مطالعه تجربي اثر ميزان سفت‌شدگي پيچ‌هاي پيونده‌هاي گوي‌سان بر طول كمانش اعضاي فشاري سازه‌هاي شبكه‌اي فضاكار

An Experimental Study of the Influence of the Degree of Bolt Tightness on the Effective Length of Compression Members of Double Layer Space Structures Composed of Ball Joints

تاكنون تحقيقات اندكي در زمينه مطالعه تجربي اثر ميزان سفت شدگي پيچ‌هاي اتصالات پيچي سازه‌هاي فضاكار بر رفتار اتصالات و پاسخ سازه انجام شده است. در اين مطالعه، آثار ميزان سفت شدگي پيچ‌ها بر شرايط سرحدي انتهايي و طول موثر كمانش اعضاي فشاري به طور تجربي مطالعه شده است. بيش از چهل آزمايش بر مدول‌هاي هرمي‌شكل متشكل از هشت عضو لوله‌اي با اتصال پيچي از نوع سيستم ِمرو (MeRo) انجام شده است. اين آزمايشها با ميزان متفاوت سفت شدگي پيچ‌هاي اتصالات تكرار شده اند و حاكي از تاثير قابل ملاحظه ميزان سفت‌شدگي پيچ ها بر طول موثر كمانش و ظرفيت باربري اعضا در نمونه هاي آزمايش بوده اند. در اين مقاله، ضمن تشريح و تفسير نتايج آزمايشها، توصيه‌هاي اجرايي جهت ارتقا ظرفيت باربري و اعتمادپذيري سازه‌هاي فضاكار شبكه اي ارايه شده است.

A search of the published literature reveals that studies related to the effects of the degree of bolt tightness on the behavior of space structural joints and consequently on the member behavior and the overall response of space structures are rather limited. An experimental investigation of the effects of bolt tightness on the rigidity of double layer grids, carried out previously by the authors, had revealed the significance of the degree of bolt tightness on the overall rigidity of the tested structures. The present research is concerned with an investigation of the effects of the degree of bolt tightness on the end conditions and consequently on the effective lengths and the slenderness ratios of compression members of double layer space structures. An experimental program of work was conducted on more than 40 test specimens consisting of typical pyramidal modules of representative double layer grids to observe the influence of the degree of bolt tightness on the effective length and load carrying capacity of diagonal compression web members of such modules. Each module was fabricated by practically identical tubular members and the matching MERO type ball joints. However, different modular specimens were constructed using tubular members with different diameters and cross sectional properties. In each individual test specimen, it was attempted that the bolts connecting the members to the ball joints be tightened by the same applied torque to achieve an identical degree of tightness in all its joints. However, different values of applied torques were used to tighten different test specimens. Following the preparation of the test specimens tightened with different applied torques, a monotonically increasing concentrated load was applied at the upper node of each specimen. The tests were continued up to failure and in each test the displacement of the central top node just underneath the applied load was measured as the main component of the response of the test module. The observations indicate that the degree of bolt tightness has a significant influence on the effective lengths and consequently the load carrying capacity of compression members of such modules. However, over-tightening may cause failure of connection sleeves. Also, bolt pre-stressing shall not be made to a degree that causes brittle fracture of pre-stressed bolts or premature tearing of member to end cone welded connections under additional applied tension.