earthquake-resistant design of steel buildings

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Earthquake-resistant design of steel buildings

Francisco López Almansa

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Objectives

Earthquake-resistant design of RC buildings. Francesc López Almansa. Barcelona

Earthquake-resistant design of steel and composite steel-concrete buildings

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Index

Major regulations 4Steel building structures 5Earthquake-resistant steel building structures 7Moment-resisting frames 16Concentrically-braced frames 21Eccentrically-braced frames 27Special Truss Moment Frames 29Outrigger walls 30Bibliography 31Internet Sites 33


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Major regulations


EUROPE− EC-8 [EN-1998-1 2004]. Chapters 6 (“Specific Rules for Steel

Buildings”) and 7 (“Specific Rules for Composite Steel-Concrete Buildings”)

− EC-8 [EN-1998-1-2 2020]. Chapters 11 (“Steel Buildings”) and 12 (“Composite Steel-Concrete Buildings”

USA− ANSI/AISC 341-16: Seismic Provisions for Structural Steel

Buildings. American Institute of Steel Construction 2016− IBC. Spread in different sections− ASCE/SEI 7-16. Chapters 11 through 23 (14.1 is specific for

steel)− FEMA

SPAIN− NCSE-02. Art. 4.6− EAE. Chapter 13

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Steel building structures (1)


There are two major type of steel buildings: single-story(long span-length, suitable for industry, sports hall, multi-purpose, etc.) and multi-story(normal span length, suitable for housing, administrative, teaching, sanitary, storage, parking, etc.)

Single-story buildings can be all-steel, provided that the roof is light (not accessible)

In multi-story buildings, the (one-way) walkable steel slabs are topped by a concrete layer (steel deck)

Columns can be either all-steel or composite steel-concrete (tubes filled with concrete, profiles embedded into concrete); in this case, the buildings are termed as composite steel-concrete

Other combinations (e.g. concrete columns and steel decks) are also possible

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Steel building structures (2)


General advantages of steel: (i) high compressive and tensile strength, (ii) weldability, (iii) low cost, (iv) abundance, (v) recyclability and reusability

General disadvantages of steel: (i) corrosion, (ii) fire behavior, (iii) poor sustainability

General advantages of steel construction: (i) lightweight, (ii) construction rapidity, (iii) less volume of structural members (mainly columns), (iv) long spans

General disadvantages of steel construction: (i) high cost, (ii) corrosion, (iii) fire behavior, (iv) buckling, (v) poor sustainability, (vi) vibration of composite slabs

Steel structures posses some earthquake-resistant advantages: lightweight and ductility. Both advantages are more relevant in all-steel buildings

Main earthquake-resistant disadvantages of the steel construction are: low damping (mainly in modern buildings with curtain walls), and fragility of the connections

The steel decks are not optimal for seismic behavior, since negative bending moments are more frequent (both for lateral and vertical shakes) and the topping concrete layer can be tensioned

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Earthquake-resistant steel building structures (1)


Types of steel structures in seismic areasMoment-resisting

frameConcentrically-braced frame

Eccentrically-braced frame

Diagonal (X) bracing also exists

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EC-8 Chapter 6. Types:a) moment resisting framesb) frames with concentric

bracingsc) frames with eccentric

bracingsd) inverted pendulum

structurese) structures with concrete

cores or concrete wallsf) moment resisting frames

combined with concentric bracings

g) moment-resisting frames combined with infills

EC-8 Chapter 7. Types (a) to (d) are alike to Chapter 6; also: (e) composite structural systems, (f) composite steel plate shear walls

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EC-8 Chapter 7. Type (e) (composite structural systems) is decomposed into three sub-types:− Type 1. Steel or composite frame with concrete infill panels− Type 2. RC wall with encased steel sections as vertical edge reinforcement− Type 3. Steel or composite beams coupling two or more RC or composite walls

EC-8 Chapter 7. Type (f) (composite steel plate shear walls, SPSW). A vertical steel plate continuous over the height of the building with RC encasement on one or both faces of the plate and of the structural steel or composite boundary members

xy

xz

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ASCE 7/16. Types: MRF, CBF, EBF, BRBF (Buckling-Restrained Braced Frame), SPSW (Special Plate Shear Walls)

MRF and CBF can be Ordinary, Intermediate and Special (equivalent to DC L, M and H in the EC-8)

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R factors for Selected Steel Systems (ASCE 7/16):

SMF (Special Moment Resisting Frames): R = 8

IMF (Intermediate Moment Resisting Frames): R = 4.5

OMF (Ordinary Moment Resisting Frames): R = 3.5

EBF (Eccentrically Braced Frames): R = 8 or 7

SCBF (Special Concentrically Braced Frames): R = 6

OCBF (Ordinary Concentrically Braced Frames): R = 3.25

BRBF (Buckling Restrained Braced Frame): R = 8 or 7

SPSW (Special Plate Shear Walls): R = 7

Undetailed Steel Systems in Seismic DesignCategories A, B or C R = 3

(AISC Seismic Provisions not needed)

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q factors in the Chapter 6 of EC-8 For DC L, 1.5 ≤ q ≤ 2 Default values of the ratio αu / α1: If a pushover analysis is carried out, higher values (not exceeding 1.6) can be

considered In buildings that are not regular in plan or in elevation, values are smaller

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The European regulations (EC-3) classifies the sections into 4 classes: 1, 2, 3 and 4

Classes “1 and 2”, “3” and “4” correspond approximately to “compact”, “semi- compact” and “slender” in the US, respectively (AISC)

Class 1 allows plastic global and local analyses, class 2 allows only plastic local analysis; in ordinary design, this hue is not crucial

In earthquake-resistant design, difference between classes 1 and 2 is relevant

Class 1 parallels roughly “seismically compact” in the US

Material considerations (EC-8)− In the dissipative zones, the actual maximum yield

strength of the steel is upper bounded: fy,max ≤ 1.1 γov fy(6.2(3)c)

− fy: nominal yield strength, γov: overstrength factor (its recommended value is 1.25)

− EC-8 (6.2(3)b) recommends to use steels S355 for non-dissipative members (columns) and to use steel S235 for dissipative members (beams and braces). Then, γov = 1

− Even better to use S460 in columns

Class 1Class 2

Class 3

Class 4

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Protected Zones are those where plastic hinges are expected to form

Shall comply with the following:− No welded shear studs are permitted− No decking attachments that penetrate

the beam flange are permitted (no power-actuated fasteners)

− No welded, bolted, screwed, or shot-in attachments for edge angles, exterior facades, partitions, duct work, piping, etc. are permitted

Requirements on sectional class of dissipative elements:

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Single-story all-steel buildings These buildings are not strongly affected by earthquakes due to their inherent

lightweight and high ductility; wind forces can be more demanding Bracing in lateral walls and in roof is necessary, mainly in longitudinal direction Vertical bracing provides lateral resistance; horizontal bracing provides rigid

diaphragm effect

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Moment-resisting frames (1)


“Seismic frame”

Not all the members of the frames participate in the lateral resistance

Some parts (termed as seismic frames) are designed to resist earthquakes

The other parts are designed only for gravity loads

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The seismic frames must be placed in both directions symmetrically and separated (to provide symmetry and torsion strength and stiffness, as if they were bracing elements)

In each seismic frame, the columns are oriented according to its plane (no column can be shared between several seismic frames)

The “strong column-weak beam” condition (4.4.2.3(4) Σ MRc ≥ 1.3 Σ MRb) applies only in the rigid connections of the seismic frames

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In moment-resisting frames, the beam-column connections are crucial to the seismic resistance

In the North Ridge earthquake (1994), many buildings experienced severe damage because of brittle premature failure of the connections

A huge research activity was undertaken and a number of pre-qualified connections were designed and published (FEMA 350)

Both bolted and welded connections are provided

Bolted-welded connections (parallel combination of both) are not permitted

Out-of-joint failure is promoted Similar activity was undertaken in Japan after

the Kobe earthquake (1994) Recently, Equal Joints initative in Europe

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EC-8 6.6 (Design and detailing rules for moment-resisting frames)

For plastic hinges in beams (class 1 and 2): MEd ≤ MplRd; NEd ≤0.15 NplRd; VEd ≤ 0.5 VplRd; VEd = VEdG + VEdM; VEdM = (MplRdA+ MplRdB) / L

Columns: NEd = NEdG + 1.1 γov Ω NEdE; MEd = MEdG + 1.1 γovΩ MEdE; VEd = VEdG + 1.1 γov Ω VEdE; VEd ≤ 0.5 VplRd

γov: overstrength factor (1.25); Ω = minimum value of the ratio between the flexural strength and demand for all the beams with dissipative zones (plastic hinges)

For preliminary design, a convenient choice for the non-dimensional slenderness is λ < 0.2 in columns and λLT < 0.4 in columns and beams (λ = λ

π𝑓𝑓y𝐸𝐸

, where λ = 𝐿𝐿k𝑖𝑖

is the mechanical slenderness, Lk is the buckling length and i is the sectional radius of gyration in the direction under consideration, given by 𝑖𝑖 = 𝐼𝐼

𝐴𝐴)

Reverting the moment sign in beams, requires lateral bracing of the bottom flanges (under compression)

To prevent the failure of the beam-column panel, web reinforcement can be necessary

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Concentrically-braced frames (1)


Diagonal (X) bracing Chevron (V) bracing Modified

chevronZipper column

Diagonal (only tension) bracing

Modified diagonal K

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If the braces were located on the same bay (e.g. vertically superposed) the bottom supporting columns would experience strong tension/compression forces due to the cumulative effect of the axial forces generated by the braces; obviously, this result is more intense in tall buildings

An effective solution is to distribute the braces along all the bays

Warning! This layout complicates the construction details, since some segments of the beams need to carry relevant axial forces

Comparison between top and bottom sketches shows that the horizontal layout of the braces generates a higher lever arm (e.g. the horizontal distance between the vertical reaction forces), thus leading to smaller vertical forces

Frequently, the “empty” spaces are also filled with braces The same consideration can be applied to chevron

bracing

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Braces are connected to the main frame through intermediate gusset plates

These connections can be either bolted or welded

In the frame plane, the braces might be considered clamped in their both ends(although are conservatively? assumed as hinged); in the orthogonal direction the braces are simply supported (pin-ended) due to the low transverse stiffness of the gusset plates

In X bracing, braces are connected in the mid point; this prevents in-plane displacements, but the restraint against out-of-plane motion is only partial (tension increases the transverse stiffness of braces)

Summary on the buckling length of X braces. In the frame plane is equal to half of actual length (smaller values can be considered) and in the orthogonal direction has some value in between the whole length and half of it

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The axial forces in the beams that are connected to braces can be significant

Braces shall be designed as their yielding precedes any other failure In X bracings, only tension braces shall be taken into account (no

need to check the compression resistance) In V bracings, both braces shall be taken into account (designed for

compression resistance) In V bracings, beams should be designed to resist the vertical force

after buckling of the compression brace In X bracings, the non-dimensional slenderness should be limited to:

1.3 ≤ λ ≤ 2. American code prescribes (for Special CBF) that λ ≤4π. Japanese code does not include any limitation

In other bracings, non-dimensional slenderness should be ≤2 Buckling resistance of beams or columns: NplRd (MEd) ≥ NEdG + 1.1

γov Ω NEdE γov: overstrength factor (1.25); Ω = minimum value of ratio between

axial strength and demand for all the braces

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Buckling-restrained braces are devices to be connected to steel or concrete frames as ordinary concentric braces

Are composed of a thin steel core that yields under tension; under compression, its buckling is restrained by a stockier casing

The sliding of the core inside the casing prevents shear stress transfer

The casing is formed by steel (all-steel devices) and, sometimes, also mortar

The sliding interface can be just made of air

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Eccentrically-braced frames (1)


This approach is not very clever, since important damage is concentrated in the main structure, thus being totally irreparable

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Eccentrically-braced frames (2)

Earthquake-resistant design of RC buildings. Francesc López Almansa. BarcelonaEarthquake-resistant design of RC buildings. Francesc López Almansa. Barcelona

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Special Truss Moment Frames (1)


The STMF is a seismic load-resisting system that consists of trusses as horizontal members and specially designed segments that are expected to withstand large cyclic deformation during seismic events

This system allows detailing for controlled damage in the special segments of open web trusses

During lateral loading, the truss will be subjected to constant shear and varying axial forces in the chord members. The maximum axial forces in the chord members occur near the ends of the truss, while the minimum forces occur in the middle zone of the truss. Therefore, the special segment is located in the middle of the truss to minimize the adverse effect of the axial forces in the chord members. The shear in the special segment is resisted through axial forces in the diagonal members and flexural shear in the chord members. If the diagonal members are not present in the special segment, then the entire seismic shear is resisted by flexural shear of the top and bottom chord members.

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Outrigger walls (1)


Horizontal trusses connecting the core of a tall building to its perimeter, as to guarantee a joint behavior

The full width constitutes the effective depth

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Bibliography. Books Akiyama H. Earthquake-Resistant Design for Buildings. Tokio University Press 1988. Ambrose J.E., Vergun D. Diseño simplificado de edificios para cargas de viento y sismo.

Limusa 1986. Bazán E., Meli R. Diseño sísmico de edificios. Limusa 2002. Bozorgnia Y., Bertero V.V. Earthquake Engineering: from Engineering Seismology to

Performance-Base Engineering. CRC Press 2004. Bozzo L.M., Barbat A.H. Diseño sismorresistente de edificios. Ed. Reverté 2000. Chandrasekaran S. et al. Seismic Design Aids for Nonlinear Analysis of Reinforced

Concrete Structures. CRC Press 2010. Datta T.K. Seismic Analysis of Structures. J. Wiley 2010. Dowrick D.J. Earthquake Resistant Design for Engineers and Architects. J. Wiley

1977. Fajfar P., Krawinkler H. Seismic Design Methodologies for the Next Generation of

Codes. Balkema 1997. García L.E. Dinámica Estructural Aplicada al Diseño Sísmico. Universidad de Los Andes

(Bogotá) 1998. Naeim F. The Seismic Design Handbook. Van Nostrand Reinhold 2002. Newmark N.M., Rosenblueth E. Fundamentos de ingeniería sísmica. Diana 1978. Paulay T., Priestley M.J.N. Seismic Design of Reinforced Concrete and Masonry

Buildings. John Wiley 1992. Priestley M.J.N., Seible F., Calvi G.M. Seismic Design and Retrofit of Bridges. John

Wiley 1996. Rosenblueth E. Design of Earthquake Resistant Structures. Pentech Press 1980. Wakabayashi M. Earthquake Resistant Design for Buildings. McGraw-Hill 1986.


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Bibliography. Codes NCSE-02. Norma de Construcción Sismorresistente: Parte General y Edificación.

Ministerio de Fomento 2002. NCSP-07. Norma de construcción sismorresistente. Ministerio de Fomento 2007. Seismic Provisions for Structural Steel Buildings. AISC (American Institute on

Steel Construction) 2016. Prequalified Connections for Special and Intermediate Steel Moment Frames for

Seismic Applications. AISC (American Institute on Steel Construction) 2005. FEMA 356. Pre-standard and commentary for the seismic rehabilitation of

buildings. Federal Emergency Management Agency 2000. ACI 318-14. Building Code Requirements for Structural Concrete. ACI (American

Concrete Institute ) 2014. ASCE/SEI 7-16. Minimum Design Loads for Buildings and Other Structures.

ASCE (American Society of Civil Engineers) 2016. Fardis M.N., Carvalho E., Elnashai A., Faccioli, Pinto Plumier A. Designers’

Guide to Eurocode 8. Thomas Telford 2005.


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Internet Sites http://www.asce.org/ http://www.concrete.org/ http://www.aisc.org/ https://www.atcouncil.org/ http://www.fema.gov/ http://peer.berkeley.edu/ http://earthquake.usgs.gov/ http://mceer.buffalo.edu/ http://eurocodes.org.ua/ http://www.roseschool.it/ http://mae.cee.illinois.edu/software_and_tools/zeus_nl.html http://www.civil.canterbury.ac.nz/eq/eqeng.shtml


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