Progressive collapse mechanism of large-span structures and an innovative collapse-resisting countermeasure

Transcription

1 Progressive collapse mechanism of large-span structures and an innovative collapse-resisting countermeasure State Key Laboratory of Disaster Reduction in Civil Engineering ZHAO,Xianzhong YAN,Shen Tongji University

2 Progressive collapse of building structures Research on frame structures Failure Research Design method Physical test Numerical analysis Theory Failure mechanism Catenary action

3 Progressive collapse of large-span structures Lessons of history June 4, 1979 Collapse of Kemper Arena in Kansas City Analysis demonstrated that failure of just one of four bolts per hanger resulted in a large prying load on the three remaining bolts. Once they failed, the hanger shed its load, overloading adjacent hangers. August 1, 27 Collapse of I-35W Mississippi River bridge Gusset plates U1 at four locations on the main trusses were under-designed originally and had already developed edge buckling during or prior to 23 due to addition of dead load of 5 cm wearing surface and curbs resulted in tragic and brittle progressive collapse of I-35W

4 Progressive collapse of large-span structures Overview of our research Planar system Planar trusses Spatial system Single-layer latticed domes

5 Progressive collapse of planar trusses Experimental investigation Three truss models were tested with one diagonal member suddenly removed Test setup truss A: welded joints truss B: diagonals pin-connected to chords truss C: diagonals rigid-connected to chords

6 Progressive collapse of planar trusses Numerical investigation Improved Nonlinear Dynamic Analysis Procedure Micro-based fracture model of steel material and steel members R Modeling member fracture in dome test

7 Catenary action Progressive collapse of planar trusses Collapse-resisting mechanism Arch action

8 Initial failure Collapse-resisting countermeasure An innovative idea Collapse? Unbalanced force! Key element R Countermeasure A: enhance structural members? R Countermeasure B: release unbalanced force? Barrier lake Landside dam

9 Strain ( ) BC1 BC2 BC3 BC4 BC BC1_PJ BC2_PJ BC3_PJ BC4_PJ BC5_PJ BC3 others Collapse-resisting countermeasure Concepts of a new joint In tested truss developing catenary action along the bottom chord R Unbalanced forces exist between neighboring bottom chord members R BC3 has the largest tensile force (key element) and is likely to fail due to excessive tension How to release the unbalanced forces in the bottom chord? The concept of the new joint involves the following attributes: R Under design loads, the joint behaves the same as a commonly used truss joint R When catenary action is being developed in the bottom chord under a progressive collapse scenario, the mechanism of the joint is activated to realize a uniform tension in the bottom chord members R A pinned connection is adopted between diagonal members and the bottom chord to prevent early buckling of the diagonal members

10 Pinned slidable joint (PS joint) Prototype design and working mechanism Under design loads, the friction generated by the preloaded bolts and the shear resistance provided by the locking rod are combined to provide the sliding resistance. When catenary action is developed, if the unbalanced force of a joint exceeds the sliding resistance, the locking rod breaks and the steel blocks can slide along the bottom chord to largely release the unbalanced force. The ear plate - pin - lug plate design

11 Two modes, two design schemes Pinned slidable joint (PS joint) Control of sliding resistance R The locking rod lags behind the preloaded bolts in providing sliding resistance R The kinetic friction is smaller than the maximum static friction R strong bolts, weak rod - if the design sliding resistance of a PS joint (R) can be fully achieved by the static friction generated by the preloaded bolts FS R n P R SL S1 FS F K S S 1 S K L R weak bolts, strong rod otherwise, if the number or the size of the preloaded bolts is insufficient FS R S S R F L 2 K S R n P L K When immobilized, sliding resistance is provided by static friction. If the static friction is overcome by unbalanced force, there is a drop of sliding resistance R

12 truss-psj R The geometric and material properties was kept the same as that of truss-pj with non-slidable pinned connector Test on truss with PS joints Test model and set up truss-pj Test setup R Side supporting using plexiglass plates R Static loading on top joints R DM2 removal in.6 s R 3-D dynamic displacement using videogrammetric technique (18 Hz) R Dynamic strain using dynamic strain data acquisition system (1 Hz)

13 Force (kn) Sliding resistance design R Based on the test results of truss-pj. R BJ1 & 4-7.5kN; BJ2 & 3 5. kn SR IF Force (kn) SR IF Force (kn) Test on truss with PS joints Sliding resistance BJ1 BJ2 BJ3 BJ4 SR IF Force (kn) SR IF Sliding resistance implementation R BJ1 & 4: R Bolts Φ6: R Rod Φ2.5: R BJ2 & 3: R Bolts Φ6: 7.5kN P 1 P kN 4.3 S Wrench torque: T K P d kN 6mm = 2.39 kn mm L = 4.9mm 256MPa = 1.26kN 1 7.5kN 3.75kN Wrench torque: T 1.33kN mm R Rod Φ2.5: S = 1.26kN 2.5kN L

14 Joint BJ2 slid along the bottom chord for more than 14 mm Test on truss with PS joints Test results R Sliding mechanism of BJ2 is activated at.24 s, releasing the unbalanced force R Other joints stay immobilized 8 4 sliding starts at.24s sliding resistance 8 4 sliding resistance for BJ1&4 sliding resistance for BJ2&3 Force (kn) -4-8 truss-pj truss-psj BJ Force (kn) -4-8 BJ1,3,4 BJ1 BJ3 BJ

15 A more robust development of the catenary action in the bottom chord 13 Strain ( ) Strain ( ) BC1 BC2 BC3 BC4 BC Strain ( ) BC1_PJ BC2_PJ BC3_PJ BC4_PJ BC5_PJ Bottom chord DM3 DM4 (min) DM6 (max) DM7 DM9 Diagonal member Test on truss with PS joints Test results PS joints facilitate the adaptation of the remaining structure towards a new balanced state with a near-optimal deformation shape R truss-psj behaved more like a cable-strut structure than a typical truss R the internal forces in almost all members are smaller than that of truss-pj Strain ( ) Top chord TC1 TC2 TC3 TC

16 Multi-scale model with complicated details Numerical investigation FE modelling considerations R Beam elements, shell elements, brick elements, discrete rigid bodies. R Contact and impact R Assembly loads modeling pretension of bolts can not be employed in association with the explicit solver Abaqus/Explicit. The static friction generated by the four preloaded bolts are modeled by a stiff shear stud providing an equivalent shear resistance.

17 The FE model is sufficiently accurate in representing the sliding behavior of the PS joints, the dynamic responses of the remaining structure and the collapseresisting mechanism of the tested truss Numerical investigation FE results and model validation Displacement ( ) test data FE results Force (kn) sliding starts at.21s sliding resistance test data FE results A method for determining the design sliding resistance for the PS joints to be used under all progressive collapse scenarios

18 Design of sliding resistance Multiple collapse scenarios This method is based on considerations: R No sliding should occur in the intact truss under design loads lower limit of sliding resistance under design loads R Also need to avoid setting the sliding resistance unnecessarily too high upper limit of sliding resistance under catenary action R PS joints may not be needed for all bottom joints under design loads vs. under catenary action R PS joints keep immobilized under arch action following bottom chord member loss lower limit of sliding resistance under arch action

19 A five-step procedure Design of sliding resistance Multiple collapse scenarios 1 Calculate the sliding resistance demand under design loads R Linear static analysis 2 Calculate the sliding threshold under different catenary action scenarios R Alternate Path (AP) analysis, removing the top chord members one at a time R Nonlinear static analysis 3 Determine the usage of PS joints 4 Calculate sliding resistance demand under arch action for joints where PS joints are used R AP analysis, removing bottom chord members one at a time 5 determine the design sliding resistance R R =1.1 max F, F BC BJm BJm BJm

20 Design of sliding resistance Example PS joints in truss-psj are re-designed to facilitate a more general application of the PS joints under all member loss scenarios Step 1 Step 2 Step 3 Step 4 Step 5

21 Numerical investigation Different member-loss scenarios Removal of a diagonal member DM2 Force (kn) R Different sliding resistance compared with the test programme R Sliding start times is different, but the sliding distance and the associated rebalanced state are not affected. R It is reasonable to design the sliding resistance just above the minimum sliding resistance demand to ensure timely function of the sliding mechanism of the PS joints BJ2 & 3 sliding starts at.16s sliding resistance BJ2(truss-PSJ-new) BJ3(truss-PSJ-new) BJ2(truss-PSJ) Displacement ( ) truss-psj-new TJ3 truss-psj

22 Numerical investigation Different member-loss scenarios Removal of a top chord member TC1 R The sliding start time, sliding distance and the final re-balanced state are all very close to the DM2 removal case R PS joints play a similar role in maximizing the function of the catenary action as they do under the diagonal member loss scenario 4 sliding starts at.17 s sliding resistance 2 Force (kn) -2-4 BJ2 & 3 BJ2 BJ

23 Numerical investigation Different member-loss scenarios Removal of a bottom chord member BC3 R No PS joints have slid, thus the PS joints do not introduce any detrimental effect on arch action. 4 2 sliding resistance Force (kn)

24 Summary Collapse-resisting of large-span structures PS joints provide proactive resistance for truss structures Thank you for your attention! Progressive collapse mechanism of large-span structures and an innovative collapse-resisting countermeasure

25 Test setup Progressive collapse of single-layer domes Experimental investigation dome A: loaded up to 4% of its ultimate loading-carrying ability dome A: loaded up to 75% of its ultimate loading-carrying ability 注荷坏稳

26 Progressive collapse of single-layer domes Numerical investigation Algorithm for fracture simulation of beam elements R Based on local stress states of beam elements under different loading conditions R Multi-scale calculation from micro-voids to beam elements Accurate simulation of the fracture process in the test programme