confer bucharest 2007 belev

8/13/2019 Confer Bucharest 2007 Belev

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EUROPEAN AND NORTH AMERICAN PROVISIONS FOR DESIGN OF

STEEL AND COMPOSITE SHEAR WALLS

Irena Hadjiyaneva1

and Borislav Belev2

SUMMARY

The steel and composite shear walls are relatively new seismic-force-resistive structural systems, which

have demonstrated excellent performance in laboratory testing and under real-life earthquake excitation.

Their stiffness, ductility and reliable cyclic response in the inelastic stage have attracted the attention of

various researchers, design professionals and code writers during the recent years. The paper presents an

overview of the basic design provisions for steel and composite shear walls contained in Eurocode 8,

AISC Seismic Provisions and Canadian CSA-S16-01 along with comparison and discussion of a few most

popular approaches to their modeling and analysis.

INTRODUCTION

Since 1970, steel plate and composite steel shear walls have been applied in USA and Japan in regions of

high seismicity and/or wind loading. The main function of a shear wall is to resist a portion of the horizontal

storey shear and overturning moment induced by lateral loads. This resistance is provided by a vertical

cantilever system of boundary elements columns and floor beams tied with steel or composite infills. The

behaviour of the vertical girder depends on the web (infill) slenderness. Under transverse loading a slender

steel web buckles, while a stiffened one remains working in pure shear. There are two possible ways to

stiffen the web panels: by a conventional set of steel stiffeners or by concrete encasement. Alternativeapproach to reach the web yielding in shear prior to its buckling is that of using a low-yield steel for the web

panel.

ADVANTAGES OF THIS STRUCTURAL SYSTEM

Ductility: The main advantage is the ductile manner of energy dissipation by shear yielding or stable post-critical tension-field action. All steel plate shear walls (SPSW), except the low-yield steel plate shear walls

(LYSW) have relatively high initial stiffness, therefore they are very effective in limiting the storey drifts.

Reduced self-weight: Compared to a reinforced concrete shear wall, a composite wall with the same shear

capacity (and most likely with larger shear stiffness) will have a smaller thickness and self-weight. The

smaller footprint of the steel and composite shear wall is very advantageous from architectural point ofview, providing more useable floor space particularly in tall buildings. The lesser weight of steel and

composite shear wall will result in smaller foundations, smaller seismic forces and reduction of construction

costs.

Faster construction: Steel shear walls can be produced in situ or in shop-welded sub-assemblage units in

order to reduce the erection period. A composite steel shear walls (CSSW) can have cast in place or pre-cast

concrete encasement. In particular, if pre-cast concrete cover is used, the bolted shear connection could be

executed at any convenient time during construction.

Potential for retrofit interventions: Shop-welded steel and pre-cast composite shear walls can constitute a

feasible system for upgrade of existing buildings in cases of inadequate lateral stiffness, insufficient force

resisting capacity or presence of damaged frame joints.

1Ph.D. Student, Dept. of Steel and Timber Structures, UACEG, 1046 Sofia, Bulgaria, e-mail: [email protected]. Prof., Dept. of Steel and Timber Structures, UACEG, 1046 Sofia, Bulgaria, e-mail: [email protected]


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Proven performance: At least two buildings that use SPSW as primary lateral force resisting system have

undergone major earthquake ground shaking. Both buildings survived with insignificant structural damage

[Astaneh-Asl, 2001]. The system also has been laboratory-tested extensively since the 1970s, see [Driver et

al., 1997], [Astaneh-Asl, 2001]. The SPSW system has been recognized in the National Building Code of

Canada (NBCC) since 1994 and was included in the Seismic Provisions of the American Institute of Steel

Construction [AISC Seismic, 2005] in 2005.

DISADVANTAGES

Fire protection required: Slender steel plates do not possess adequate fire resistance and need to be

protected. In this respect, the composite steel shear walls have an advantage due to their concrete

encasement.

Difference between design and actual strength:All mentioned shear walls have unavoidable significant

overstrength resulting from the minimum design requirements. This leads to higher seismic shear forces

applied to the foundation.

Stiffness: In terms of stiffness, most deformable are the LYSW, then the unstiffened SPSW, stiffened

SPSW and CSSW (in ascending order) and this must be taken into account. For example, SPSW systemsare less stiff than the concrete shear walls primarily due to their flexural flexibility. Therefore, when using

SPSW in tall buildings, the engineer must provide additional flexural stiffness. In both The Century and the

U.S. Federal Courthouse projects, large composite concrete-filled steel pipe columns were used at all

corners of the core wall to increase the flexural stiffness of the system as well as its overturning capacity

[Seilie and Hooper, 2005].

Impact of construction sequence: Excessive initial compressive force in the steel plate panel may delay

the development of the tension-field action. It is important to choose a construction sequence that would

avoid excessive compression in the panel. In the U.S. Federal Courthouse project, the welding of the plate

splice connections was delayed until most of the dead load deformation occurred in order to relieve the pre-

compression within the steel plate shear wall panel.

Insufficient coverage by design codes: The SPSWs and CSSWs are relatively new systems, and not all

types of them are covered by the codes. For example, Eurocode 8, Part 1 [CEN, 2004] has provisions for the

CSSWs only.

SOME EXAMPLES OF APPLICATION

Steel plate shear walls in a 35-storey office building in Kobe, Japan [Astaneh-Asl, 2001]

The structural system in this building consists of a dual system of steel moment frames and shear walls. The

shear walls are reinforced concrete in the three basement levels and composite walls in the first and second

floors. Above the 2nd

floor the walls are stiffened steel shear walls. Inspections of the buildings indicated

that the damage after the earthquake in 1995 was minor and consisted of local buckling of stiffened steel

plate shear walls on the 26thstorey and a permanent roof drift of 225 mm in northern and 35 mm in western

direction.

Figure 1. Plan and elevations of 35-storey Kobe building (left), and Plan layout of 18-storey hospital in San Francisco (right)


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Composite shear walls in a 18-storey hospital in San Francisco [Astaneh-Asl, 2002]

The composite shear walls in this building consist of steel plates with concrete cover on both sides.

Boundary columns are rolled or welded built-up wide-flange sections. Floor beams in the shear wall panelsare welded plate girders. The shear connections consist of ties passing through holes in the steel plate and

web of plate girder.

The described buildings are schematically shown on Fig. 1.

METHODS OF ANALYSIS

Refined analysis approaches

In order to predict the hysteretical response of test specimens and assess the accuracy of simplified finite

element (FE) models, complete non-linear models with planar FEs have been used. Non-linear material

properties, initial imperfections and buckling phenomena are included in the analyses. These comprehensive

models are useful for scientific research, but for design purposes simpler methods are needed.

Simplified analysis approaches

Steel Plate Shear Walls

For preliminary proportioning of horizontal (beam) and vertical (column) boundary elements and plate

thickness estimation, a single diagonal strut idealization of the infill plate was proposed in [Thorburn et al.,

1983]. Further, the so-called tension strip model for analyzing thin-panel SPSW was proposed by the same

researchers. Based on the theory of pure diagonal tension concept of Wagner, it represents the diagonal

tension field developed after plate buckling as a series of discrete pin-ended strips inclined with the same

orientation (Fig. 2). The number of bars/strips required for realistic modeling depends upon the panel

geometry, but in general, 10 bars per storey are sufficient. This approach was further developed by variousCanadian researchers and checked against experimental results [Driver et al., 1977].As a design procedure,

it is included in the Canadian Steel Design Code [CSA, 2001].

Figure 2. Parallel strip model representation of a typical steel plate shear wall [Rezai et al., 2000]

An important parameter depending both on the storey geometry and on the stiffness of the boundary

framing members is the angle of inclination of the tension strips , which is estimated from the equation

4

3

.1

2tan

11 . . 360. .

c

b c

t L

A

ht h A I L

+

=

+ +

, where (1)


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Ac is the cross section area of verticalboundary elements (VBE);

Ic moment of inertia of vertical boundary elements (VBE);

Ab - cross section area of horizontalboundary elements (HBE).

L- the distance between VBE centerlines; and

h- the distance between HBE centerlines (see Fig. 2).

Rezai et al. proved that the angle of inclination of the tension field depends on h-to-L ratio. If 0.8h

L ,

then the model with parallel inclined strips must be used, otherwise a modified model seemed more

appropriate [Rezai et al., 2000].

The shear force lateral displacement relationship (overall capacity curve of the shear panel) is obtained

through summation of the respective capacity curves of the boundary frame and the plate (infill) acting in

parallel as shown on Fig. 3. The resulting shape of the curve is thus a favourable tri-linear one.

Figure 3. Shear response of wall panel and its components

Composite Steel Plate Shear WallsIn the Commentary of [AISC Seismic, 2005] a simplifying method for preliminary determination of concrete

encasement thickness is proposed. The underlying criterion is avoidance of buckling of the steel infill prior

to its yielding in shear. The overall buckling of a composite panel can be checked using a transformed

section stiffness of the wall (see Fig. 4). One approach to doing this is to transform concrete wall to vertical

and horizontal stiffeners. Composite steel plate shear walls shall be designed to yield through shear of the

steel plate [Astaneh-Asl, 2002].

Figure 4. The equivalent stiffener concept


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DESIGN PROVISIONS FOR SPSW AND CSSW SYSTEMS IN EUROCODE 8, AISC SEISMIC

PROVISIONS AND CAN/CSA S16-01

Behaviour factor q of Eurocode 8 and response modification factors R of the NorthAmerican codes

Eurocode 8, Part 1 contains provisions for composite steel plate shear walls in which the lateral shear

resistance is supplied by the steel plate only. For these CSSWs the values of the behaviour factor qare given

in Table 1.

Table 1. Extraction from Eurocode 8, Part 1

Ductility ClassSTRUCTURAL TYPE

DCM DCH

f) Composite steel plate shear walls 13 u 14 u

The default value of

u/

1is

u/

1= 1.2, but may be obtained from a nonlinear static (pushover) global analysis.

The latest AISC Seismic Provisions [AISC Seismic, 2005] contain design rules for Special plate shear walls

(i.e. high ductility wall system) and for Composite steel plate shear walls. The code provisions are based on

the FEMA 450 recommendations [FEMA, 2003], Table 2.

Table 2. Extraction from FEMA 450, Chapter 4

Basic Seismic-force-resisting

SystemResponse Modification Factor,

RSystem Over-Strength Factor

oDeflection Amplification

Factor, CdSpecial steel plate shear

walls7

26

Composite steel plate shear

walls 6.5 2.5 5.5Dual system with special

moment frames and special

steel plate shear walls8 2.5 6.5

Dual system with special

moment frames and

composite steel plate shear

walls

7.5 2 .56

The Canadian Steel Design Code CAN/CSA-S16-01 [CSA, 2001]alsocontains provisions for steel plate

shear walls. Herein two types of SPSWs are considered: Ductile (with R=5) and Limited-Ductility plate

walls with R=2.

Design rulesComposite Steel Plate Shear Walls

Both Eurocode 8 and AISC Seismic Provisions require that the composite steel plate shear walls shall be

designed to yield through shear of the steel plate. Despite the different limit state design format and

notations in the formulas for the design shear resistance of a composite wall, the results are very similar

because they are based on pure steel yielding in shear. An important prerequisite is that the steel plate

should be stiffened by one- or two-sided reinforced concrete encasement and it shall be reliably attached via

headed studs or other mechanical connectors in order to prevent local buckling of the steel plate (and

separation between the two wall components).

The detailing rules of the two design codes are also very similar and could be summarized as follows:


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1. The concrete thickness should be not less than 200 mm when it is provided on one side and 100 mmon each side when provided on both sides. The minimum reinforcement ratio in both directions shall

not be less than 0,25%.

2. The steel plate shall be continuously connected on all edges to the structural steel framing andboundary members with welds and/or bolts to develop the full yield strength of the plate in shear.

3. The connections between the plate and the boundary members (columns and beams), as well as theconnections between the plate and the concrete encasement, shall be designed so that full yield

strength of the plate can be developed.

4. The boundary members could be either composite or structural steel, and shall be capacity-designed,taking into account the possible overstrength of the composite infill.

5. There is no limitation of the storey aspect ratio of the composite steel shear walls.6. In [AISC Seismic, 2005] an explicit criterion is introduced to prevent local plate buckling between

the studs. When one sided RC encasement is provided, the ratio b/t should comply with:

/ 1.1 /v y

b t k E F (2)

( )

2

5 5 / / vk L h= + , where (3)

b= distance between the studs, and t= plate thickness. For a 10 mm thick steel plate made of Grade S355

steel, the allowable maximum stud spacing is about 550 mm.

7. Both design codes impose requirements for the cross section class of the boundary beams andcolumns if they are unencased or partially encased. For the flange outstand of I- and H-cross

sections the limitation on the c/tf ratio for special, i.e. very ductile composite shear walls is

actually the same as that of EC8 for ductility class DCH. For the case of fully encased boundary

members, Eurocode 8 refers to the design and detailing rules for composite systems with RC shear

walls having composite steel-concrete boundary elements.

Steel Plate Shear Walls

Herein the provisions contained in [AISC Seismic, 2005] are mainly discussed because [CEN, 2001] has no

provisions for pure steel plate shear walls. The design shear strengthis calculated as follows:

2sin42.09.0 cfwyn LtfxV = , where (4)

Lcf = clear distance between the vertical boundary elements (column flanges)

= angle of web yielding, as defined by Equation (1).

The detailing rules of [AISC Seismic, 2005] could be summarized as follows:

1. The panel aspect ratio shall be in the range 0.8 / 2.5L h< . The storey height hand bay widthLshall be taken between the centerlines of HBE and VBE, respectively.

2. Openings in the web should be surrounded by HBE and VBE, extending to the full width and heightof the wall.

3. Columns, beams, connections them and connections of the web to surrounding elements should bedesigned for the expected (i.e. increased by overstrength factors) yield strength, in tension,

developed by the web at an angle .Furthermore, the HBE shall be designed under the assumption

that the steel infill (web) provides no support for gravity loads.

4. The cross sections of the VBE and HBE must be seismically compact which approximatelycorresponds to Class 1 of Eurocode 3.

5. The strong column weak beam concept must be maintained at all frame joints as implied forspecial moment resisting frames (S-MRF).

6. All HBE-to-VBE connections shall be designed, in principle, as connections in ordinary momentresisting frames. It should be noted that partial strength connections are allowed if their resistance

exceeds 50 % of the respective resistance of the connected member and the design shear force at the

beam ends shall include the effects from the expected yield strength in tension of the webs.


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7. Lateral bracing of the HBE shall be provided to both flanges of the I-section as required for S-MRFs.

8. The VBE (i.e. columns) should have moment of inertia about the axis perpendicular of web planenot smaller than min,cI , derived by the formula given in clause 17.4g of [AISC Seismic, 2005].

9.

The column splices shall be located as close as possible to one-fourth of the storey height above thefloor, and comply with the requirements for members that are part of the seismic-load-resistive

system.

The design and detailing rules of [CSA, 2001] are more stringent for the ductile steel plate walls and more

relaxed for the low-ductility ones. For brevity, they will not be discussed herein. Direct comparison with the

US provisions is not straightforward due to the different formats of Limit state design. It must be noted,

however, that the latest US provisions [AISC Seismic, 2005] reflect entirely all essential findings of the

Canadian researchers who have pioneered the studies and practical implementation of this structural system.

CONCLUSIONS

1. SPSWs and CSSWs have undergone many tests and analytical studies, and most of them provedtheir significant ductility and robustness.

2. The composite steel plate shear walls are a feasible alternative to the pure steel shear walls becausethe concrete encasement provides adequate stiffening and fire protection to the steel plate. In

general, the AISC Seismic Provisions and EC8 have almost identical design and detailing rules for

this specific type of shear wall systems. However, the EC8 clauses are more general and difficult for

the design engineer to follow. Some important issues such as the stiffness of the composite shear

wall system to be used in the global static and dynamic analyses are not addressed in detail.

3. The design rules for SPSWs contained in [AISC Seismic, 2005] and [CSA, 2001] are also similar.They follow the tension-field concept applicable to the thin unstiffened steel infills. The boundary

members (beams and columns) and their connections shall be capacity-designed to carry the gravity

loads and provide adequate end anchorage to the steel web yielding in shear in its post-bucklingstage. The authors do not have any logical explanation why Eurocode 8 [CEN, 2004] has no

provisions for this new and promising seismic force resistive system.

REFERRENCES

1. American Institute of Steel Construction. (2005), SeismicProvisions for Stuctural Steel Buildings.2. Astaneh-Asl, Ab. (2001), Seismic Behaviour and Design of Steel Shear Walls, Steel Tips.3. Astaneh-Asl, Ab. (2002), Seismic Behaviour and Design of Composite Steel Plate Shear Walls, Steel Tips.4. Berman J., and M. Bruneau. (2003), Plastic Analysis and Design of Steel Plate Shear Walls, Journal of Structural

Engineering, ASCE, vol. 129, No. 11.

5. Canadian Standards Association (2001), CAN/CSA S16-01. Limit State Design of Steel Structures.6. CEN (2004), EN 19981. Eurocode 8: Design of structures for earthquake resistance . Part 1: General rules, seismicactions and rules for buildings.7. Driver R., Kulak G., Kennedy, D, and A. Elwi. (1997), Seismic Behaviour of Steel Plate Shear Walls, Structural

Engineering Report 215, Univ. of Alberta, Alberta, Canada.

8. FEMA (2003), NEHRP Recommended Provisions For Seismic Regulations For New Buildings and OtherStructures, FEMA 450.

9. Kulak G., Kennedy D., Driver R., and M. Medhekar. (2001), Steel Plate Shear Walls An Overview, Engineering Journal,First Quarter.

10. Rezai M., Ventura C.,and H.Prion. (2000), Numerical Investigation Of Thin Unstiffened Steel Plate Shear Walls,Proceedings, 12WCEE, NZ.

11. Sabouri-Ghomi S., Ventura C., and M. Kharrazi. (2005), Shear Analysis and Design of Ductile Steel Plate Walls, Journalof Structural Engineering, ASCE, vol. 131, No. 6.

12. Seilie I., and J. Hooper, (2005), Steel Plate Shear Walls: Practical Design and Construction,Modern Steel Construction,April.

13. Thorburn, L.J., Kulak, G.L., and C.J. Montgomery (1983), Analysis of Steel Plate Shear Walls,Structural EngineeringReport 107, Univ. of Alberta, Alberta, Canada.

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