Influence of Frame Structure on Frame Stiffness:
The Case Study of Rock Triaxial Testing Machines In the first two articles of this series, we shared the basic concepts, importance and common measurement methods of testing machine stiffness. As mentioned in the first article, the stiffness of the testing machine frame has the most significant impact on the overall stiffness of the machine, which in turn is closely related to its structural design. How exactly do different frame structures affect frame stiffness? Taking the rock triaxial testing machine as an example, this article will discuss the influence of testing machine frame structure on frame stiffness.
Currently, there is a wide variety of frames for triaxial testing machines on the market, and different types of frames are suitable for significantly different actual working conditions. Several common frame types and their applicable scenarios are listed below.
Testing Machine | Frame Type | Application Scenario |
Conventional Triaxial Testing Machine | One-piece “O”-shaped frame | Suitable for axial loading systems of triaxial testing machines for hard rock and concrete, with stiffness requirement ≥ 5 GN/m. |
| Spliced “O”-shaped frame |
|
| Column portal frame | Suitable for axial loading systems of triaxial testing machines for soft rock, soil, concrete, asphalt pavement materials, etc., with stiffness requirement ≥ 1 GN/m. |
| Space-adjustable portal frame |
|
True Triaxial Testing Machine | Three-direction column-type combined frame | Suitable for axial loading systems of triaxial testing machines for soft rock, soil, concrete, asphalt pavement materials, etc., with stiffness requirement ≥ 1 GN/m. |
| Four-column and spliced combined frame |
|
| Four-column and one-piece combined frame | Suitable for both soft and hard rock true triaxial testing machines, with applicable stiffness range: 2 ~ 15 GN/m. |
| Dual one-piece combined frame | Suitable for axial loading systems of triaxial testing machines for hard rock and concrete, with stiffness requirement ≥ 5 GN/m; specially noted to reach up to ≥ 15 GN/m. |
Based on the table above, we have clarified the applicable working conditions of frames for various triaxial testing machines. We now need to focus on the core point: **the reason why different frames suit different scenarios lies in their structural design, which determines their stiffness characteristics**. To fully understand how frame structure affects overall stiffness, we must first analyze the structural differences between various frame types.
Below, we start from the frame structure and analyze the stiffness characteristics of different frames, so as to understand how the testing machine frame structure affects its stiffness.
1. One-Piece Rectangular Frame
Frame Structure：
This frame is manufactured as an integral closed structure by casting or forging the upper/lower crossbeams and left/right columns into a single unit. It eliminates thread/pin clearances and simultaneously suppresses axial elongation and column bulging.
Stiffness Characteristics：
High stiffness (≥10 GN/m), low deformation, and low center of gravity. It features excellent bending, shear, and torsion resistance, and has a certain tolerance to eccentric loading.
2. Spliced Rectangular Frame
Frame Structure：
The frame is rigidly assembled into a closed structure by welding or bolting the upper crossbeam, lower crossbeam and left/right columns. It features low machining difficulty and low cost, suitable for laboratory scenarios with medium stiffness requirements, on-site disassembly/assembly or transportation restrictions.
Stiffness Characteristics：
Its stiffness along the loading direction is close to that of an integral frame.
3. Column Gantry Frame
Frame Structure：
The frame is constructed by inserting polished columns through the base and upper crossbeam, which are then locked and fixed with high-strength nuts. It is generally arranged symmetrically, available in either a two-column or four-column configuration, featuring a simple and compact structure.
Stiffness Characteristics：
It features high overall stiffness and excellent anti-overturning capacity. The stiffness is mainly affected by the crossbeam thickness, column cross-sectional area and span.
4. Space Adjustable Gantry Frame
Frame Structure：
The frame consists of a worktable, upper crossbeam, columns, and lifting drive mechanism. The upper crossbeam can be moved up and down by hydraulic or electric devices to adjust the test space height, so as to meet the testing requirements of specimens with different heights.
Stiffness Characteristics：
The test space allows stepless adjustment. The stiffness varies with the height of the crossbeam: the lower the crossbeam, the higher the stiffness.
5. Three-Directional Column Combined Frame
Frame Structure：
It consists of three independent four-column (or two-column) frames. The structure is simple and easy to assemble, with high interchangeability and low processing difficulty. It supports expandable and adjustable space.
Stiffness Characteristics：
The minimum stiffness among the three directions shall be taken as the verification index for the overall machine stiffness. The stiffness characteristics of a single direction can refer to those of the column portal frame.
6. Four-Column & Spliced Combined Frame
Frame Structure：
The frame consists of a set of four-column frame and a dual-shaft four-cylinder spliced closed frame. It features a simple structure, flexible assembly, low processing cost for individual components, and a short production cycle.
Stiffness Characteristics：
The stiffness in the four-column direction is lower than that in the other two directions. Therefore, the **minimum stiffness in the four-column direction** shall be taken as the verification index for the overall machine stiffness, with reference to the stiffness characteristics of the column portal frame.
7. Four-Column & Monoblock Combined Frame
Frame Structure：
The frame consists of a four-column frame and a dual-axis four-cylinder closed frame. The dual-axis frame can be either a welded monoblock frame or a cast (or forged) monoblock frame. The four-column frame is relatively independent of the frames in the other two directions. To enhance the overall stiffness, the column section diameter is generally designed to be relatively large.
Stiffness Characteristics：
The frame features a wide design range of stiffness (2 GN/m ~ 15 GN/m). The overall stiffness of the machine is verified against the stiffness in the four column direction.
8.Dual Monoblock Combined Frame
Frame Structure：
The frame is composed of two cast (or forged) monoblock frames installed in a nested manner. A typical structure is a single-axis frame fitted inside a double-axis frame. By modifying the structure to add a sealing function, a **two-rigid one-flexible frame** can be realized.
Stiffness Characteristics：
This frame represents the highest stiffness combination among all frame types. The stiffness in each direction can reach that of a monolithic frame, making it suitable for applications requiring high stiffness (≥ 10 GN/m).
9.Self-Aligning Five-Side Enclosed Frame
Frame Structure：
This frame integrates the advantages of both monolithic frames and four-column space-adjustable frames. It generally consists of two load-bearing beams:One beam is fixed with cylinders covering three directions and five surfaces to form a closed chamber, while the other beam is movable. When moved to the test position, it can also complete the automatic centering function.
Stiffness Characteristics：
The frame features a compact structure and small volume, making it suitable for applications requiring medium-to-high stiffness (≥5 GN/m).
Spanish 
English 
Russian