10NCEE

Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska

SEISMIC DEFORMATION DEMANDS ON SHEAR WALLS IN FRAME-WALL SYSTEMS İ. Kazaz1 ABSTRACT In this study a detailed parametric study was conducted to investigate the seismic deformation demands in terms of drift ratio, plastic base rotation and compressive strain in frame-wall systems. The level of frame-wall interaction was varied by changing the wall index in a generic frame-wall system and its relation with the seismic demand at the base of the wall was investigated. The wall indexes of analyzed models were in the range of 0.2-2% interval. The seismic behavior frame-wall models were determined using nonlinear time-history analyses. Analyses results revealed that the increased wall index led to significant reduction in the top and inter-story displacement demands especially for 4-story models. The calculated average roof drift decreased from 1.5% to 0.5% for 4-story model. The average drift ratio in 8- and 12-story models has changed from approximately 1.1% to 0.75%. However, the main effect of increased wall area was observed on the dispersion of the deformation demands. As the wall amount in the system increased, the dispersion in the calculated roof drift due to ground motion uncertainty decreased considerably. When walls were assessed according to plastic rotation limits defined in FEMA 356, it is seen that the walls in frame-wall systems with low wall index could seldom survive the design earthquake without major damage. Concrete compressive strains in low wall index frame-wall structures were much higher than the limit allowed for design (εc = 0.0035). Above the wall index value of 0.75% nearly all walls assure at least life safety (LS) and immediate occupancy (IO) performance states. It is proposed that in the design of dual systems where frames and walls are connected by link beams, the minimum value of wall index should be 0.5%, in order to prevent excessive damage to wall members.

1

Associate Professor, Dept. of Civil Engineering, Erzurum Technical University, Erzurum, Turkey.

Kazaz İ. Seismic deformation demands on shear walls in frame-wall. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

Seismic deformation demands on shear walls in frame-wall systems İ. Kazaz1

ABSTRACT In this study a detailed parametric study was conducted to investigate the seismic deformation demands in terms of drift ratio, plastic base rotation and compressive strain in frame-wall systems. The level of frame-wall interaction was varied by changing the wall index in a generic frame-wall system and its relation with the seismic demand at the base of the wall was investigated. The wall indexes of analyzed models were in the range of 0.2-2% interval. The seismic behavior frame-wall models were determined using nonlinear time-history analyses. Analyses results revealed that the increased wall index led to significant reduction in the top and inter-story displacement demands especially for 4-story models. The calculated average roof drift decreased from 1.5% to 0.5% for 4-story model. The average drift ratio in 8- and 12-story models has changed from approximately 1.1% to 0.75%. However, the main effect of increased wall area was observed on the dispersion of the deformation demands. As the wall amount in the system increased, the dispersion in the calculated roof drift due to ground motion uncertainty decreased considerably. When walls were assessed according to plastic rotation limits defined in FEMA 356, it is seen that the walls in frame-wall systems with low wall index could seldom survive the design earthquake without major damage. Concrete compressive strains in low wall index frame-wall structures were much higher than the limit allowed for design, εc = 0.0035. Above the wall index value of 0.75% nearly all walls assure at least life safety (LS) and immediate occupancy (IO) performance states. It is proposed that in the design of dual systems where frames and walls are connected by link beams, the minimum value of wall index should be 0.5%, in order to prevent excessive damage to wall members.

Introduction Low-to-midrise reinforced concrete moment resisting frame buildings compose large portion of the building inventory in Turkey. After the M7.4 Kocaeli Earthquake use of shear walls in combination with reinforced concrete moment resisting frames has gained a boost. These buildings are mostly constructed with 4-12 stories in regions of high seismicity and used for residential accommodation. Dual systems are preferred in earthquake resistant design, because the interaction of these two distinct structural forms enables the control of drifts in the building due to their inherent characteristics. The interaction between the frames and walls of structures with link beams is more significant than in the classical form of frame-wall structure in which the frames are parallel to the walls, i.e. walls are located inside a frame [1]. The shear and moment transferred from beams can significantly change the moment profile causing a reduction in the inflection height of the 1

Associate Professor, Dept. of Civil Engineering, Erzurum Technical University, Erzurum, Türkiye

Kazaz İ. Seismic deformation demands on shear walls in frame-wall systems. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

wall. Frame wall-interaction poses a serious problem for reinforced concrete structural walls especially in situations where the frame part of the structural system becomes stiffer as compared to the walls. Kayal [2] investigated the effect of wall-column stiffness ratio, which is defined as the ratio of the flexural rigidities of the shear wall and the column (EIw/EIc), among with additional parameters such as the ratio of beam and column stiffness, load ratio (lateral to vertical load). One of the significant conclusions emerged from this study is that nonlinear idealization of the flexural characteristics of shear walls became more pronounced when shear walls are located in stiff frames since the actual characteristic of shear walls, which is “shear behavior” has to be activated. Although walls are stiff elements and has a limiting effect on the deformations when elastic action is considered, in the inelastic range after plastic hinge formation at the base of the wall, system behavior changes completely to lead to significant deformation demands. This may have profound effects on the behavior of dual systems [3]. Because of the rigid-body displacement of such walls, rotations along the height of the wall, of the same order as that at the foundation, will be introduced at every level. In this study a detailed parametric study was conducted to investigate the seismic deformation demands in terms of drift ratio, plastic base rotation and compressive strain at the base of walls in frame-wall systems. Parameters of Investigation and Design of Walls The elements of analytical framework is composed of development of simple lumped-parameter structural models of frame-wall systems for NLFEA, determination of the parameters that affect the shear wall response and identification of procedures to include these effects in the analyses. The parameters affecting the wall response are not only related to characteristics wall properties like length (Lw), height (Hw), axial load ratio (P/Po), but also 3D structural interaction effects arising from differences between the dominant deformation modes of walls and frames. The general plan configuration shown in Fig. 1(a) was used for all the frame-wall structures in the parametric investigation. The structure is composed of nine 3-bay frames in the transverse direction and three 8-bay frames in the longitudinal direction. By increasing the number of walls allocated into the central bay in the transverse direction different frame-wall arrangements were obtained leading to different wall indexes. Later using the stiffness properties of these buildings single wall-equivalent frame models that depend mainly on a specific wall length and a wall index were developed. The purpose is to reduce the size of the finite element models and to create a workable framework for the parametric investigation. Shear walls of lengths 3, 5 and 8 meters were used. The building heights that were considered to determine the aspect ratio of the shear walls consists of 4, 8 and 12 story structures. The interstory height was considered to be constant along the height of the building as 3 m. Assuming the dimensions of the columns are 0.6x0.6 m and the beams are 0.6x0.4 m robust beam and column elements were used to assure that the desired frame-wall interaction develops effectively. It was considered that the composed frame-wall systems have wall indexes ranging from 0.002 to 0.02. A design procedure that depends on shear-flexure beam continuum formulation of framewall structures [4] was used to quantify the seismic shear force distribution among the wall and frame components of the generic frame-wall models and to calculate the amount of flexural reinforcement at the boundary elements of the walls. The formula for the lateral deflection of the frame-wall structure (combined shear-flexure beam) reads as

d4y dx 4

−α 2

d2y dx 2

=

w( x ) EI

where

α2 =

GA + η EI

(1)

In these equations GA is the equivalent story shear rigidity of the frame component and EI is the flexural rigidity of shear wall members [5], η holds for the equivalent flexural rigidity of beams framing to wall [4]. This term especially increases the accuracy of the calculations in cases where the transverse beams have substantial flexural capacity so increase the flexural resistance of the wall by the reactions from transverse beams. The main parameter that determines the dominant deformation mode of a shear-flexure beam is the parameter αH. The deflected shape of structures composed of structural walls as lateral load resisting system can be approximated by using values of αH between 0 and 2. The deflected shape of dual systems or braced systems can be calculated by values of αH typically between 1.5 and 6. For buildings composed of only moment-resisting frames the values of αH between 5 and 20 can be used. (a) Bare Frame

(b) FW1

6m

6m

lw

lw

6m

6m 8@6 m = 48 m

8@6 m = 48 m

Figure 1. Plan view of frame-wall configurations Dependent on the plan shown in Fig. 1(a) and using different configurations of shear walls, building height and wall length various dual structures, where frames and walls interacting at different levels, were obtained. The behavior of each of these structures was characterized with αH parameter. Then the entire multibay-multistory frame-wall structure is reduced to a representative single wall and equivalent frame structure as shown in Fig. 2(a) using the procedure defined below. The stages of model construction can be defined as follows 1. For a particular system the behavior factor (αH), wall length (Lw) and wall index (p) and the number of stories (N) are known. 2. The fundamental period of structure (T1) is determined by Eq. 2 which is developed for frame-wall buildings [6]. T1 = 0.00406

H N Lw

1

(

p 1.8752 + (α H )

2

)

(2)

3. For an assumed or given wall index (p) the floor area per wall is calculated as A f = pL w t w for predetermined wall dimensions. In this study a constant wall thickness (tw) that is equal to 0.25 m is adopted for all the rectangular walls. 4. The story masses are calculated on the basis of ms = Af .(1 t/m2).

5. The static equivalent triangular lateral load pattern specified in the Turkish Seismic Code [7] is used in the loading of the continuum model. Using the knowledge produced on dynamic and static characteristic of frame-wall structures, variations in stiffness and yielding strength along the height of the single wall-equivalent frame structure models are calculated. Total equivalent seismic load (base shear) acting on the entire frame-wall structure is obtained by

Vt =

W A(T1 ) R

(3)

where W is the total weight of the structure and R is the seismic load reduction factor (structural behavior factor) which is taken as 6. Spectral acceleration coefficient, A(T1), is calculated according to spectral shape given in Fig. 3 and assuming local site class Z3 (firm soil) in seismic zone 1 (Effective ground acceleration coefficient, Ao = 0.4). 6. Comprehensive descriptions, assumptions, derivations and case studies of the continuum formulation can be found in Kazaz and Gülkan [4], so only the final expressions for the bending moment and shear force at the base of wall component under triangular distribution of lateral loads are given below. These equations facilitate the design of models used in the parametric study here. The bending moment at the base of the wall can be calculated using the following expression: Vt sinh(α H 1 ) M Bo =

α

−

Vt hcc4 3 EI c f

w ⎡ α hcc sinh(α H 1 ) ⎤ + cosh(α H 1 ) ⎥ − 3 1 [α hcc cosh(α H 1 ) + sinh(α H 1 ) − α H ] ⎢ 2 ⎣ ⎦ α H 3 ⎛ ⎞ ⎛ hcc hcc3 ⎞ − + − 1 α sinh( α ) 1 h H ⎜ ⎟ cc ⎜ ⎟ cosh(α H 1 ) 1 4 EI w f ⎠ 2 EI w f ⎠ ⎝ ⎝

(4)

in which Vt = total seismic base shear, hcc= contra-flexure height on the columns at the base story taken as 70 percent of the story height, H = total building height, H1 = H-hcc, EIw = total flexural rigidity of walls and the flexibility factor for the base story f is calculated as f =

hcc3 h3 h − cc − cc 6 EI w 3 EI c GAw

(5)

In this equation EIc is the total flexural rigidity of all columns in the base story, GAw is the total shear rigidity of the walls. The amplitude of triangularly distributed lateral load is w1 = 2VH (H2 − hcc2 ) . The base shear force on the wall component can be calculated using t Vw = M Bo

hcc2 h3 − Vt cc 2 fEI w 3 fEI c

(6)

When multiple shear walls exist in the system the calculated wall component bending moment and shear force are distributed according to flexural rigidities of each wall. In the calculation of member forces and moments cracked section stiffness is used. 7. Considering the moment demands that arise during the dynamic response, a linear bending moment envelope is used in design. Additionally from the base of wall in a region that

has a height equal to H cr = max [ Lw , H w 6 ] but not greater than 2Lw a constant moment distribution is assumed considering the tension shift effect [7]. 8. It is assumed that the length of boundary elements is 0.2Lw at the edges and flexural reinforcement is distributed uniformly in the boundary element. The percentage of vertical web reinforcement is a constant value equal to minimum value 0.0025 and concrete compressive strength is 25 MPa. In the boundary element the minimum amount of longitudinal reinforcement ratio is taken as 0.01. 9. Elastic design analysis under lateral loads have demonstrated that nearly entire lateral load resistance of frame-wall buildings is provided by the structural walls as the wall index increases in the system. Although design elastic analysis results point to negligible effect of frame elements in seismic resistance, in reality there usually exist a considerable strength in frame elements due to minimum amount of longitudinal reinforcement requirement in a column (ρb)min = 0.01. So the actual force reduction factor (R) in the system may turn out to be smaller than the one intended initially in the design of frame-wall systems. Modelling of Single Wall-Equivalent Frame System For nonlinear analysis a discrete nonlinear type of simplified frame-wall model as presented in Fig. 2(b) was developed for the finite element analysis. The model was generated in SeismoStruct [8]. 3D displacement based beam-column elements capable of modeling members of space frames with geometric and material nonlinearities were used to model shear walls. The sectional stress-strain state of beam-column elements was obtained through the integration of the nonlinear uni-axial material response of the individual fibers into which the section has been subdivided, accounting for the spread of inelasticity along the member length and across the section depth. The frame component was modeled as combination of rigid frame elements interconnected with shear links. The wall and frame components connected with rigid links and a rotational spring was introduced to account for the link beam moments. The story distortion angle (story drift)–shear force relation for the link elements was idealized with the Takeda hysteresis model.

(a) Figure 2.

(b)

a) Single wall-equivalent frame, b) Finite element model of the generic single wallequivalent frame-model

Aschheim [9] states that the roof yields drift range between 0.5 and 0.6 percent regardless of the number of stories. The yield story distortion (γy) was assumed to be 0.55 percent. The yield curvature of link beams was calculated by the relation φy =1.7εy/hb, where εy is the yield strain of the reinforcement and hb is the depth of the beam. For the materials and section geometries adopted in this study (εy = 0.0021 and hb= 0.6 m) the beam curvature at yield takes the value of 0.006 rad/m, which also agrees with the moment-curvature analyses of typical beam sections. The Takeda hysteresis model characterizes the behavior of rotational springs used to model link beams. The yield strength of frames was distributed over the stories so that all would yield simultaneously under the static design earthquake forces. Seismic Input In the selection of acceleration time-series to be used as input to dynamic analyses, spectral matching technique was used. It was assumed that the ground motion spectra match the elastic response spectrum of TSC [7] that is defined on firm soil site class (Z3) with PGA of 0.4 g for an earthquake that has 10 percent probability of being exceeded in 50 years. The search is based on the average root-mean-square deviation of the observed spectrum from the target design spectrum. Even when the ground motions satisfactorily match the target spectral shape, their intensity may vary significantly. A scale factor defined as the average of the ratio of spectral ordinates of target and matching spectra at periods 0.1s, 0.4s and 0.85s was introduced. So, 215 ground motions compiled by Kazaz [10] were screened for finding the spectrum compatible traces. 10 records obeying the given limitations were selected and used as seismic input for response analyses. The acceleration response spectra of these scaled ground motions are presented in Fig. 3. In Table 1 peak ground values of unscaled strong motions records are given together with applied scale factors. Table 1. No 1 2 3 4 5 6 7 8 9 10

Earthquake Imperial Valley Kocaeli Northridge Northridge Whittier Narrows Cape Mendocino Northridge Northridge Loma Prieta Northridge

Year 1979 1999 1994 1994 1987 1992 1994 1994 1989 1994

Catalog data of the selected ground motions Mw 6.5 7.4 6.7 6.7 6.1 7.1 6.7 6.7 6.9 6.7

Station Keystone Rd., El Centro Array #2 Duzce Los Angeles, Brentwood V.A. Ho. Pacoima-Kagel Canyon 7420 Jaboneria, Bell Gardens 89324 Rio Dell Overpass - FF 24389 LA - Century City CC North 24283 Moorpark - Fire Sta. Hollister Differential Array LA - Fletcher Dr.

Rd (km) 16.2 17.1 23.1 10.6 16.4 18.5 25.7 28 25.8 29.5

PGA PGV (cm/s2) (cm/s) 309 32.7 308 50.7 182 24.0 424 50.9 216 28.0 378 43.9 218 25.2 189 20.2 274 35.6 235 26.2

SFSpec 1.77 1.52 2.69 1.20 1.98 1.43 2.11 2.56 1.47 2.00

Results of Analyses The seismic demand characterized by the code spectrum compatible ground motions were applied on a set of generic frame-wall buildings that represents broad range of frame-wall combinations of relative strength. Equivalent frame-single wall models that cover a wall index range of 0.002 to 0.02 were used in the analyses. The purpose of the analyses is to investigate the

effect of wall amount (represented by wall index, p) on the deformation demands. The performance of wall elements is evaluated by comparing the base rotations and strains with respect to code specified limits. The response parameters under investigation are plotted in Figs. 4 to 6. These parameters include maximum roof drift ratio, average base rotation maximum compressive strain at the extreme fiber of the base section. The response parameters are mainly plotted as a function of wall index to see the effect of wall amount on these parameters. 2.0 Mean Spectrum TSC 2007

SA (g)

1.5

Ao=0.4 S(T)=1+1.5T/TA S(T)=2.5

T