LOADING SYSTEMS : Static Structural Testing

Transcription

1 LOADING SYSTEMS : Static Structural Testing Static Loading Systems Structural testing (static means slow loading) Test of components: Beams (bending) Columns (axial) Beam-columns (both) Devices (connectors etc ) Test of structures: Single loads Multiple loads Purpose of testing: Test set-up: Determine strength limits Determine flexibility/rigidity of structure Define and construct models Implement proper boundary conditions (Instrument model and references) Implement loading

2 Question #1: What types of loading need to be applied? Static time is not a factor (infinitely slow) Quasi-static very slowly applied loading in one direction (monotonic) Quasi-static - reversed cyclic loading Dynamic (random) loading shake table excitations

3 Component Testing: 1 beams (bending, shear, stc ) 2 columns (axial, bending, shear, P-Δ, creep, etc ) 3 special devices (dampers, isolators, etc ) Subassembly Testing: 1 beam-column-slab-joint cruciform 2 parts of frames Loading Conditions: 1 single loads, i.e. axial load for column buckling 2 multiple loads, i.e. P-Δ effect, axial and shear loads 3 base loads, i.e. earthquake simulations Purpose of Testing: 1 determine strength limits (crack, yield, ultimate, etc ) 2 determine flexibility/stiffness of structures 3 verify analytical tools 4 verify failure theories Experimental Setup Process 1 design and construct model 2 implement proper boundary conditions 3 instrument model 4 implement loading and reaction load requirements

4 Example setup: Calibrate bending moment load cells i.e. Typical loadcell Try: It is desired to put a constant moment over the load cell Now we can calibrate loadcells! Set gain (signal amplification) such that the output voltage corresponds to the applied moment.

5 Static Loading Devices 1 Universal Testing Machines Note that model specimens are attached to heads by special grips. Manufacturers include: Tinius Olsen (electrically driven) Instron (electrically driven) MTS-Uniaxial (hydraulic) MTS-Bi-Axial &Torsion (hydraulic) 2 Calibrated Mechanical Weights 3 Hydraulic Jacks (cylinders) requires power supply 4 Mechanical Jacks Open loop 5 Prestressing i.e. for axial loads to simulate gravity loads (Note: the prestressing changes directions with the spaecimen)

6 6 Lever for load multiplication: L +L L 1 2 N= F 2

7 Loading Arrangements Single point loads: -Vertical and lateral. i.e. Distributed loads: -Two equal loads -Two unequal loads

8 -Uniformly distributed loads - P/8 (typ.) -Triangular load distribution -

9 Load Measuring Devices 1) Calibrated weights with a balance 2) Pressure transducer F ~ P * A Area of piston = constant) i.e. 3) Mechanical transducer P ~ Δ (K = constant) 4) Stress-strain based transducer (subject of latter presentation)

10 Servo controlled devices for loading 1) Actuator: This is an open loop system that controls the linear or rotary motion of a load in response to an input command. (Usually operates based on a command without precise knowledge of the magnitude of the load. Load may vary up to 10% ; Open loop) Servosystem: Here feedback is used with a servoactuator to produce a closed loop system. Transducers - Servomotor - Modulator - provide feedback high level energy converter such as an electrical or hydraulic motor. provides a conversion of low power input command (for the servoactuator) or the error signal (for the servosystem) to a high power output that operates the servomotor

11 Hydraulic Servomotors (actuators) Hydraulic servomotors convert hydraulic power into mechanical motion. Types: Requirements: Linear motion Rotary motion Low fluid leakage Low friction Seals are critical Servo-system Components Type Modulator Servomotor Transducer Electromechnanical Amplifier Servomotor δ, v, T AC, DC Linear/ Rotary Driver Brushless servomotor Translator/ Stepper motor driver Electrohydraulic Servovalve Hydraulic motor δ, v, F, p, T cylinder Electropneumatic Servovalve Air motor, δ, v, F, p, T cylinder Where; δ = position, v = velocity, F = force, p = pressure, T = torque

12 Servo controlled Devices for Loading or Δy=y A -y D =error Block Diagram of Servo System Loop adjusts only for the difference of desired versus achieved value y A=H y D - Gy A y A=Hy D- HGy A y A 1 + HG = Hy D H y A= y 1 + HG D HG>>1 H is defined as the Plant Gain (transfer function) G is defined as the Loop Gain If G very large, then HG+1 => HG Y A = Y D / G. If we premultiply Y D by G then the Y A = Y D Gain G should be adjusted to obtain a linear rule

13 Servocontrol can be done using any mechanical variable Displacement (displacement control) Force (force control) Strain Differential Pressure (Velocity) (Acceleration) (etc ) Actuator needs to have o Servovalve (which converts electrical signal into differential pressure) o Pressure (at constant flow) from a power supply (use 3000psi). o Total actuator capacity is (A area of piston): F =A in lbs 2 p psi

14 o Sufficient oil flow: Definition: Flow is first derivative of volume: dv dl Q= =A =A v where v is oil velocity dt dt A in Q gpm = 2 v in sec 3.85 More accurate for an actuator: Q = A v + K df/dt K depends on the compressibility of the fluid. o Accumulation (reservoir) Note that pumps supply a constant oil flow. i.e. Q=constant Instantaneous flow is the derivative of the volume at any time The accumulator is an oil reservoir for the accumulation of excess or supply for need of extra oil flow.

15 More accurate: The differential equation governing the actuator piston can be expressed as follows. V qs( t) Ax t ( t) F a( t) klefa ( t) 4 A x t (t) Velocity of the table; A The effective piston area. F a (t) Force into the actuator. V Volume of the actuator load chamber; Is the bulk modulus of the oil; F a (t) Is the time derivative of the force in the actuator k le proportionality actor to quantify friction, seals, and other generated losses

16 Hydraulic Servomotors (actuators) Servocontrolled schematic: I= current to servovalve, E = voltage signal (program or feedback) ΔF L K 2+K3s 2 K2B+A 1 2ζ s + s+1 ω 2 s 2 2 ns ωns ΔE c ΔE s K a Δl KA 1 2 K2B+A 1 2 2ζs 2 s + s+1 ns ωns ω ΔY 1 s ΔY K ρ

17 Servocontrolled Hydraulic Actuator

18 Sizing the required servovalve for an actuator: Know: Desired Force (Max) Desired Velocity (Max) Available Pressure (Max) Q=? = F v / 3.85 Δp i.e 20 Kips actuator, max velocity 15 in / sec, Δp=3 ksi Q = 20 x 15 / 3.85 x 3 = 27 gpm -> 30 gpm References: Nachtigal, C.L., Instrumentation and Control, Wiley & Sons, 1990

19 Reference loads Constant value loading Low rate loading (ramp) Load Displacement Saw-tooth loading Load Displacement Signals for loading Manual Function generator (analogue) Digital generator converter (analogue)

20 Static Loadings for Structural Testing Lateral and vertical. Biaxial (or triaxial) Homework: Define a method to control all actuators to move a rigid body in unidirectional motion (any direction)

21 Load History for Case Study: Loading near collapse with cyclic static loading. Need to identify yielding limit. Need to plan inelastic loading Need to stop test upon collapse 1 Choose force control for elastic range 2 Load up to ¾ of calculated yielding force 3 Determine stiffness (slope) in elastic range 4 Calculate approximately the yielding displacement (by extrapolation) 5 Switch to displacement control and develop cycles in terms of displacements (can be constant rate, with T1 T2, or constant time, with T1 = T2) 6 Increase cycles as desired (two of each alternating with two low cycles in between). Test Protocols: 1. Provide testing requirements for qualification testing 2. Specific to types of structures 3. Can be used also for exploration testing

22 The protocol of SAC Project 1990: CUREE Publications: SAC Steel Project... SAC/BD-00/10, Loading Histories for Seismic Performance Testing of SMRF Components and Assemblies, by H. Krawinkler, A. Gupta, R. Medina, and N. Luco project-sac/sac_atc-bd00.html - 28k [ More results from ] For example, a steel moment resisting frame during an earthquake.

23 Additional Quasi-Static Loadings should be looked in reference: CUREE Publications: Wood frame Project Reports... W-26, Seismic Demands for Single- and Multi-story Wood Buildings H. Krawinkler, F. Zareian, L. Ibarra, R. Medina, and SJ Lee, 2003, 146 pages k - Aug 29, 2004 [ More results from ]

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25 Example of testing of components (recall portal method) Actual substructure response Δ Drift % = 100 L Rotation Typical test configuration: This arrangement ignores P- effects.

26 It focuses on plastic hinges in beams, columns and in the beamcolumn joint region. Exterior substructure: