Multietajate En

  • View
    215

  • Download
    0

Embed Size (px)

Text of Multietajate En

  • 8/9/2019 Multietajate En

    1/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    1

    TALL BUILDINGS

    Depending on the place where the building is located and also on its height, the

    action of the wind can be more or less severe than the seismic action. As a result,

    one of the three requirements:

    resistance;

    rigidity;

    ductility;

    becomes more important to satisfy.

    In situations where the action of the wind is more severe, the rigidity requirement is

    more important, because the response of the structure must remain in the elastic

    range. On the other hand, in situations where the seismic action is more severe, the

    ductility requirement is more important, to be able to dissipate the energy.

    Specific requirements are:

    structural requirements;

    technological and functional requirements.

    1.1.1. Steel structural systems

    The structure of a building usually consists of a set of plane frames, to ensure

    spatial behaviour. Basically, there are three types of plane frames (Fig. 1.1). In all

    these three cases, the beam-column connection must be rigid.

    MRF EBF CBF

  • 8/9/2019 Multietajate En

    2/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    2

    MRF CBF EBF

    MRF moment resisting frame

    CBF concentrically braced frame

    EBF eccentrically braced frame

    Fig. 1.1.Types of plane frames

    The eccentrically braced frames (EBF) were created by Egor Paul Popov

    (1977) at the University of California, Berkeley (died April 19, 2001). A comparison of

    the main features of these three systems is presented in table 1.1.

    Table 1.1.Characteristics of structural systems plans

    MRF CBF EBF

    Resistance good good good

    Rigidity low very good good

    Ductility very good low good

    Commentaries on the loading state and on the energy dissipation.

    The most recent structural solutions try to better control the response of the

    structure. They can be classified as:

    passive

    o base isolation

    o dampers

    o tuned mass dampers

    o buckling-restrained braced frameso the use of low yield steel

  • 8/9/2019 Multietajate En

    3/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    3

    active

    Commentaries separation of functions:

    resistance moment frames;

    rigidity braces; ductility dampers.

    Depending on the height of the building and mainly on the ratio between the

    height Hand the dimension in plan B, the most commonly used structural solutions

    for resisting horizontal forces are the following ones:

    moment frames (Fig. 1.2a);

    frames with vertical bracings (Fig. 1.2b);

    frames with vertical bracings and outriggers (Fig. 1.2c); structures with beam-walls (Fig. 1.2d);

    tubular structures (Fig. 1.2e);

    multi-tube and mega-frame structures (Fig. 1.2f);

    tube in tube

    bundled tubes.

    (a) (b) (c) (d) (e) (f)

    Fig. 1.2.Types of structural systems

    2B

    H

    4B

    H

    4B

    H

    6B

    H

    6B

    H

    8

    B

    H

  • 8/9/2019 Multietajate En

    4/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    4

    The location of seismic systems in a building depends on structural,

    architectural, technological and functional requirements. Generally, codes (EN 1998-

    1, P100-1:2013) classify seismic structural systems in:

    moment frames (Fig. 6.1 EN 1998-1);

    Fig. 1.3.Moment resisting frames

    frames with concentric bracings (Fig. 6.2 EN 1998-1) (Fig. 6.3 EN 1998-1);

    Fig. 1.4.Frames with concentric bracings

    frames with eccentric bracings (Fig. 6.4 EN 1998-1);

  • 8/9/2019 Multietajate En

    5/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    5

    Fig. 1.5.Frames with eccentric bracings

    inverted pendulum structures (Fig. 6.5 EN 1998-1);

    Fig. 1.6.Inverted pendulum structures

    structures with concrete cores and concrete walls (Fig. 6.6 EN 1998-1);

    Fig. 1.7.Structures with concrete cores and concrete walls

    moment frames combined with concentric braces (Fig. 6.7 EN 1998-1);

  • 8/9/2019 Multietajate En

    6/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    6

    Fig. 1.8.Moment resisting frame combined with concentric bracing

    moment frames combined with infills (Fig. 6.8 EN 1998-1);

    Fig. 1.9.Moment resisting frame combined with infills

    1.2. SEISMIC DESIGN PRINCIPLES OF METAL STRUCTURES

    1.2.1. The capacity design concept

    The modern seismic design approach is based on the idea that much of the

    energy induced by the earthquake in the structure is dissipated by means of plastic

    mechanisms; generally, it is not efficient to resist this action only in the elastic range,

    unless under very special circumstances, such as the case of nuclear power plants

    etc. Current trends are to better control energy dissipation, meaning the behaviour of

    the structure in the post-elastic range. The basic idea is to use a plastic mechanism

    which is chosen, directed and controlled by the engineer, and to ensure conditions

    so that all other elements remain essentially in the elastic range until the moment

    when the mechanism has exhausted its capacity of dissipation. Subsequently to thisprinciple, plastic deformations are accepted only in selected areas generally located

  • 8/9/2019 Multietajate En

    7/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    7

    in beams in a neighbouring area to the joint and, in the end, at the base of the

    columns or at their top end.

    The designer choses the potentially plastic zones on the structure (their type

    and location) and the rest of the structure must remain basically in the elastic range

    while the chosen plastic zones keep on plastifying. This means that the rest of the

    structure must be able to resist, basically in the elastic range, the loads (efforts)

    generated by the action that fully plastifyies all the chosen potentially plastic zones.

    Following this, all the parts of the structure that are outside of the potentially plastic

    zones must resist efforts associated to the capacity of the chosen plastic

    mechanism, considering not the minimum values of the plastic resistance of these

    zones but probable (expected) ones. This is basically the meaning of the capacity

    design concept.

    Remark: A higher resistance of the potentially plastic zones can either put in danger

    the non-dissipative zones of the structures (as failures may occur there) or lead to

    oversize of the entire structure.

    1.2.2. Basic concepts

    Ductility

    Generally, ductility is the ability of energy dissipation in the plastic range. It is

    expressed as ratio between ultimate displacement and the value of the same

    displacement corresponding to the yielding limit.

    Plastic hinge

    It is a concept that defines a cross-sectionof the structural member in which

    the ability to resist an increasing bending moment has been exhausted. Plastic

    hinges do not exist in reality; plastic deformations develop over a certain length of

    the element, so they are not located in one section, as the model presumes.

    Class of the cross-section

    Depending on the occurrence of local buckling of the components of a cross-

    section that is subjected to bending moment, four classes of cross-sections are

    defined in EN 1993-1-1. Depending on the class of the cross-section, the loading

    state of the structural member can be determined using elastic or plastic analysis

  • 8/9/2019 Multietajate En

    8/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    8

    and the resistance of the cross-section can be expressed in the plastic, elastic or

    critical range (Fig. 1.10).

    Fig. 1.10.Evolution of the stress state in a cross-section subject to bending moment

    1.2.3. The main steps of seismic design of the structure

    The capacity design concept tries to better manage the behaviour of the

    structure in the elastic and in the plastic range till failure. The following steps are

    recommended to be followed to achieve a good behaviour of the structure:

    1. Chosethe structural systemand the model of the structure.

    2. Estimate the loadsin each loading case.

    3. Estimate the masses. The masses are calculated corresponding to the loading

    situation that contains the seismic action. They are therefore associated to the

    gravitational loads that are defined by long-term factors applied to the nominal

    values of the loads.

    4. Pre-size structure by choosing the position of the plastic zones, implicitly the

    plastic mechanism of energy dissipation. Cross-sections are proposed for all

    structural members.

    5. Performinga modal analysis of thestructure, for determining the eigenperiods

    and the distribution of equivalent static forces corresponding to each eigenmode.

    6. Performing a static analysis of the structure. In this step, a number of

    eigenmodes are taken into account to estimate the resulting seismic forces.

    max< fy max= fy max= fy max= fy

    y y

    z

    z

    class 4

    class 3

    class 2

    class 1

  • 8/9/2019 Multietajate En

    9/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    9

    7. Chosethe cross-sections of themembers in plastic zonesto resist the efforts

    generated by the action of the seismic base shear (Fb).

    8. Estimate the plastic reserve () of the chosen plastic mechanism.

    9. This reserve is taken into account by amplifying the seismic force by the

    factor(while the gravitational loads remain unchanged). Do another static analysis

    for this amplified seismic force (Fb). The rest of the structure (except the

    selected plastic areas) must remain essentially in the elastic range while the

    chosen plastic mechanism dissipates energy, so all theother memberswill be

    sized to resist the efforts generated by this amplified seismic force. This

    amplified seismic force (Fb) can also be determined using a virtual work

    equation:Lext= Lint (1.1)

    where:

    Lextis produced by seismic force;

    Lintis produced by efforts in the dissipative elements.

    In the absence of a more rigorous analysis, some codes recommend penalising

    values. For instance, the American code ANSI/AISC 341-10 and the Romanian

    code P100-1:2013 recommend the following values for the global overstrength

    factor of the structure(Tab. 1.2).

    Table 1.2.Values for the global overstrength factor of the structure([3] Tab. F.1)

    Structural system

    a) Moment frames 3,0

    b) Concentrically braced frames 2,0

    c) Eccentrically braced frames 2,5d) Inverted pendulum structures 2,0

    e) Dual frames

    - moment frames + concentrically braced frames

    - moment frames + eccentrically braced frames

    2,0

    2,5

    f) Frames with buckling-restrained braces 3,0

  • 8/9/2019 Multietajate En

    10/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    10

    These recommendations apply in the situation where the structure complies with

    the rules of conformations given in the codes ANSI/AISC 341-10 or P100-1:2013

    for a good post-elastic behaviour of the structure.

    10. Chosethe cross-sections of themembers outside the plastic zonesto resist

    the efforts generated by the action of the amplified seismic force (Fb).

    11. Return to step 5 and perform a new static analysisfor the new cross-sections

    of the structural members, taking into account the seismic base shear (Fb)and,

    respectively, the amplified seismic force (Fb) and follow steps 5-10 to tune up

    the structure.

    12. Perform a push-over analysis(post-elastic analysis)to verify that the chosen

    plastic mechanism is developed and the required ductility can be obtained.13. Improve the structure based on the results of the push-over analysis (by

    changing the cross-sections of some members) and follow steps 5-12 to tune up

    the structure for as many times as it is necessary.

    14. Perform a time-history analysis (dynamic analysis) using accelerograms,

    which can be either real or fabricated. It can be done in the elastic or in the

    plastic range.

    15. Improve the structure based on the results of the time-history analysis (by

    changing the cross-sections of some members) and follow steps 5-14 to tune up

    the structure for as many times as it is necessary.

    1.3. ESSENTIAL SEISMIC REQUIREMENTS FOR THE METAL STRUCTURES

    FOR TALL BUILDINGS

    1.3.1. Classifications

    Present day codes distinguish between the types of structures, depending on

    their plastic behaviour. Basically, there are two types of structural concepts:

    low dissipative structural behaviour;

    dissipative structural behaviour.

    These two types of behaviour are well distinguished in the European standard EN

    1998-1 [10], in the Romanian code P100-1:2013 [11] and in the American code

  • 8/9/2019 Multietajate En

    11/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    11

    ANSI/AISC 341-10 [12]. Subsequently, three types of structures, depending on their

    post-elastic behaviour, can be distinguished in the three standards [10], [11]:

    DCL (limited ductility class) [10], [11] capable to undergo minimum plastic

    deformations under the effects of the seismic action; EN 1998-1 [10] acceptsthem in the case of buildings without isolation of the base, only in regions with

    low seismicity; in ANSI/AISC 341-10, they are calledordinary structures(OMF,

    OCBF, OEBF etc.);

    DCM (medium ductility class) [10], [11] capable to undergo limited plastic

    deformations under the effects of the seismic action; in ANSI/AISC 341-10, they

    are calledintermediate structures(IMF, ICBF, IEBF etc.);

    DCH

    (high ductility class) [10], [11] capable to undergo significant plasticdeformations under the effects of the seismic action; in ANSI/AISC 341-10, they

    are calledspecial structures(SMF, SCBF, SEBF etc.).

    Table 1.3. Design concepts, structural ductility classes and upper limit reference

    values of the behaviour factors([3] Tab. 6.1)

    Design concept Structural ductility

    class

    Range of the reference

    values of the behaviourfactor q

    Concept a)

    Low dissipative structuralbehaviour

    DCL (Low) 1,5 - 2

    Concept b)

    Dissipative structural behaviour DCM (Medium)4

    also limited by thevalues of Table 1.4

    DCH (High) only limited by thevalues of Table 1.4

    Table 1.4.Upper limit of reference values of behaviour factors for systems regular in

    elevation([3] Tab. 6.2)

    Structural type Ductility Class

    DCM DCHa) Moment resisting frames 4,0 5u/1

  • 8/9/2019 Multietajate En

    12/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    12

    b) Frame with concentric bracings

    - Diagonal bracings

    - V-bracings

    4,0

    2,0

    4,0

    2,5

    c) Frame with eccentric bracings 4,0 5u/1

    d) Inverted pendulum 2,0 2u/1

    e) Structures with concrete cores or concrete walls See R.C. section

    f) Moment resisting frame with concentric bracing 4,0 4u/1

    g) Moment resisting frames with infills

    - Unconnected concrete or masonry infills, in contactwith the frame

    2,0 2,0

    - Connected reinforced concrete infills See composite section

    - Infills isolated from moment frame (see momentframes)

    4,0 5u/1

    1.3.2. Some structural requirements for seismic resistant steel structures

    1. The structure will follow the principle strong column weak beam to avoid

    the appearance of floor mechanisms.

    2. Kbracings where the intersection of the diagonals is located on the column are

    not accepted in seismic resistant structures (Fig. 1.11) because of the

    unbalanced horizontal force that acts on the column after buckling of the

    compressed diagonal. In the case of V (or inverted V) bracings, the beam

    containing the intersection point of the diagonals must be continuous in that point

    and sized to resist gravitational loads neglecting the presence of the braces.

    Fig. 1.11.Recommendations for the bracing system

    3. All structural members and their connections must remain essentially in the

    elastic range under the action of the seismic base shear force (Fb).

  • 8/9/2019 Multietajate En

    13/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    13

    4. For earthquakes that can generate forces superior to the seismic base shear

    (Fb), the structure must be able to dissipate energy using a plastic mechanism.

    The mechanism must be chosen, managed and controlled by the designer and

    plastic deformations must develop only in chosen zones. The beginning of plastic

    behaviour of the structure must be done under the action of the force Fb.

    5. The state of efforts and deformations induced on the structure by the maximum

    ground acceleration (peak ground acceleration on the location) must be

    estimated. A non-linear dynamic analysis is recommended for this purpose. In

    this state the structure shall comply with the following requirements:

    all structural members and their connections must be essentially in the

    elastic range except the chosen plastic zones;

    all lateral displacements (Fig. 1.12) must meet the "drift" requirements

    given in codes;

    Fig. 1.12.Limitation of lateral displacement ("drift")

    all plastic deformations in the plastic zones must not exceed the ultimate

    plastic deformation allowed for these zones (material and cross-section

    limitations).6. No other ultimate limit state (general instability, brittle fracture etc.) should be

    reached before the formation of the plastic mechanism.

    7. The acceptable plastic deformations are as follows:

    plastic hingesin the members in bending:

    at the ends of the beams, preferably not in the beam-column

    connection (Fig. 1.13a), even though EN 1998-1 accepts it;

    different research programs try to investigate the plastic behaviourof these connections;

    v

    h

  • 8/9/2019 Multietajate En

    14/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    14

    at the ends of long linksin eccentrically braced frames (EBF);

    at the ends of intermediate links in eccentrically braced frames

    (EBF); in this case the influence of the shear force on the plastic

    moment resistance cannot be neglected. at the base of the frameor at the top of the columns(Fig. 1.13b,

    c) in the upper storey of multi-storey buildings after the formation of

    at least 50% of the plastic hinges in the beams;

    at the top and bottom of columns in single storey buildings in

    which NEd in columns conform to the inequality: NEd/Npl,Rd < 0,3

    (Fig. 1.13b, c); the same approach can be used for inverted

    pendulum structures, provided that the earthquake resistantstructure possess more than one column in each plane;

    (a) (b) (c)

    Fig. 1.13.Recommended locations for plastic hinges

    yielding of the tension diagonal in frames with concentric bracings;

    shear yielding in the webs of short beams and short links.

    8. The following are forbidden in potentially plastic zones:

    change the cross-section of the element;

    to have holes;

    to support other secondary (or main) structural members.

    9. Plastic deformations are not allowed in the following locations:

    all along the columns (except for the locations at point 7);

    in the joints (panel zones) of the structure (see the notes below);

    in the anchor bolts of the columns;

    in the connections of members (see the notes below).

    Note 1: More recent codes do not explicitly forbid plastic deformations in the

    panel zone. EN 1998-1 and P100-1:2013 do not say anything about the panel

    rather not

    recommended

    recommended

    recommended

  • 8/9/2019 Multietajate En

    15/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    15

    zones. ANSI/AISC 341-10 has specific provisions concerning the (column) panel

    zone (Fig. 1.14).

    Note 2: ANSI/AISC 341-10 limits the slenderness of the panel zone. Many

    tests demonstrated that significant ductility can be obtained by shear yielding of

    the panel zone (ANSI/AISC 341-10, page 9.1-201) but excessive panel zone

    distortions can unfavourably affect the beam-to column connection. The

    Japanese code also accepts yielding in the panel zone, based on the results of

    tests.

    Note 3: EN 1998-1 accepts to dissipate energy in semi-rigid connections

    (connections where a moment-rotation relation is given) if their behaviour is very

    well controlled.

    Fig. 1.14.The column panel zone in the joint of a structure

    10. The cross-sections of the potentially plastic zones (class 1 cross-sections) must

    allow the development of plastic deformations without local buckling or other

    forms of instability. Plastic deformations shall be directed to the chose potentially

    plastic zones.

    Generally, codes make a connection between the chosen behaviour (ductility)

    factor q and the slenderness of the cross-sections of structural members. For

    instance, EN 1998-1 and P100-1:2013 require the following:

    Table 1.5.Requirements on cross-sectional class of dissipative elements depending

    on Ductility Class and reference behaviour factor([3] Tab. 6.3)

    Ductility class Reference value of

    behaviour factor q

    Required cross-

    sectional class

  • 8/9/2019 Multietajate En

    16/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    16

    DCM

    1,5 < q 2 class 1, 2 or 3

    2 < q4 class 1 or 2

    DCH q > 4 class 1

    11. The joints and the connections of seismic-resistant structures should remain

    essentially in the elastic range for a good plastic behaviour of the entire structure

    (see notes at point 9). Local buckling of the joint is not acceptable.

    12. Floor mechanisms or other partial (local) mechanisms are not accepted in multi-

    storey buildings subjected to seismic actions.

    13. The steel grades that are used must fulfil special requirements in order to provide

    a good plastic behaviour of the entire structure.

    1.3.3. Requirements for steel grades used for seismic-resistant structures

    Because of the required good plastic properties, codes provide special

    requirements for steel grades used for seismic-resistant structures. P100-1:2013, as

    well as previous versions of EN 1998-1 and ANSI/AISC 341-10, contain precise

    values for the most important mechanical characteristics of steel. Newer versions of

    EN 1998-1 and ANSI/AISC 341-10 tend to be more relaxed, as they make reference

    to other design codes. The most important requirements are the following ones:

    1. Generally, codes limit the superior value of the yielding limit of steel. As a result,

    the commonly used steel grades for seismic-resistant structures are: S235, S275

    and S355. One of the reasons for limiting the yielding limit of steel was that one

    of the causes of many brittle fractures during the Northridge earthquake (1994)

    was that the bigger yielding limit resulted in increased requirements for

    connection.

    2. The steel grade and the welding material must have an adequate tenacity to

    avoid brittle fracture. P100-1:2013 requires 27J on a Charpy specimen at the

    minimum design temperature. EN 1998-1 no longer contain explicit values and it

    makes reference to other codes. ANSI/AISC 341-10 is more precise in defining

    the zones where special requirements apply.

  • 8/9/2019 Multietajate En

    17/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    17

    3. In potentially plastic zones, the actual maximum yielding strength, fymaxshall not

    exceed a value that must be written on plans. This limitation is expressed as

    follows:

    yovmaxy f1,1f (EN 1998-1) (1.2)

    yovmaxy ff (P100-1:2013) (1.3)

    yyye FRF (ANSI/AISC 341-10) (1.4)

    utye FRF (ANSI/AISC 341-10) (1.5)

    where:

    fy nominal value of the yielding limit (ex. 235N/mm2for S235);

    ov overstrength factor;

    the recommended value in EN 1998-1 is 1,25;

    P100-1:2013 recommends different values: 1,40 for S235; 1,30 for

    S275 and 1,25 for S355. However, these values must be correlated

    with the locally adopted valueM0=1,1;

    RyFy expected yield stress;

    RtFu expected tensile strength.

    The values of Ryand Rtin ANSI/AISC 341-10 [4] are given in table 1.6.

    Table 1.6.Ryand Rtfor steel and steel reinforcement materials([4] Tab. A3.1)

    Application Ry Rt

    Hot-rolled structural shapes and bars:

    - ASTM A36/A36M

    - ASTM A1043/1043M Gr. 36 (250)

    - ASTM A572/572M Gr. 50 (345) or 55 (380),

    ASTM A913/A913M Gr. 50 (345), 60 (415), or 65 (450),

    ASTM A588/A588M, ASTM A992/A992M

    - ASTM A1043/A1043M Gr. 50 (345)

    - ASTM A529 Gr. 50 (345)

    - ASTM A529 Gr. 55 (380)

    1,5

    1,3

    1,1

    1,2

    1,2

    1,1

    1,2

    1,1

    1,1

    1,1

    1,2

    1,2

    Hollow structural sections:

    - ASTM A500/A500M (Gr. B or C), ASTM A501 1,4 1,3

    Pipe:

    - ASTM A53/A53M 1,6 1,2

  • 8/9/2019 Multietajate En

    18/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    18

    Plates, Strips and Sheets:

    - ASTM A36/A36M

    - ASTM A1043/1043M Gr. 36 (250)

    - A1011/A1011M HSLAS Gr. 55 (380)

    - ASTM A572/A572M Gr. 42 (290)

    - ASTM A572/A572M Gr. 50 (345), Gr. 55 (380),

    ASTM A588/A588M

    - ASTM A1043/1043M Gr. 50 (345)

    1,3

    1,3

    1,1

    1,3

    1,1

    1,2

    1,2

    1,1

    1,1

    1,0

    1,2

    1,1

    Steel Reinforcement:

    - ASTM A615, ASTM A706 1,25 1,25

    4. P100-1:2013 requires:

    20,1ff yu ;

    %20u ;

    the elongation at the end of the yielding plateau must be superior to 1,5%;

    EN 1998-1 and ANSI/AISC 341-10 no longer contain such explicit limits.

    5. Steel must have a good weldability.

    6. Bolts to be used are 8.8 and 10.9. 12.9 bolts generally have a brittle failure. For

    in-plane loaded connections, the shear resistance must be superior to thebearing resistance in order to have a ductile failure. For end-plate connections,

    failure mode 1 (complete yielding of the flange) is the most ductile one.

    1.3.4. Requirements for the connections of seismic resistant steel structures

    Generally, codes require as basic principle the idea that joints must be able to

    transfer the resistances of the elements, taking into account the possibility that the

    resistance of the element is greater than its nominal value. To meet this

    requirement, codes contain recommendations about conformation, calculation and

    technological requirements for joints. Below are presented some of the most

    important requirements:

    1. The conformation of the joint detail must limit the zones where notch effects

    could arise, important residual stresses could develop or where plastic

    deformations could occur in the joints (Fig. 1.15).

  • 8/9/2019 Multietajate En

    19/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    19

    Fig. 1.15.Example of joint that avoids notch effects

    2. In a connection with weld seams and bolts, loads shall not be shared between

    these two different connecting means.

    3. It is generally considered that full penetration butt welds are able to transfer the

    required resistance.

    4. EN 1998-1 [10] and P100-1:2013 [10] require for the resistance of joints realised

    with fillet welds and bolts that:

    fyovd R1,1R ((6.1) EN 1998-1; (6.1) P100-1:2013) (1.6)

    where:

    Rfy the plastic resistance of the connected member, calculated based on the

    nominal value of the yielding limit;

    ov overstrength factor;

    Rd resistance required to the joint.

    5. Given that welding requires more advanced technology and more skilled workers

    (the quality of welding depends primarily on these requirements), the American

    and European practice is to realize the shop connections by welding and the site

    connections by bolts, thus avoiding welding on site. On the contrary, in Japan,

    most of the joints on the site are made by welding.

    6. Given that the weld introduces residual stresses and it can be a weak point in the

    structure, in the joints of a structure it is generally recommended for the column

    to be continuous (Fig. 1.16a). Yet there situations where, for technical reasons, it

    is accepted that the beam is continuous (Fig. 1.16b). This type of detail is not

    accepted in Romania.

    No Yes

    1:5

    1:5

  • 8/9/2019 Multietajate En

    20/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    20

    (a) (b)

    Fig. 1.16.Types of beamcolumn joints

    7. It is recommended for the shear resistance of bolts to be superior to the bearing

    resistance, in order to have a ductile failure of the connection. To fulfil this

    requirement, EN 1998-1 [10] and P100-1:2013 [10] require for the shear

    resistance to be 1,2 times superior to the bearing one.

    1.4. THE BEAMS OF SEISMIC RESISTANT STEEL STRUCTURES

    1.4.1. Conformation principles

    The majority of codes recommend realising the beams with three zones (Fig.

    1.17):

    a rigid zone (r) at each end of the beam, near the beam to column joint;

    a plastic zone (p), neighbour to the rigid one;

    a central elastic zone (e).

  • 8/9/2019 Multietajate En

    21/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    21

    Fig. 1.17.Zones on a beam sized by the seismic action

    Considering the notations (Fig. 1.17):

    r

    EdM the maximum bending moment in the rigid zone (r

    E,Ed

    r

    G,Ed

    r

    Ed MMM += );

    rp

    EdM the bending moment in the cross-section between the rigid zone and the

    plastic one ( rp E,Edrp G,EdrpEd MMM += );

    pe

    EdM the bending moment in the cross-section between the plastic zone and the

    elastic one ( pe E,Edpe

    G,Ed

    pe

    Ed MMM += );

    during an earthquake, the gravitational loads remain unchanged, while the effects of

    the seismic action can increase.

    Following this, we consider a monotonic increase of the bending moments

    caused by the seismic action. Failure of the beam-column connection (cross-section

    r) must be avoided ( rRdr

    Ed MM < ). Consequently, the first cross-section fully plastic is

    (rp) (on the right hand side in figure 1.17) ( plrprp MM = ). If the bending moment caused

    by the seismic action increases, the plastic deformations develop towards cross-

    section (pe). In the state when the bending moment in cross-section (pe) is equal to

    the plastic resistance of the cross-section ( plpepe MM = ) we have plastic stress

    distribution on the entire cross-section all along zone (p), where we accepted the

    development of a plastic hinge. In this loading state, cross-section (r) must still

    ME

    r rep p

    MG

  • 8/9/2019 Multietajate En

    22/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    22

    remain basically in the elastic range ( rRdr

    Ed MM < ). If the bending moment caused by

    the seismic action continues to increase, rotations occur in the already formed

    plastic hinge and, at a certain moment, a second plastic hinge will begin to form.

    This second plastic hinge must not be located in zone (r) (on the left hand side in

    figure 1.17) but in zone (p). Following a similar scenario, the entire cross-section

    goes plastic all along zone (p) (on the left hand side in figure 1.17). If the beam is

    properly sized, at this stage, zones (r) must be still basically in the elastic range and

    zone (e) must be in the elastic range.

    To get this behaviour, the cross-section in the plastic zone (p) must be class

    1, the cross-section in the rigid zone (r) may be class 1 or 2 (preferably class 1) and

    the cross-section in the elastic zone (e) may be class 1, 2 or 3 (preferably class 1 or

    2). The length of the rigid zone (r) (Fig. 1.17) is also very important. Basically, it must

    not be too long, to reduce the increase of the bending moment between the plastic

    hinge and the beam to column connection. If the plastic hinge is too much away

    from the column face, in order to avoid the weak column, the requirements for the

    column cross-section increase. On the other hand, the rigid zone must be sufficiently

    long to allow a good stress distribution in the flange between the plastic hinge cross-

    section and the beam to column connection.Basically, plastic deformations in the structure must begin to develop under

    the action of thebase shear force (Fb) (when the equivalent seismic force reaches

    this value), so, the first plastic hinge must form in a structure in a beam once that the

    value of the seismic force overcomes thebase shear force (Fb).

    1.4.2. Types of practical solutions for beams

    There are basically two types of practical solutions to direct the formation of

    the plastic hinge in the beam at a certain distance from the beam to column

    connection:

    strengthening the rigid zone (Fig. 1.18) relative to the rest of the beam; the

    solution in figure 1.18a shows an additional risk of lamellar tearing of the material

    in the flange of the column, because of the welding seams;

  • 8/9/2019 Multietajate En

    23/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    23

    (a) (b)

    Fig. 1.18.Examples of solutions for strengthening the rigid zone (FEMA 351 [4] Fig.

    2.3, FEMA 350 [4] Fig. 3.23)

    reducing the beam cross-section (Fig. 1.19) in zone (p); this solution is also

    known as RBS (reduced beam section) or dog bone.

    RBS solution was patented in the United States by the manufacturer ARBED of

    Luxembourg in 1992. After the Northridge earthquake (1994) they gave up their

    intellectual property rights, which facilitated the evolution of this solution. The detail

    in figure 1.19a reduces the stress concentrations. Tests showed that plastic

    deformations develop all along the plastic zone. The detail in figure 1.19c tries to

    follow the bending moment diagram and to generate a uniform yielding state along

    the plastic zone. Tests showed that, after some plastic deformations, brittle fractures

    may occur in the reduced sections in figures 1.19b and 1.19c because of the corner

    areas, which are load concentrators.

    Fig. 1.19.Examples of solutions for the reduced beam section

    (a)

    (b)

    (c)

  • 8/9/2019 Multietajate En

    24/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    24

    1.4.3. Example of RBS design

    Generally, there are two types of requirements of major importance in the

    choice of the cross-sections of beams (and other structural members). They do not

    refer to the reduced beam section zone (RBS) but to the beam cross-section:

    the cross-section of the beam must be able to resist the bending moment

    generated by the load combination containing amplified seismic forces;

    r

    E,EdT

    r

    G,Ed

    r

    Ed MMM += ((6.6) P100-1:2013) (1.7)

    r

    E,Edov

    r

    G,Ed

    r

    Ed M1,1MM += ((6.6) EN 1998-1) (1.8)

    where:

    r

    EdM the total bending moment in the beam to column connection;

    r

    G,EdM the bending moment generated by gravitational loads;

    r

    E,EdM the bending moment generated by seismic loads;

    T the value of the overstrength of the structural system (see Tab. 1.2);

    the minimum value of the plastic reserve of all plastic hinge cross-sections;ov the overstrength factor (of the material in the plastic hinge zones);

    the structure must fulfil the drift requirement (Fig. 1.12);

    limh

    v (1.9)

    the displacement v in relation (1.9) is calculated without reducing the forces by

    the behaviour factor q.

    Generally, the drift requirement (1.9) is more difficult to be fulfilled for moment

    frames. The reduction of the cross-section in the plastic hinge zone reduces the

    rigidity of the frame. Researches carried out by Grubbs at the University of Texas

    [16] showed that a 50% reduction of the flanges in the RBS zone reduces the rigidity

    of the frame by 6 7%, while a 40% reduction of the flanges in the RBS zone

    reduces it by 4 5%.

    In a first iteration, the cross-sections of members (beams, columns, braces

    etc.) are chosen, based on the drift requirements. Class 1 and 2 cross-sections can

    be sized either in the elastic or in the plastic range.

  • 8/9/2019 Multietajate En

    25/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    25

    The main steps of design are the following ones:

    1.4.3.1. Estimation of the position of the plastic hinge

    If the loads generated by gravitational loads are less than 30% of the

    resistance of the cross-section of the beam, the American code FEMA 350 [17]

    recommends the following relations.

    Fig. 1.20.Moment frame with plastic hinges on the beams (FEMA 350 [4] Fig. 3.23)

    If gravitational loads generate more than 30% of the loads in the beam cross-

    section, a plastic analysis of the structure is necessary to determine the position of

    the plastic hinges. FEMA 350 [17] recommends values for the distance sh(Fig. 1.21)

    for different types of details of beam to column connection. Generally, this value is

    around:

    2

    h

    2

    hs bch += (1.10)

    where:

    hc the height of the cross-section of the column ;

    hb the height of the cross-section of the beam.

    deformed frame

    plastichinges

    initialform

    h

    L

    L1

  • 8/9/2019 Multietajate En

    26/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    26

    Fig. 1.21.Location of the plastic hinges

    Depending on the type of detail, the distance shmay increase up to:

    bc

    h h2

    hs += (1.11)

    Generally, the length of the plastic hinge in steel structures beams is about half of

    the beam height (hb/2) [17]. FEMA 350 [17] recommends the following values for the

    dimensions in figure 1.22:

    Fig. 1.22.Geometry of RBS (reduced beam section)

    ( ) fb75,0...50,0a = (1.12)

    ( ) bh85,0...65,0b = (1.13)

    1.4.3.2. Estimation of the probable value of the bending moment in the plastic hinge

    plastic hinge

    L1

    L

    sh

    hb hc

    ab

    c

    c8

    bc4R

    22

    +=

    bf

  • 8/9/2019 Multietajate En

    27/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    27

    The aim of this sizing procedure is to limit the maximum bending moment that

    may occur in the cross-section of the beam to column connection at a value inferior

    to 85100% of the plastic moment resistance of the cross-section (Fig. 1.23). The

    following steps need to be followed to fulfil this requirement:

    1. the value of cis proposed (Fig. 1.23);

    fb25,0c (1.14)

    2. the plastic strength modulus Wpl,RBSof the reduced section (RBS) is determined;

    3. the probable valueof the plastic bending moment prRBS,plM of the reduced cross-

    section (RBS) is determined (Fig. 1.23); this estimation intends to manage the

    possibility for the plastic bending moment in the plastic hinge cross-section to be

    bigger than the nominal value;

    in the spirit of EN 1998-1 and P100-1 :2013:

    RBS,plyovpr

    RBS,pl Wf1,1M = (1.15)

    where:

    fy the nominal value of the yielding limit;

    ov overstrength factor; the recommended value in EN 1998-1 is 1,25;

    1,1 a safety variation of 10%;

    in the American code FEMA 350 [17]:

    yeypr

    pr

    RBS,pl FZRCM = ((3.1) FEMA 350 [17]) (1.16)

    where:

    Fy the nominal value of the yielding limit;

    Ze the plastic strength modulus Wpl,RBS;

    Ry factor given in ANSI/AISC 341-10 [4] (see Tab. 1.6);

    Cpr factor taking into account strain hardening and other phenomena

    that can lead to overstrength; FEMA 350 [17] recommends the relation

    y

    uy

    prF2

    FFC

    += ((3.2) FEMA 350 [17]) (1.17)

    Fu the nominal value of the ultimate strength;

    Cpr= 1,2 may be used in the absence of other information;

  • 8/9/2019 Multietajate En

    28/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    28

    Fig. 1.23.Bending moment diagrams

    1.4.3.3. Estimation of the shear force in the plastic hinge

    The maximum shear force is calculated writing (Fig. 1.24):

    2

    Lq

    L

    MMV 1

    1

    pr

    RBS,pl

    pr

    RBS,pl

    RBS

    +

    += (1.18)

    2

    Lq

    L

    MMV 1

    1

    pr

    RBS,pl

    pr

    RBS,pl

    RBS

    +

    += (1.18)

    Bending moment resistance of the beam (MRd)

    Necessary ending moment resistance

    Probable moment

    L1

    L1

    L

    sh

    q

    VRBS VRBS

    prRBS,plM

    prRBS,plM

    pr

    RBS,plM

    RBS,plM

  • 8/9/2019 Multietajate En

    29/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    29

    Fig. 1.24.Equilibrium of the beam segment between the plastic hinges

    1.4.3.4. Estimation of the requirements for the critical cross-section

    The bending moment in the critical cross-sections rEdM at the face of the

    column (Fig. 1.25a) and cEdM in the axis of the column (Fig. 1.25b) is estimated,

    based on the following relations:

    ++=2

    baVMM RBS

    pr

    RBS,pl

    r

    Ed (1.19)

    hRBSpr

    RBS,plcEd sVMM += (1.20)

    The (plastic) resistance of the beam cross-section is:

    0Mypl

    r

    Rd,pl fWM = (1.21)

    The following requirement is checked:

    ( ) r Rd,plr

    Ed M00,1...85,0M = (1.22)

    If rEdM is bigger thanr

    Rd,plM , cmust be increased (Fig. 1.22) and return to 1.4.3.2.

    (a) (b)

    Fig. 1.25.Estimation of the requirements for the critical cross-sections

    1.4.4. Checks

    sh

    r

    EdM c

    EdM

    VRBS VRBS

    +2

    ba

    pr

    RBS,pl

    M prRBS,pl

    M

  • 8/9/2019 Multietajate En

    30/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    30

    1.4.4.1. Resistance checks

    The three zones of the beam (rigid zone, plastic zone, elastic zone) must be

    checked with the following relations:

    00,1M

    M

    Rd,pl

    Ed ((6.2) EN 1998-1; (6.2) P100-1:2013) (1.23)

    15,0N

    N

    Rd,pl

    Ed ((6.3) EN 1998-1; (6.3) P100-1:2013) (1.24)

    50,0V

    V

    Rd,pl

    Ed ((6.4) EN 1998-1; (6.4) P100-1:2013) (1.25)

    For class 3 cross-sections, the plastic values of the resistance must be replaced by

    the elastic ones. The axial force limitation (1.24) is necessary only in the plastic

    zone. The values of the resistance in relations (1.23), (1.24) and (1.25) are

    calculated as follows:

    0MyplRd,pl fWM = for class 1 and 2 cross-sections (1.26a)

    0MyelRd,el fWM = for class 3 cross-sections (1.26b)

    0MyRd,pl fAN = (1.27)

    0M

    yv

    Rd,pl

    3fAV

    = (1.28)

    where:

    A the area of the cross-section of the beam;

    Av the shear area of the cross-section of the beam;

    M0 partial (safety) factor for resistance of the cross-section; M0= 1,0

    (EN 1998-1; EN 1993-1-1) ; M0= 1,1 (P100-1:2013);

    Wpl the plastic strength modulus of the cross-section of the beam;

    Wel the elastic strength modulus of the cross-section of the beam;

    For seismic structures, the use of class 1 and 2 cross-sections is recommended.

    The efforts in relations (1.23), (1.24) and (1.25) are as follows:

    for the plastic zone (p);

    p

    E,Ed

    p

    G,Ed

    p

    Ed MMM += (1.29)

    p

    E,Ed

    p

    G,Ed

    p

    Ed NNN += (1.30)

  • 8/9/2019 Multietajate En

    31/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    31

    p

    M,Ed

    p

    G,Ed

    p

    Ed VVV += ((6.5) EN 1998-1; (6.5) P100-1:2013) (1.31)

    \ p M,EdV the design shear force associated to the application of the plastic

    bending moments with opposite signs in both plastic hinge zones;

    L

    MMV

    p

    B,Rd,pl

    p

    A,Rd,plp

    M,Ed

    += (1.32)

    L the span of the beam; it should be the distance between the two plastic

    hinges design shear force associated to the application of the plastic bending

    moments with opposite signs in both plastic hinge zones (L1in figure 1.24);

    for the rigid zone (r) and for the elastic zone (e);

    E,EdTG,EdEd MMM += (1.33)

    E,EdTG,EdEd NNN += (1.34)

    E,EdTG,EdEd VVV += (1.35)

    where:

    G,EdE effort (M, N, V) caused by gravitational loads;

    E,EdE effort (M, N, V) caused by seismic loads;

    M

    ovT 1,1 = (1.36)

    T the global overstrength factor;

    =

    i,Ed

    i,Rd,plM

    M

    Mmin (1.37)

    1.4.4.2. Lateral torsional buckling check

    The lateral stability of the beam shall be checked presuming that a plastic

    hinge occurred near the most loaded end of the beam. Both flanges of the beam

    must be blocked against lateral displacements in the plastic hinge zone. Codes

    generally require for these lateral supports to be able to resist a certain force; for

    instance, P100-1:2013 recommends a force equal to 0,06ovfytfbf. Lateral torsional

    buckling of the beam must be prevented on the entire length of the beam.

    Recommendations for the maximum distance between lateral supports can be foundin EN 1993-1-1.

  • 8/9/2019 Multietajate En

    32/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    32

    1.4.5. Constructional recommendations for beams

    1. None of the following is not permitted in the plastic zone (p):

    to support secondary beams;

    to change the cross-section of the beam;

    to drill holes.

    2. The connection between the beam flange and the column flange (beam web in

    the same plane with the column web) shall be realised by fully penetrated but

    weld (Fig. 1.26).In figure 1.26, tbfis the thickness of the flange and (FEMA 350 [4]):

    1 bevel as required for selected groove weld procedure;

    2 max (tbf ;13mm) (+ 0,5tbf ; 0,25tbf );

    3 0,75tbf tbf19mm (6mm);

    4 minimum radius 10mm (plus not limited, minus 0);

    5 3tbf(13mm);

    FEMA 353 [XX] covers the requirements for fabrication details, including cutting

    methods and smoothness.

    Fig. 1.26.Recommended welding detail for beam flange (FEMA 350 [4] Fig. 3.5)

    3. The connection between the web of the beam and the column can be realised

    either by fillet weld or by butt weld. The bolted connection should be avoided (in

  • 8/9/2019 Multietajate En

    33/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    33

    combination with flanges welded connection). If, for technological (erection)

    reasons, a bolted connection is used on site, eventually, welding seam must be

    realised (Fig. 1.27).

    Fig. 1.27.Connection detail (ANSI/AISC 358-10 [4] Fig. 8.1)

    4. It is desirable for the beam-column connections to be realised in the shop (by

    welding), so that a very good behaviour can be obtained even for very severe

    earthquakes. Under these circumstances, it is recommended for the site

    connection to be placed in the elastic zone of the beam and, consequently, the

    columns arrive on site with the rigid and plastic zones of the beams already

    attached. The site connections are made by high strength bolts in slip

    connections. The drawback is related to transport, as it is not always easy for

    such column parts. However, it is to note that many builders prefer an end-plate

    beam-column connection and this cannot be neglected.

    5. The beam-column connection must be able to transfer loads corresponding to

    the resistance of the connected part (beam), taking into account the overstrength

    of the material in the beams.

    6. Lateral torsional buckling of the beam must be prevented. Lateral stability of the

    top flange is ensured by the reinforced concrete slab by means of the studs that

    connect them to avoid relative slip. Special attention must be given to the bottom

    flange, as lateral supports must be offered (for instance, by means of a

    connection to the slab). It is recommended to avoid any connection between

  • 8/9/2019 Multietajate En

    34/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    34

    the slab and the top flange in the plastic zone (and in the rigid zone) [17]

    (tests showed that the important increase of the cross-section because of this

    connection, as concrete is in compression and steel in tension, can lead to brittle

    fracture in the bottom flange when the beam is subject to cyclic loading [17]). An

    increased bending moment resistance of the cross-section of the beam also

    increases the risk of forming a plastic hinge on the column, instead of having it

    on the beam, as desired.

    7. It is recommended to have stiffeners at both ends of the plastic hinge (Fig. 1.28).

    The thickness of the stiffener, tst, must be at least 75% of the thickness of the

    web and bigger than 10mm. It is also recommended to be bigger than bst/15,

    where bstis the width of the stiffener. The stiffener width must obey the following

    relation:

    stwst btb2 =+ (1.38)

    where twis the thickness of the web.

    Fig. 1.28.Stiffeners on the plastic hinge zone

    1.5. THE COLUMNS OF SEISMIC RESISTANT STEEL STRUCTURES

    1.5.1. General recommendations

    The cross-sections of columns can be one of the following ones:

    open cross-sections (Fig. 1.29a);

    hollow cross-sections (Fig. 1.29b).

    They can be either hot-rolled or built-up by welding.

    tw

    tr br

    bf

  • 8/9/2019 Multietajate En

    35/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    35

    (a) (b)Fig. 1.29.Common cross-section solutions for columns

    Some of the most important advantages of hollow sections are the following ones:

    the geometrical characteristics of the cross-section and the stiffness about the

    two main axes are comparable;

    they have a much better behaviour to accidental torsion, compared to the open

    ones;

    an improved resistance against fire and against corrosion.

    In many cases, hollow sections are filled with concrete, which leads to a better

    rigidity and an increased fire resistance.

    The main drawback of hollow sections is related to more complicate joint details

    when connecting the column to the beams, to braces or to the foundation.

    The cross-sections of columns should be class 1 or class 2. In zones where

    plastic deformations are accepted (near the connection to the foundation and at the

    top end) the cross-section should be class 1.

    The connections of columns should be placed neither in strongly loaded

    zones (neighbour to joints), nor in potentially plastic ones. Generally, this

    recommendation is obeyed if the connection of the column is placed in a zone

    between H/5 and H/3 above the floor (where H is the storey height), which

    corresponds to a distance of 0,8 1,2m.

    The column connections may be either welded or bolted; bolted connectionsuse high strength bolts in slip connections and the transfer of loads can be realised

    either through the splices or by contact between parts.

    Columns should be connected at each floor in the plane normal to the frame

    plane.

    The cross-section of the part of column where plastic deformations are

    accepted should have a shape that allows reducing the amount of stresses caused

    by the axial force and the shear force, compared to stresses caused by the bendingmoment. The length of the plastic zone should be limited at 1,5hc, where hcis the

  • 8/9/2019 Multietajate En

    36/36

    STEEL STRUCTURES FOR TALL BUILDINGS

    height of the column cross-section. Stiffeners should be placed all along the plastic

    zone at a gap less than 0,5hcamong them.

    1.5.2. Recommendations for column checks

    1.5.2.1. General checks