Push (& Pull) to the Limit
Push (& Pull) to the Limit
Eric Lundquist remembers when the Detroit auto industry had access to testing labs in which an entire car could be crushed, simply to determine which bolt failed first. The precision of the tests and the treasure trove of data the industry could mine was unparalleled. “I thought it would be neat to be able to do that with trusses,” Lundquist said. “I came up with this hair-brained idea that if the auto companies can do this, we can.”
Lundquist, CEO of Allwood Building Components, in Richmond, Michigan, was one of the chief fundraisers who worked to procure seed money for SBCRI, SBCA’s state-of-the-art testing facility that has been providing the component industry with testing services and publicly-accessible performance data for the past decade.
It wasn’t an easy start. For the first few years, every testing station successfully built and every testing standard successfully implemented prompted further additions and refinements to the lab’s capabilities. There was always new data to collect and new problems to solve. For example, early on it was discovered that the smallest connection or positioning guide could change the load path and alter the “load in” and “load out” measurements. This, in turn, made accurate analysis of the data a challenge.
Through trial and error, new testing equipment was developed to consistently track accurate load paths during a test. In 2009, the lab became ISO/IEC 17025 accredited, proving it meets the very rigorous international standards for testing and calibration laboratories conducting full-scale load path assessment tests and certifying that SBCRI’s capabilities and expertise are unique in the world of structural testing.
Full-scale testing is what had really been missing from the industry, Lundquist explained. Early, company-specific testing, he said, simply didn’t take into account the whole building system. A roof or a floor—or a whole house—“it’s not just single components,” said Lundquist. Until SBCRI’s testing capabilities and philosophy came along, no one was analyzing systems to understand how all the pieces in a roof, floor or building worked together.
“The whole idea,” said Lundquist, “was to create a program so that you could test a set of components to destruction and have data.” He says the current lab stations have far outgrown that original vision, as well as his own experience. His first day in the truss industry, he recalls, was spent lifting boxes of nails onto a truss rigged with a dial gauge, measuring how much nail-weight the truss could take before it collapsed.
Today, hydraulic actuators have replaced the manual hoisting of nail boxes, and load cells, string potentiometers, and sophisticated computing systems have replaced dial gauges and clip boards. The goals of testing, however, haven’t changed: SBCRI is ready to conduct rigorous scientific testing and provide both the data and analysis to advance the entire structural building components industry.
Lateral Wall Testing Station:
Shear and lateral load testing of single wall panels.
A load cell attached to a hydraulic actuator measures the horizontal force on the bottom chord of the truss, and string potentiometers measure displacement throughout the assembly. This station can apply up to 30,000 pounds to fully-sheathed panels through a truss connected to the wall top plate. Portal frame walls (which simulate one large opening for garage or sliding glass doors) and perforated walls (which simulate multiple openings for windows or doors) are most commonly tested here.
Small-scale and single-element testing.
A typical load frame configuration for a flexure bending test. SBCRI has three load frame stations that can apply up to 140,000 pounds of compression (pushing) force and up to 100,000 pounds of tension (pulling) force to assemblies from as small as 2x2x2 inches or as large 4x6x24 feet.
Tension Testing Station:
Pulling single elements apart to test material properties.
This station is the newest at SBCRI, and it is critical for determining the properties of wood fiber, steel and composite materials. It can apply 100,000 pounds of tension force to material with cross-sections up to 3x11 inches. It is unique in that it can apply compression (pushing) force as well as tension (pulling) force. The load pads at either end eliminate any crushing of the material being tested as it is loaded, preventing stress concentrations that can cause fractures near the grips.
Individual Truss Testing Station:
Testing how actual performance compares to design values.
Known at SBCRI as “Big Blue,” this station is the most common truss testing machine for the component manufacturing industry. Linear actuators apply point loads to the top chord of the truss. Load cells in bearing locations measure the force applied through the truss prior to failure. This machine was donated by Trussway and can test components up to 50 feet long and 12 feet high. Testing at this station is typically a precursor to testing trusses in a full-system assembly where it is important to know the stiffness of the member in a single state and compare that to its composite stiffness in an assembled state.
Testing the transverse pressure resistance capabilities of wall assemblies and wall sheathing materials.
This station is used to test the ability of various sheathing products, windows and doors, and even sheet metal to resist wind loads. It can apply up to 500 pounds per square-foot of negative or positive pressure. The negative pressure test applies the vacuum force to pull sheathing away from the panel frame. The positive pressure test applies a vacuum force to pull wall panel sheathing downward between stud cavities. The resulting data defines fastener and material performance and various failure modes.
Modular Testing Bays:
Testing large-scale, real-world assemblies.
These stations are the primary focus of SBCRI, where fully assembled systems are tested for performance under a wide variety of loading conditions. For example, the stations can test roof and floor truss sections, complete with sheathing and bracing; two-story structures with various wall and roof truss configurations; and prefabricated, insulated concrete panels connected to a poured concrete footing. These tests measure shear strength, shear modulus, the effect of seismic or cyclic loading, load paths and composite behavior. The large footprint of these setups provides a realistic scale to make testing truly representative of jobsite conditions. The resulting data is then used to evaluate load paths (with SAP 2000 3D models) and gain a more detailed understanding of connections and related material performance properties.