Fatigue Resistant Sockets for Dynamic Loads Cable Structures Cable structures technology Info Nr. 3 4-2012 SEIL- UND HEBETECHNIK GMBH DR.-KARL-LENZ-STRASSE 66 D-87700 MEMMINGEN TELEPHONE +49 (0) 83 31-937-393 TELEFAX +49 (0) 83 31-937-624 E-MAIL cablestructures@pfeifer.de INTERNET www.pfeifer.info
In this information paper presents its two new open spelter sockets type 700 and type 710 In 1999 Seil- und Hebetechnik GmbH, as the first manufacturer on the market, obtained national technical approvals for its cable tension members. They were specifically designed for predominantly static loads. Therefore they are perfectly suitable for pedestrian bridges, stadium roofs, other wide span structures and zoological gardens. Recently achieved the European technical approval for this product line (ETA -11/0160). It is available as a free download on our web site www.pfeifer.de/en/cable-structures/. Besides the mentioned applications the cables are also included in structures, such as road bridges or other structures which are dynamically loaded. Numerous fatigue tests have shown that an open spelter socket that is economically designed for static loads is only suitable to Figure 1: Open spelter socket Type 700 Requirement class according to EN 1993-1-11 paragraph 2: Fatigue load Corrosive load, not exposed to the outside environment Corrosive load, exposed to the outside environment a limited extent for dynamic loads. However, usage within road bridges and other structures under cyclic loads requires materials with specific characteristics. These characteristics are fulfilled by the cables, but not to the full extent with our open spelter sockets type 802. For this reason designed a new open spelter socket, which we would like to present to you now: The new open spelter sockets type 700 and type 710 The new open spelter socket type 700 represents a one-piece socket with a fixed forked cable connection. The force in the cable is transferred through a movable pin to the steel structure. A bolted lid prevents the pin from falling out. This type is perfectly suitable for applications with periodically occurring loads, which do not require length adjustment. No significant fatigue load Class 1 Class 2 Fatigue load mainly caused by longitudinal forces Fatigue load caused by longitudinal forces and bending (wind and rain) Class 3 Class 4 Class 5 Open spelter sockets type 802 fulfil the requirements of the classes 1 and 2. Open spelter sockets type 700 now fulfil all requirement classes 1 to 5. Thus, the fatigue loads are also covered. 2
Open spelter sockets type 710 consist of a conical socket with internal thread type 800, a threaded rod with right-hand and left-hand thread and a forked connector with internal thread. This type is used where length adjustment of the cable tension member is necessary. When dimensioning the connection in the steel structure it has to be considered that the dimensions of the plate thickness must be in accordance with the specifications. If there are no exact values given the notch class classification from table 8.1 (13) DIN EN 1993-1-9 can be used for the calculation. Apart from the end fittings type 700 and type 710 there are several other sockets from the range of products, which are suitable for dynamic loads. For an anchorage of cable tension members with adjustable length we provide the proven and tested cylindrical socket type 810 with spherical nut and spherical disc type 813 / 814. This combination fulfils all demands on cable tension members when they are exposed to dynamic loads. Figure 2: Adjustable open spelter socket type 710 Requirement classes according to EN 1993-1-11 paragraph 2: Fatigue load Corrosive load, not exposed to the outside environment Corrosive load, exposed to the outside environment No significant fatigue load Class 1 Class 2 Fatigue load mainly caused by longitudinal forces Fatigue load caused by longitudinal forces and bending (wind and rain) Class 3 Class 4 Class 5 Open spelter sockets type 710 fulfil the requirements of the classes 1 to 4. The fork connector of the type 710 corresponds to the geometry of the socket type 700. The two types 700 and 710 thus fit together well. The sophisticated shape of these connections follows our corporate design of the product families PG and PE used for Cable tension members. Thus, the cable types PV (PV = fully locked cable) and other Pfeifer cable types can be arranged side by side in a building following one lead design intent. Figure 3: Cylindrical socket type 810 with spherical nut and disc type 813 / 814 Design and calculation criteria: Structural analysis: The structural analysis and verification of the open spelter sockets is, as far as possible, performed according to DIN 18800 / DIN EN 1993. In accordance with this standard the socket, the connection sheet, the fork and the pin are verified. Fatigue analysis: The calculation of the dynamically stressed cable tension members is based on the standard DIN EN 1993-1, part 9 and part 11. In accordance with this standard, full locked cables and spiral strands with poured anchorings with metal and resin fillings are allocated to a notch class category with the reference value of =150 N/mm² (DIN EN 1993-1-11 table 9.1). This reference value specifies the fatigue strength for 2 million load cycles and a maximum force of 0.45 x Z Bk (Z Bk = characteristic breaking force of the cable). 3
Fatigue tests are performed according to EN 1993-1-11 paragraph A.4.1 with a stress cycle of σ =1.25 x The standard DIN EN 1993-1-9 table 3.1 recommends safety factors for the fatigue strength (γ Mf ). If, for instance, the design concept safety against fatigue failure without notice is chosen for the cable tension member and the resulting damage is chosen as high, then the resulting γ Mf -factor is 1.35. Therefore the limiting value of the stress range for the cable tension member is / γ Mf = 150 N/mm² / 1.35 = 111 N/mm² The engineer has to calculate the damage equivalent constant stress range for the respective cable tension member. The reference value is 2 million load cycles ( σ E,2 ) This value has to be determined according to DIN EN 1993-1-9 paragraph 6. The design value results from the multiplication of σ E,2 with the safety factor γ Ff for the damage equivalent stress range. The fatigue verification is fulfilled if the design value is smaller than or equal to the limiting value. Fatigue verification according to DIN EN 1993-1-9 paragraph 8: γ Ff * σ E,2 / γ Mf Illustration of the dependence of the stress cycles N from the longitudinal stress range (Wöhler curve from EN 1993-1-11 (9.2)): In the example illustrated the curve for a cable tension member with the notch class classification of = 150 N/mm² is shown. As explained above, this reference value is valid for 2 million stress cycles with a maximum force of 0.45 x Z Bk. In accordance with EN 1993-1-11 paragraph 9.2, for the inclination of the fatigue strength curve the value of m=4 can be assumed for N 2 x 10 6 and the value of m = 6 for N > 2 x 10 6. It is therefore possible to read the anticipated fatigue life as a function of the longitudinal stress range from the diagram. Equation for the determination of the bearable stress range σ z : With N c = 2 x 10 6 = 150 N/mm² N z = number of load cycles m = 4 up to 2 x 10 6 load cycles and m = 6 above 2 x 10 6 load cycles Equation for the determination of the number of bearable load cycles N z : With N c = 2 x 10 6 = 150 N/mm² ( Δσ Z := m Δσ m C Δσ N C Z :=N C ( Δσ Z ( N C N Z σ z = stress range m = 4 up to 2 x 10 6 load cycles and m = 6 above 2 x 10 6 load cycles m ( 4
FEM calculation: The sockets type 700 and 710 were constructed in CAD programmes in 3D. These drafts served as a basis for the subsequent FEM calculations. Several optimising steps lead to the present design. Symbols used (in accordance with EN 1993-1-9) σ = stress range (longitudinal stresses) σ C = reference value for fatigue strength with N c = 2 x 10 6 Load cycle σ E = damage equivalent constant stress range related to n max γ Mf = γ factor for the fatigue strength γ Ff = γ factor for the damage equivalent stress range σ E Figure 4: FEM calculation of open spelter socket The aim of the analysis was to develop cable tension members which are suitable for dynamic stress (for example road bridges) have an aesthetically pleasing shape can be produced efficiently. These demands are entirely fulfilled with the new types 700 and 710. 5
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