Resistance of plug & play N type RHS truss connections

Abstract The article presents concept of II generation N type RHS plug & play truss connection made in non-welded technology. Three connections of the representative truss are analyzed. Resistance calculations of these truss joints, using the component method, are shown. Test results and the theoretical resistance of these connections are compared.


Introduction
Modern techniques introduced by AM (Advanced Manufacturing), such as: cutting metals with laser 3D, CNC laser cutter, 3D printers, and automation of production processes lead to the need to reconsider again the current methods and techniques of connecting elements in steel structures. In this paper, a new concept of plug & play, non-welded N type joint for the rectangular hollow section (RHS) steel trusses is presented. A "lock" was made by laser cut in the truss RHS chord appropriate slots, in which it is inserted a "key", which is branch compressed RHS member with special slots and ends prepared by laser cutting. Tension bracing was fixed to chord member by anchor blocks manufactured by AM technology and they have been twisted on their treaded ends by nuts. Assembly of these elements eliminates the need to use welds to connect the bracing members with the chord what is the typical technology used so far [1]. Loads from bracings to the chord are transferred only by squash and shear. The component method which is frequently used for the resistance *Corresponding Author: Jerzy K. Szlendak [1] has been recently used for the RHS truss T joints welded [2] and no welded developed as plug & play [3]. This method is also used for resistance estimation of studied herein plug & play N type RHS joints. However, it is necessary to include some different components, compare to that which are introduced before [3].

Concept of II generation joints
New concept of non-welded N type RHS II generation joint is different of non-welded N RHS I generation one [4]. The tension bracing is not made with two flat bars as before, but with single round rod threaded at the ends which allows to be screwed. Sometimes when the compressive force could be present in a usually tensile bracing then it could be made with RHS, however in its ends should be welded threaded round bars to be screwed as in case of full length round rods. The anchor block consists of two parts. The first have special "teeth" fixed this part to truss chord member and the second part through the cylindrical surface enables transfer the load stress from different inclination angle and simultaneously it is a support for washer and nut. The branch with RHS as a "key" is inserted in the slots of "socket" made in the chord member and also it has its own slots in which are fixed "teeth's" of top and bottom anchor blocks. This limits the deflections of chord flanges. The "teeth" in upper anchor block put into the branch allows the better merging the branch to chord member. Sections of the joint are shown in Figure 2.    Table 1) is equal to 0.8 or more. The component resistance N b,Rd is equal to: where: b = 2 · hn + 10 · t 0 -assumed area of web in compression stress [1], χ -buckling coefficient calculated for the part of the chord web width the dimension b · t 0 . The buckling length is assumed to be equal to h 0 − t 0 . This is advisable because of the overall load capacity. The resistance of this component is equal to: where: A V -shear area of socket in upper and bottom chord flanges, A Vp -shear area of the bottom anchor block "teeth".

Chord socket under bearing stresscomponent N6
Tension force in tension brace causes stress in chord socket from anchor block. The overall connection resistance is equal to the sum of the resistance of the top and bottom part of connection. The resistance component, when slope angle of tension brace is considered, is equal to: where: f dbh -Hertz contact stress from, Ag -stress area of anchor block "teeth" perpendicular to the direction of force.

Branch member inside the chord under compression -component N7
Excessive load of the branch can lead to failure of weakened cross section of branch inside the chord tube. This section under compression consists only with four angles. The resistance of this component is equal to: where: A -area of four angles, χ -buckling coefficient from [1]. where: Ap -area section branch.

"Teeth" of anchor blocks under punching shear -component N8
Tension force in tension brace causes shear in "teeth" anchor block. The overall resistance is equal to the sum of the where: A V -shear area of "teeth" in bottom and upper anchor block.

Tension bracing under tensioncomponent N9
When the parameter β is small, and chord member has high rigidity e.g. thick flanges, this could involve failure of tension bracing. The resistance of his component is equal to: N t,Rd = As · f yb (7) where: As -sectional area of tension bracing

Experimental tests
In Table 1 the geometry of the nine tested specimens and their mechanical properties are given. From 24 meter double-grate RHS truss, Figure 7 has been selected 3 different representative connections. Because truss has have constant spacing of nodes equal to 2 m tension diagonals in tested joints have a different angles of inclination to chord member. The vertical branch was loaded and unloaded several times by a 200 kN hydraulic jack and in the same time the tension brace by a 500 kN one. The chord was loaded by constant longitudinal force equal to 200 kN for stabilization of it during the test procedure. LVDT gauges and ARAMIS system were used to measure the dis-  placements of samples. The load and displacements of vertical compressed branch and tension bracing were continually registered during the loading and unloading process until failure. In Table 2 the theoretical resistances of the joint are given. These predictions of the joint resistances have been estimated from all components and its minimum value is finally provided. The components N1+N2, N3, N4, N7, described failure phenomena causes by vertical force in compressed brace while components N6, N8 and N9 from force in tension bracing. In Figure 8 to Figure 13 are presented test results and also the theoretical estimations obtained from the decisive components of the joint resistance. Components N1+N2 are adopted from estimation done for T RHS joints [2].  6 Practical applications and future prospects 1. Non-welded N type RHS II generation joint are easy to manufacturing automation. 2. However the real manufacturing tolerances of the RHS members could be a problem to make the slots and holes in proper places. More advanced preparation of such operations will be needed. 3. These joints may bring closer the truss steel structures with large spans and oversized dimensions for their delivery in elements. They could be easy to make and cheap in transport and could significantly reduce the overall costs. 4. It is suggested to apply hot dip galvanizing as the corrosion protection of structures with such joints. 5. Structures with such joints can be mounted as temporary because they are easy to assemble and disassemble