The degree to which construction materials stand up to earthquakes or hurricanes is key to whether building structures are safe and economical. But clearly, waiting until disaster strikes to assess performance is inefficient and potentially deadly. A better path is to evaluate key components in the lab.
Authored by: Bruce Besch Edited by Kenneth J. Korane Key points Resources |
That’s the goal of the Wood Truss Council of America’s (WTCA) new Structural Building Components Research Institute (SBCRI) in Madison, Wis. The testing center replicates real-world behavior of construction products and systems and generates data unattainable at other facilities or with other test methods.
Typical industry research to date has focused on single components and, in many cases, subsets of these components, such as a section of a wall or roof truss. But what’s really of interest is how these components act as a system.
The facility’s test bed measures 30 × 90 ft and is 32 ft high large enough to hold a two-story house. Thus, SBCRI can put to test a complete structural framework. During these tests, hydraulic actuators simultaneously apply static and dynamic loads in vertical and horizontal planes. Typical tests analyze roof systems and simulate loads generated by the building’s own weight, or heavy snow; highwind lateral loads on the walls; and wind-induced uplift loads on trusses. Undulations in the test-bed floor simulate lateral and vertical displacements during an earthquake.
Turning to electrohydraulics
Testing of large-scale structures requires not only hydraulic’s high forces, but precise control of many actuators, simultaneously, in three axes. This sort of performance mandated an electrohydraulic system. For this, Keith Hershey, director of Research & Development for SBCRI, turned to Cleveland-based Parker Hannifin and distributor Ritter Technology LLC.
The initial design called for 52 vertically mounted, Parker 2HX electrohydraulic actuators specifically engineered for motion-control systems. The cylinders have built-in wave-scale position-feedback transducers (2-μm resolution) and low-friction pistons and rod seals to minimize stick-slip during incremental low-force moves. Each generates forces from 8 to 15,000 lb with 20-in. stroke and positioning accuracy to 0.001 in.
Eight horizontal actuators apply lateral and sinusoidal forces to simulate standard earthquake loads as defined in ASTM E 2126-05 (Load Testing for Shear Resistance of Walls for Buildings). Parker DF Plus Voice Coil servovalves, with a step response <9 msec, manage hydraulic flow to precisely control actuator position and force.
Four PD075 piston pumps deliver 30 gpm each at 3,000 psi. The pumps couple to 60-hp electric motors and mount vertically in a 1,500-gallon reservoir to save space and limit noise. Manifold-mounted, 5-μm filters on each pump keep the servosystem clean, and a 15-gallon bladder accumulator maintains constant pressure as load demands vary. Smaller 1-gallon bladder accumulators provide shock suppression during start-up and emergency stops.
Precisely synchronizing so many hydraulic actuators is beyond the capability of many general-purpose motion controllers. So designers opted for an RMC75E motion controller from Delta Computer Systems, Vancouver, Wash., because of the unit’s processing power, speed, and ease of integration with the hydraulic components.
Adamation — a controls integrator based in Maumee, Ohio — developed the network architecture and software. A distributed-control system guides test set-up (actuator placement and calibration), runs the tests, and collects all load distribution and deflection data. And because the new facility will pioneer new ways for testing building components and assemblies, control of the actuators had to be easily modified and reconfigured.
Programs were written for more than 10 different control functions. For example, one test, developed per a customer request, ramps the load on a roof up to a set value over 3 hr to simulate snow loading. The test transitions to cyclic loading with a known displacement and frequency on just a portion of the roof, followed by a decaying load to zero.
Thorough testing
To conduct a test, technicians first position hydraulic actuators around the component or structure. Load cells and string pots mounted at points of interest on the structure measure force and deflection.
Bar codes on actuators, cable harnesses, and controllers let operators use a wireless handheld PC to scan in location, tuning, and scaling parameters, load set-up information into the appropriate controller, and jog the cylinders to a “home” position. The operator selects the appropriate test from a pulldown menu and sets position, cyclic frequency, and force requirements — or can create entirely new test parameters. Data sampling rate can range from every 10 msec to every 24 hr. And software lets the operator simulate the test from start to finish to ensure there are no glitches before the real action begins.
Currently, there are five control panels and 60 axes of position and force control. But the hydraulic system is sized to handle up to 206 additional actuators to meet future growth needs.
WTCA and the Truss Plate Institute fund testing on behalf of the structural building components industry, and SBCRI conducts proprietary testing for individual companies as well. Critical tests of cutting-edge materials and components — from engineered lumber to fiber-reinforced products and light-gauge steel — are in the works.
SBCRI officials say the level of testing the facility provides will lead to more efficient design, installation, and use of all structural building components. It will give engineers a better understanding of loads through all the interconnected parts of a building. And, ultimately, help the industry manufacture better products, increase both safety and cost effectiveness, as well as challenge and improve building codes.