brb防屈曲支撑相关论文global restraint in ultra-lightweight buckling-restrained braces_W

1、Global Restraint in Ultra-Lightweight Buckling-Restrained BracesPeter Dusicka, P.E., M.ASCE1; and John Tinker, P.E., S.E., M.ASCE2Abstract: A concept for an ultra-lightweight buckling-restrained brace was conceived, and a prototype was designed that utilized an aluminum core and bundled glass fiber-

2、reinforced polymer pultruded tubes for the buckling restraint. Prediction of global stability in compression was made using analytical methods based on single-degree-of-freedom (SDOF) and previously established Euler buckling models. Detailed finite-element simulations of the proposed prototypes uti

3、lized a constitutive model calibrated from experimentally obtained reversed cyclic coupon testing of 6061-T6511 aluminum alloy at 24% total strain amplitude. Analytical formulations were compared with monotonic and cyclic numerical results from a parametric study varying restrainer stiffness, end mo

4、ments induced by frame drift, and core reduced section length. The study concluded that SDOF and Euler formulations may underestimate the required restrainer stiffness by a factor of two or greater. The resulting ultra-lightweight brace prototypes satisfying global buckling restraint were calculated

5、 to weigh 27 and 41% of traditional mortar-filled tube and all-steel buckling-restrained brace (BRB) configurations, respectively. DOI: 10.1061/ (ASCE)CC.1943-5614.0000320. 2013 American Society of Civil Engineers.CE Database subject headings: Aluminum (material); Bracing; Buckling; Ductility; Fiber

6、 reinforced polymer; Hysteresis; Seismic design.Author keywords: Aluminum; Bracing; Buckling; Ductility; Fiber reinforced polymer; Hysteresis; Seismic design.J. Compos. Constr. 2013.17:139-150.IntroductionSmall- to medium-size concrete or steel buildings constructed according to deficient legacy cod

7、es constitute a large portion of existing backlog of structures requiring seismic retrofit. A number of retrofit solutions are available to address these deficient struc- tures. However, many solutions impose great difficulties for material handling and installation of traditional lateral elements,

8、such as shear walls or conventional steel braces, because of limited access for heavy lifting equipment such as cranes and forklifts. In such cases, an ultra-lightweight lateral bracing system could allow for easier manual transport, lifting, erection, and connection of required components to the ex

9、isting structure without significant equipment mobilization and deconstruction of existing walls for access. By minimizing disruption to building occupants, the build- ing may remain partially viable during construction, resulting in decreased owner cost and thus increasing the feasibility of electi

10、ve upgrades. The proposed system combines the technologies of con- ventional buckling-restrained braces (BRB), aluminum seismic dissipaters, and fiber-reinforced polymers (FRP) to accomplish the goals of decreased installation weight, system compactness, and efficient energy dissipation. All of thes

11、e technologies are relatively new additions to the building engineering toolkit, and1Associate Professor, Dept. of Civil and Environmental Engineering, Portland State Univ., P.O. Box 751, Portland, OR 97207 (corresponding author). E-mail: 2Project Structural Engineer, AECOM Technology

12、Corporation,800 LaSalle Ave., Suite 500, Minneapolis, MN 55402.Note. This manuscript was submitted on March 22, 2012; approved on July 23, 2012; published online on August 7, 2012. Discussion period open until July 1, 2013; separate discussions must be submitted for individual papers. This paper is

13、part of the Journal of Composites for Construction, Vol. 17, No. 1, February 1, 2013. ASCE, ISSN 1090-0268/2013/1-139- 150/$25.00.exploration into their potential for seismic applications is being pursued by numerous researchers.Attempts to refine metallic seismic dissipaters originally pro- posed b

14、y Skinner et al. (1975) have recently strayed from the traditional steel core and mortar-filled steel tube restrainer BRBs developed throughout the 1980s and 1990s (Watanabe et al. 1988; Wada et al. 1989; Watanabe and Nakamura 1992; Black et al. 2002, 2004). Many variations have been presented (Xie

15、2005; Palazzo et al. 2009), but those termed lightweight and con- structed of bolted or welded all-steel components for both the core and restrainer are the most numerous (Mazzolani et al. 2004, 2009; Tremblay et al. 2006; Usami et al. 2008; DAniello et al. 2008, 2009; Chao and Chen 2009; Ju et al.

16、2009). Competing concepts have been characterized as beneficial because of decreased instal- lation cost, having replaceable cores, having the ability to use low-skilled labor for installation, being compact for installation in confined spaces, and use in existing building retrofits.Aluminum as an i

17、ndustrial material has been around for more than a century, but its incorporation into the primary structural elements of buildings has been relatively slow, with use primarily constrained to secondary systems such as curtain walls, and aux- iliary structures such as awnings or canopies. However, at

18、tempts to utilize its potential ductility and absence of cyclic hardening in seismic force dissipating systems have begun to appear. Shape memory braces constructed with superelastic aluminum alloys that allow a structure to recenter after a seismic event with little perma- nent deformation (Mazzola

19、ni et al. 2004), replaceable shear links constructed of low-yield point aluminum installed in concentri- cally-braced frames or special truss moment frames (Rai and Wallace 2000), and replaceable aluminum plate shear panels (Rai 2002; Mazzolani et al. 2004; Brando et al. 2009) have all been proposed

20、 and tested with moderate success.Fiber-reinforced polymer has successfully been used in structures since the 1970s, and has been commonly employed in applications bonded to concrete or steel members requiringDownloaded from by Hebei University of Engineering on 01/12/15. Copyright A

21、SCE. For personal use only; all rights reserved.JOURNAL OF COMPOSITES FOR CONSTRUCTION ASCE / JANUARY/FEBRUARY 2013 / 139strengthening or repair (Zhao and Zhang 2007). Although, more pertinent applications recently have been developed that increase ductility of steel members. For instance, bonded un

22、idirectional sheets wrapped around special truss moment frame chord members enhanced cyclic response of plastic hinge behavior (Ekiz et al. 2004), FRP strips bonded to compression elements of flexural members (Accord and Earls 2006), webs of wide-flange Tee (WT) compression members (Harries et al. 2

23、009), and hollow structural section (HSS) columns (Shaat and Fam 2006, 2007, 2009) have also been reported to delay local buckling of elements subjected to compression. Applications in which the FRP is not bonded to the substrate are few, but are emerging as an effective method for delaying or precl

24、uding compression buck- ling. Pilot tests of a slender single steel angle retrofitted with a pultruded FRP square tube and wrapped with glass FRP (GFRP) fabric was experimentally loaded in cyclic push-pull and delayed buckling to 35% of the nominal tensile yield prior to connection failure (Dusicka

25、and Wiley 2008). Small-scale monotonic experi- ments and finite-element modeling of rectangular steel bars sur- rounded by PVC or mortar blocks and wrapped with carbon FRP (CFRP) fabric have achieved monotonic compression loads up to 153% of nominal tensile yield of the core (Ekiz and El-Tawil 2008)

26、. Experimental large-scale cyclic tests of pinned and semi- fixed end steel angles similarly wrapped with mortar blocks and CFRP fabric achieved compression loads that nearly achieved yield at 90% nominal (El-Tawil and Ekiz 2009).This paper contributes to the ongoing discussion of high- performance

27、BRBs by reporting on the development of a new ultra-lightweight BRB (ULWBRB) that was conceived for a typical model building using analytical models developed from establis- hed buckling theory, experimental cyclic coupon testing of a can- didate aluminum alloy to obtain a calibrated constitutive mo

28、del, and detailed finite-element simulations. The proposed ULWBRB utilizes readily available materials to allow customization of the core-restrainer configuration, as shown in Fig. 1(a), in which the core consists of structural grade aluminum extrusion (four equal legged angles shown), and the restr

29、ainer consists of pultruded GFRP tubes that are wrapped with GFRP fabric. The GFRP tubes have fibers oriented along the length of the brace as dictated by the pultrusion process, whereas the GFRP wrap has the primary fibers oriented transversely to bundle the tubes together. Between the cores are GF

30、RP spacers that are needed to allow for gusset plate end connections while suppressing local buckling of the reduced core. Given that the restraint materials have significantly lower composite modulus than conventional BRBs, this studyconcentrates on the global restraint requirements to show the via

31、bil- ity of this ULWBRB concept. Analytical models considered both a single degree of freedom (SDOF) and an established Euler buckling model to provide an initial required restrainer stiffness for a given axial design force and core length. Monotonic numerical simula- tions of the prototype brace we

32、re performed to examine the effect of restrainer stiffness with two different core reduced section lengths and varying degrees of applied end moment. Cyclic sim- ulations were used to assess if a predictable and reliable ductility and energy dissipation was possible with monotonically obtained restr

33、aint requirements.Brace Geometry and Estimate of Strain DemandsSeismic forces, story drift, axial displacement, frame geometry, and end connections were established within the context of retrofit of a model building layout based on the SAC 3-story office building located in Los Angeles, California (

34、FEMA 2000). The building consisted of 9.14-m (30-ft) square bays and measured 36.6 by54.9 m (120 by 180 ft) with a story height hi3.96 m (13 ft). Seismic design criteria were taken from an applicable building code as follows: Ss 2.15 g, Sds 1.43 g, R 7, and Cd 5.5 (ASCE 2010). Two adjacent buckling-

35、restrained braced frames (BRBF) in an inverted v-brace configuration were centered on each of the four perimeter column lines. An equivalent lateral force procedure with 5% minimum eccentricity was used to determine the seismic base shear and distribution to the individual stories and frames. A brac

36、e design force of Pu 1070 kN (241 k) at the first level was calculated using the assumption of equal tension and compression stiffness of the BRBs.Definition of the prototype brace geometry with a two-step core profile is shown in Fig. 1(b). An end-to-end core length Lb 4.83 m (190 in.) was generate

37、d using assumed W21 111 beams and W14 176 columns to remain consistent with previous literature reports on testing of full-scale BRBFs (Fahnestock et al. 2007). Selection of the brace reduced section length Lc was sub- sequently made by considering axial stiffness of the brace required to limit the

38、inelastic story drift to a maximum of 2.5% (ASCE 2010). Calculation of the elastic story drift ratio Die=hi for a non- prismatic core neglecting the contribution of the much stiffer beams and columns was previously cited by Tremblay et al. (2006) in Eq. (1), where Lc=Lb; A1=A3, Fy = core nominal spe

39、ci- fied yield strength; Ec = core Youngs modulus; and = brace angle with horizontal.Downloaded from by Hebei University of Engineering on 01/12/15. Copyright ASCE. For personal use only; all rights reserved.Aluminum core (4 angles shown)FRP fabric wrapContinuous FRP spacersBolted en

40、d connectionsFRP bundled tube restrainerEnd wrap reinforcing as required.Brace Length (Lb)Lc3Lc2FullInter.Sect.Sect.Reduced Section (Lc)Lc2Lc3Inter.FullSect. Sect.A1Transitions (Ltr)A2A3LsRestrainer Length (Lr)LsOverlap Length (Lo)Extension(a) (b)Fig. 1. Ultra-lightweight buckling-restrained brace l

41、ayout: (a) overall brace configuration; (b) core and restrainer geometry140 / JOURNAL OF COMPOSITES FOR CONSTRUCTION ASCE / JANUARY/FEBRUARY 2013Die Fy 1 1o Cd Dcos FyLb Lc3hiEcsin cos cLcieEcBy rearranging Eq. (1) algebraically to solve for , Eq. (2) isJ. Compos. Constr. 2013.17:139-150.given. Die=

42、hi0.45% was calculated by dividing the inelasticGlobal Restraint Requirements Using SDOFstory drift of 2.5% by the deflection amplification factor Cd. Using the variables 42, Ec 69.6 GPa (10,100 ksi), 0.9, Fy 241 MPa (35 ksi), and 0.456, 0.481 was calculated, which represents a 2.31-m (91.4-in.) lon

43、g reduced section. The final reduced section length was adjusted to 2.44 m (96 in.) to give a convenient 0.5 and is referred to as the Group B brace. Another geometry was created for the parametric study to examine higher expected axial strain and stiffness with Lc1.47 m (58 in.), or 0.3, which is r

44、eferred to as the Group A brace. Table 1 summarizes details of the two prototype brace dimensions.Analytical ModelTransverse displacement of the slender core member during buck- ling imparted flexural demand on the restrainer through application of a force with an unknown distribution function wx .

45、Effort to resist this displacement was modeled as a simple span restrainer beam pinned for formulation convenience at the end of length Lr. Force interaction between the core and the restrainer was then established using a SDOF mechanical model with axially inexten- sible truss members in which an a

46、ssumed plastic hinge existed at the midlength, as shown in Fig. 2(a). Flexural stiffness of the elastic 1Die Ec sin cos 1 hiFy2restrainer served to prevent transverse bifurcation of the core, there- fore increasing the critical buckling load, Pcr. The plastic hinge was justified by first considering

47、 the internal core moment by combin-Approximation of the average inelastic strain in the two-steping elastic column Eqs. (4) and (5) for a pinned-pinned column andsolving for the internal moment at midlength Mp at a given trans-core at maximum story drift is required to determine materialstrain dema

48、nd. Using Eq. (3) and the previously defined variables,verse displacement intt in Eq. (6). Tangent modulus theory was usedc 3.22 and 2.25% was calculated for the Group A and B braces at 2.5% story drift, respectively. These values fall within strain amplitudes reported for previous BRB tests of 12%

49、for longer core lengths and 35% for shorter core lengths (Tremblay et al. 2006).to account for core material nonlinearity by replacing Ec with Ect.Mint EcIcy 004xSelection of Lc should target an appropriate inelastic strain suitable for use with established cyclic properties of the core material and

50、 brace stiffness required to meet a target design story drift. Typically,yx t sin Lr5connection details, intermediate section overlap length, and axialpLr22M t EcIt c6shortening requires 0.5 as a practical limit. In the prototype, was maximized by extension of the unwrapped tubes a distanceint2Lrof

51、Ls to delay local buckling of the intermediate section while still allowing the full section to slide through the restrainer without impacting the wrap.The resisting moment, Mres, provided by the bundled tube restrainer was calculated from equilibrium on the column half- length shown in Fig. 2(a), r

52、esulting in Eq. (7), where Er andTable 1. ULWBRB Prototype DimensionsGroupLbm (in.)Lcm (in.)Lc2cm (in.)Lc3cm (in.)Lrm (in.)Locm (in.)Lscm (in.)Ltrcm (in.)A1cm2 (in:2 )A2cm2 (in:2)A3cm2A4.83 (190)1.47 (58)119 (47)48.3 (19)3.40 (134)78.7 (31)25.4 (10)15.2 (6)51.1 (7.92)78.7 (12.2)112 (17.4)B4.83 (190)

53、2.44 (96)71.1 (28)48.3 (19)3.40 (134)33.0 (13)25.4 (10)15.2 (6)51.1 (7.92)78.7 (12.2)112 (17.4)PLr/ 2Lr/ 2PksF/2F/2PLr/2 Lr/2PDtFBuckling Length (Lr)xRestrainer, P = 0PV(x)PM(x) y(x)V(x)Buckling Length (Lr)F/2Lr/ 2MresDtF/2Axially rigid connector barsSDOF(Lr /2)2 - D2)tSDOFEulerDownloaded from ascel

54、 by Hebei University of Engineering on 01/12/15. Copyright ASCE. For personal use only; all rights reserved.Core, P 0Restrainer, P = 0Core, P 0(a) (b)Fig. 2. Analytical model layout: (a) SDOF buckling model; (b) Euler buckling modelJOURNAL OF COMPOSITES FOR CONSTRUCTION ASCE / JANUARY/FEBR

55、UARY 2013 / 141intIr = Youngs modulus and moment of inertia of the restrainer, re-column of length Lr supported by an infinite number of axially spectively. Comparison of Mp to Mres showed approximately tworigid bars connected to the restrainer, also spanning length Lr.orders of magnitude difference

56、 at a common transverse displace-ment t. In this study, the modulus of elasticity for the pultruded composite tubes was taken as Er 19.3 GPa (2,800 ksi), and Ir was calculated from four 10.8 cm by 6.35 mm (4.25 in. by 1=4 in:) square tubes acting compositely. Tangent modulus Ectof the core was taken as 1% of Youngs modulus to account for strain hardening. The sharp transition between elastic and plastic behavior negates the need for an incremental approach accounting for material nonlinearity.sBy beginning with force e