioned in Appendix “A”. In the 2002 code edition, the so called Alternate Design Method was taken out.

1.5.1 The Strength Design Method

At the present time, the strength design method is the method adopted by most prestigious design codes; including the 2008 version of the ACI building code (ACI 318-08). In this method, elements are designed so that the internal forces produced by factored loads do not exceed the corresponding strength capacities and allow for some capacity reduction. The factored loads are obtained by multiplying the working loads (service


loads) by factors usually greater than unity. The favored mode of failure is the one that ensures a controlled local failure of members in a ductile rather than brittle manner.

1.5.1.1 Shortcomings:

1. The use of elastic methods of analysis to determine the internal forces in the members, which are associated with the factored loads, is inconsistent. This is due to the fact that when the ultimate load is approached, steel and concrete are no longer behaving elastically, a basic requirement of the validity of the elastic methods of design.

2. Regardless of the method of design used, structures are expected to behave elastically or nearly under normal working loads. Under this condition, the strength method can not be used and the working stress analysis should be made to determine the deformations and crack widths.

1.5.2 The Working-Stress Design Method

Before the introduction of the strength-design method in the ACI building code in 1956, the working stress design method was used in design. This method is based on the condition that the stresses caused by service loads without load factors are not to exceed the allowable stresses which are taken as a fraction of the ultimate stresses of the materials, cf ′ for concrete

and yf for steel. In this method, linear elastic relationship between stress

and strain is assumed for both concrete and steel reinforcement. The working stress-design method will generally result in designs that are more conservative than those based on the strength design method. Now only the design of sanitary structures holding fluids is based on the working-stress design method since keeping stresses low is a logical way to limit cracking and prevent leakage.


1.5.2.1 Shortcomings

1. No way to account for degrees of uncertainty of various types of loads. Dead loads, for example, can be predicted more accurately than live loads which are usually variant and harder to predict.

2. Experimental investigations showed that analysis according to the working-stress design method does not predict actual behavior, especially, at high stresses.

3. The elastic theory does not allow for prediction of the ductility of a structural member. Consideration of ductility, however, is of a vital importance in the field of design for most dynamic effects.

4. The working stress design method does not make allowances for varying quality control, standard of construction and variations indicating the magnitude of damage that may be caused by possible failure of a particular element.

5. It has been confirmed by tests that the working stress design method does not give correct information with respect to the actual factor of safety against failure of reinforced concrete members. The factor of safety is defined as the ratio between the load that would cause the total collapse to that used as the service or working load. It has been found that the value of this factor is far different from the ratio of the strength to the so-called working stress.

1.6 Loads on Structures

All structural elements must be designed for all loads anticipated to act during the life span of such elements. These loads should not cause the structural elements to fail or deflect excessively under working conditions. Therefore, the designer must use the available codes to estimate these loads if such estimates are available. If not, the designer must use his own judgment to make these estimates which are needed for the analysis process


before embarking on the design process. The most important load types are listed below.

1.6.1 Dead Load (D.L)

The dead load is usually a load of permanent status, such as the own weight of the structure, its partitions, flooring and roofing. The exact value of the dead load is not known until the structural members have been proportioned. Once this is done, this load is calculated and used with other loads to design these members. Only then, the assumed loads are compared with the actual ones, if the difference is substantial such as in long spans, modifications of the assumed values are necessary to guarantee economy on one extreme and adequacy on the other.

1.6.2 Live Load (L.L)

The live load is a moving or movable type of load such as occupants, furniture, etc. Live loads used in designing buildings are usually specified by local building codes. Live loads depend on the intended use of the structure and the number of occupants at a particular time. The structural engineer must use a good judgment if the expected live load is not specified by the local code, or if he expects a larger value than the one specified by the code. Live loads are arranged in such a way to give maximum values for the internal forces. Table 1.1 shows typical live load values used by the ASCE 7-05.

Table 1.1: Typical live loads specified in ASCE 7-05 Apartment Buildings: § Residential areas and corridors 200 kg/m2 § Public rooms and corridors 480 kg/m2

Office Buildings: § Lobbies and first-floor corridors 480 kg/m2 § Offices 240 kg/m2 § Corridors above first floor 380 kg/m2 § File and computer rooms 400 kg/m2

Schools: § Classrooms 195 kg/m2


§ Corridors above first floor 385 kg/m2 § First-floor corridors 480 kg/m2

Stairs and Exit Ways: 480 kg/m2 Storage Warehouses: § Light 600 kg/m2 § Heavy 1200 kg/m2

Garages (cars): 200 kg/m2 Retail Stores: § Firest floor 480 kg/m2 § Upper floors 360 kg/m2

Wholesale, all Floors 600 kg/m2

1.6.3 Wind Load (W.L)

The wind load is a lateral load produced by wind pressure and gusts. It is a type of dynamic load that is considered static to simplify analysis. The magnitude of this force depends on the shape of the building, its height, the velocity of the wind and the type of terrain in which the building exists. Usually this load is considered to act in combination with dead and live loads.

1.6.4 Earthquake Load (E.L)

The earthquake load, which is also called seismic load, is a lateral load caused by ground motions resulting from earthquakes. The magnitude of such a load depends on the mass of the structure and the acceleration caused by the earthquake.

The provisions of the ACI Code provide enough ductility to allow concrete structures to stand earthquakes in low seismic risk regions. In moderate to high-risk regions, special arrangements and detailing are needed to guarantee ductility.

1.7 Safety Provisions

Safety is required to insure that the structure can sustain all expected loads during its construction stage and its life span with an appropriate factor of


safety. The factor of safety is used to account for the following uncertainties:

§ Real Loads may differ from assumed design loads, or distributed differently.

§ Material strengths could be smaller than those used in the design.

§ Executed dimensions or reinforcement are less than those specified by the designer.

§ Assumptions and simplifications are made during analysis or design.

The factor of safety should account for the expected type of failure and its consequences and for the importance of the member in terms of structural integrity.

The ACI strength design method, , involves a two-way safety measure. The first of which involves using load factors, usually greater than unity to increase the service loads. The magnitude of such a load factor depends on the accuracy of determining the type of load under consideration. The second safety measure specified by the ACI Code involves a strength reduction factor multiplied by the nominal (theoretical) strength to obtain design strength. The magnitude of such a reduction factor is usually smaller than unity. The load factors and the strength reduction factors will be discussed in detail in the following section.

1.7.1 Load Factors

These load factors are required for possible overloading resulting from;

§ Magnitudes of loads may vary from those assumed in design. § Uncertainties involved in determination of internal force.

In the ACI 318-2002 Code, the load combination and strength reduction factors of the 1999 code were revised and moved to Appendix C, and remains in the ACI 318 08 code edition.


According to ACI 9.2.1, required strength U shall be at least equal to the effects of factored loads in Eqs. (1.1) through (1.7).

The effect of one or more loads not acting simultaneously is to be investigated.

a- Dead load and fluid load Combination:

( )FDU += 4.1 (1.1)

b- Dead load, fluid load, temperature load, live load, soil load, roof load,

snow load, and rain load combination:

( ) ( ) rLHLTFDU 5.06.12.1 +++++= ( ) ( ) SHLTFDU 5.06.12.1 +++++= (1.2)

( ) ( ) RHLTFDU 5.06.12.1 +++++= c- Dead load, roof live load, live load, rain load, wind load, and snow load

combination:

LLDU r 0.16.12.1 ++= LSDU 0.16.12.1 ++= LRDU 0.16.12.1 ++= WLDU r 8.06.12.1 ++= (1.3)

WSDU 8.06.12.1 ++= WRDU 8.06.12.1 ++=

d- Dead Load, wind load, live load, roof live load, snow load, and rain

load combination:

rLLWDU 5.00.16.12.1 +++= SLWDU 5.00.16.12.1 +++= (1.4)

RLWDU 5.00.16.12.1 +++=


e- Dead Load, earthquake load, live load, and snow load combination:

SLEDU 2.00.10.12.1 +++= (1.5)

f- Dead Load, wind load, and soil load combination:

HWDU 6.16.19.0 ++= (1.6)

g- Dead Load, earthquake load, and soil load combination:

HEDU 6.10.19.0 ++= (1.7)

Where

U = Required strength to resist factored loads, or internal forces

D = Dead loads, or related internal forces

F = Fluid loads, or related internal forces

T = Cumulative effects of temperature, creep, shrinkage, and differential settlement

L = Live loads, or related internal forces

H = Soil pressure, or related internal forces

rL = Roof live loads, or related internal forces

S = Snow loads, or related internal forces

R = Rain loads, or related internal forces

W = Wind loads, or related internal forces

E = Earthquake loads, or related internal forces

Regarding the above given equations, the following important notes are also given in ACI 9.2.1 and 9.2.2

a- The live load factor on L on Eqs. (1.3), (1.4) and (1.5) is permitted to be reduced to 0.5 except for garages, areas of public assembly, and all

areas where the live load is greater than 485 2/ mkg .


b- Where the wind load W has not been reduced by a directionality factor, it is permitted to use 1.3W instead of 1.6W in Eqs. (1.4) and (1.6).

c- Where earthquake load E is based on service level forces, 1.4 E is to be used in place of 1.0 E in Eqs. (1.5) and (1.7).

d- The load factor on H shall be set equal to zero in Eqs. (1.6) and (1.7) if the structural action due to H counteracts that due to W or E . Where lateral earth pressure provides resistance to actions from other forces it shall not be included in H but shall be included in the design resistance.

e- If the live load is applied rapidly, as may be the case for parking structures, loading docks, warehouse floors, elevator shafts, etc., impact effects should be considered. In all equations, substitute (L + impact) for L when impact should be considered.

For many members, the loads considered are dead, live, wind and earthquake.

Where the F, H, R, S , Lr and T loads are not considered equations (1.1) through (1.7) simplify to those given in Table (1.2) below.

Table 1.2: Required Strength for simplified load combinations

Loads Required Strength Equation NO.

Dead (D) and Live (L) D4.1

LD 6.12.1 +

(1.1)

(1.2)

Dead (D), Live (L) and wind (W)

LD 0.12.1 +

WD 8.02.1 +

LWD 0.16.12.1 ++

WD 6.19.0 +

(1.3)

(1.3)

(1.4)

(1.6)

Dead (D), Live (L) and ELD 0.10.12.1 ++ (1.5)


Earthquake (E) ED 0.19.0 + (1.7)

1.7.2 Strength Reduction Factors

According to the ACI Code 9.3.1, the nominal (theoretical) strength is multiplied by a strength reduction factor to obtain the design strength.

Design strength ≥ Required strength

The reasons for using the strength reduction factors include:

§ Allow for the probability of under-strength due to variations in material strengths and dimensions.

§ Allow for inaccuracies in the design equations. § Reflect the degree of ductility and required reliability of the member

under the load effects being considered. § Reflect the importance of the member in the structure.

In the ACI 318-2002 Code, the strength reduction factors were adjusted to be compatible with model building code.

According to ACI 9.3.2 strength reduction factors Φ are given as follows:

a- For tension-controlled sections ……….…………………….. Φ = 0.90

b- For compression-controlled sections,

• Members with spiral reinforcement …………….….……… Φ = 0.75

• Other reinforced members ………………………...………. Φ = 0.65

c- For shear and torsion ………………………………….……. Φ = 0.75

d- For bearing on concrete …………………………….……….. Φ = 0.65

e- Post-tensioned anchorage zones ………………..…………… Φ = 0.85

f- Strut and tie models …………………………………………. Φ = 0.75

An example of showing the importance of a member in a structure is that columns have smaller strength reduction factors, thus larger safety


measures than beams. This is due to the importance of columns when paying attention to their extensive type of failure which differs from the localized type of failure encountered in beams. Moreover, columns are less ductile than beams, thus requiring a larger factor of safety.

In ACI 10.3.4, sections are called tension-controlled when the net tensile strain in the extreme tension steel is equal to or greater than 0.005 when the concrete in compression reaches its crushing strain of 0.003, as shown in Fig. 1.2.a.

In ACI 10.3.3, sections are called compression-controlled when the net tensile strain in the extreme tension steel is equal to or less than yε

(permitted to be taken as 0.002 for reinforcement with 2/4200 cmkgfy = )

when the concrete in compression reaches its crushing strain of 0.003, as shown in Fig. 1.2.c.

There is a transition region between tension-controlled and compression-controlled sections, shown in Fig. 1.2.b.

(a) (b) (c)

Figure 1.2: Classification of sections for 2/4200 cmkgfy = ; (a) Tension-controlled section; (b) Section in transition between tension and compression; (c) compression-controlled section.


Example (1.1): For frame ABCD shown in Figure 1.2.a, determine the axial forces for which member AB should be designed for the following service loads are applied:

− a dead load of 1 t/m on member BC; − a live load 2.5 t/m on member BC; − a horizontal wind load of 5 tons at joint C, which may act to the

right or left on member AB and CD, respectively.

Figure 1.2.a: Frame

Solution:

The frame is analyzed using SAP 2000 structural analysis and design software for the following load combinations.

Combination (1): D + L, based on Eq. (1.2)

ton/m5.2 (2.5) 1.6 (1) 1.2 60.120.1 =+=+= LDwu

.)(0.26)2/10(2.5 comptonsFAB == , as shown in Figure 1.2.b.

Combination (2): D + W, based on Eq. (1.3)

WDU 8.02.1 +=

Wind acts to the right

.)(0.4 comptonsFAB = , as shown in Figure 1.2.c.


Wind acts to the left

.)(0.8 comptonsFAB = , as shown in Figure 1.2.d.

Combination (3): D + L + W, based on Eq. (1.4)

LWDU 0.16.12.1 ++=


.)(5.14 comptonsFAB = , as shown in Figure 1.2.e.


.)(5.22 comptonsFAB = , as shown in Figure 1.2.f.

Combination (4): D + W, based on Eq. (1.6)

WDU 60.19.0 +=


.)(50.0 comptonsFAB = , as shown in Figure 1.2.g.


.)(50.8 comptonsFAB = , as shown in Figure 1.2.h.

Studying the four combinations, member AB should be designed for an axial compression load of 26.0 tons.


Figure 1.2: (continued); Frame and loading combinations, (b) D+L combination; (c) D+W combination (Right); (d) D+W combination (Left); (e) D+L+W combination (Right); (f) D+L+W combination (Left); (g) D+W combination (Right); (h) D+W combination (Left);


Example (1.2): The beam shown in Figure 1.3.a carries a uniformly distributed service dead load of 3 t/m, and a service live load of 1.5 t/m. Determine the maximum positive and negative bending moments for which beam ABC .

Solution:

Figure 1.3.a: Beam ABC

The Beam is analyzed using SAP 2000 structural analysis and design software for the following loading cases.

Maximum negative moment:

This case is evaluated by fully loading the two spans by dead and live loads.

mtonLDwu /0.6)5.1(6.1)3(2.160.120.1 =+=+=

The maximum negative moment is given as.

( ) mtM ve .75.18.max =− , as shown in Figure 1.3.c.

Maximum positive moment:

This case is evaluated by fully loading one of the two spans by dead and live loads while loading the other span by dead load only.

For the span loaded with dead and live loads,

LDwu 60.120.1 += = 1.2 (3) + 1.6 (1.5) = 6.0 t/m

For the other span, Dwu 20.1= = 1.2 (3) = 3.6 t/m

The maximum positive moment is given as.

mtM ve .0.12(max) =+ , as shown in Figure 1.3.e.


(b)

(c)

(d)

(d)

Figure 1.3 : (continued); (b) loads causing maximum negative moment at point B; (c) corresponding bending moment diagram; (d) loads causing maximum positive moment in span BC; (e) corresponding bending moment diagram.


Example (1.3): For frame ABCD shown in Figure 1.4, determine maximum positive and negative bending moments for which member BC should be designed for when the following service loads are acting:

− a dead load of 4 t/m on member BC; − a live load 3 t/m on member BC; − a horizontal wind load of 1 tons at joint C, which may act to the

right or left on member AB and CD, respectively.

Figure 1.4.a: Frame

Solution

The frame is analyzed using SAP 2000 structural analysis and design software for the following load combinations. Combination (1): D + L load

ton/m9.6 (3) 1.6 (4) 1.2 60.120.1 =+=+= LDwu


Figure 1.4.b: Dead and live loads

Combination (2): D + W load (on members CD and AB)

WDU 8.02.1 +=

ton/m4.8 (4) 1.2 2.1)( === Dverticalwu ton/m0.8 (1) 8.08.0)( === Whorizontalwu

Figure 1.4.c: Dead and wind loads (on member CD)


Figure 1.4.d: Dead and wind loads (on member AB)

Combination (3): D + L + W load (on members CD and AB)

LWDU 0.16.12.1 ++=

ton/m7.8 1(3)(4) 1.2 0.12.1)( =+=+= LDverticalwu

ton/m0.8 ) (1) 8.08.0)( === Whorizontalwu

Figure 1.4.e: Dead, Live and wind loads (on member CD)


Figure 1.4.f: Dead, Live and wind loads (on member AB)

Combination (4): D + W Load (on members CD and AB)

WDU 60.19.0 +=

ton/m3.6 (4) 0.9 9.0)( === Dverticalwu

ton/m1.6 ) (1) 6.16.1)( === Whorizontalwu

Figure 1.4.g: Dead and wind loads (on member CD)


Figure 1.4.h: Dead and wind loads

Based on the results obtained from the four combinations, member BC should be designed for a maximum negative bending moment of 252.57 ton.m and a maximum positive bending moment of 227.43 ton.m


1.8 Problems

P1.10.1 The beam shown in Figure P1.10.1 carries a uniformly distributed service dead load of 2.5 t/m, and a service live load of 2.0 t/m. Determine the maximum positive and negative bending moments for which beam ABC should be designed for.

Figure P1.10.1

P1.10.2 For frame ABCD shown in Figure P1.10.2, determine the axial forces for which the member CD should be designed for when the following service loads are applied:

− a dead load of 1.5 t/m on member BC; − a live load 2.0 t/m on member BC; − a horizontal wind load of 8 tons at joint C, which may act either to

the right or left.

Figure P1.10.2


P1.10.3 The multi-story frame shown in Figure P1.10.3 carries the following loads:

− two 5-ton service concentrated live loads applied at points G and H;

− a service dead load of 3 t/m and a service live load of 2 t/m on member CD;

− a service dead load of 5 t/m and a service live load of 1.5 t/m on member BE.

Determine the maximum positive and negative bending moments for which member BE should be designed for.

Figure P1.10.3

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