3308 移動式クレーンの安定確保に必要な地耐力の検討(OS5-2 交通機械の安全・環境,OS5 安全,安心,防災,環境負荷低減,オーガナイズド・セッション(OS))

Abstract

Mobile cranes are heavy machinery used in construction sites. Sufficient bearing capacity is required in the supporting ground to keep the machinery horizontal. However, overturning accidents frequently occur because of ground penetration at the foundations of lower careers. A differential settlement ΔS makes machinery unstable because an overturning moment increases whereas a moment of stability decreases. Thus, the machinery overturns by reaching the limit equilibrium of moment. Therefore, a value of the acting pressure through the foundations pa should be less than a value of the ultimate bearing capacity of ground q_d so that ΔS can be reduced to negligible small values. In addition, a tall super structure of the machinery takes high center of gravity. Then, the rotational energy induced by a quick tilt causes the machinery quite unstable by reaching the limitation of kinetic equilibrium earlier than that of static equilibrium. Accordingly, brittle failure is dangerous characteristics of ground for the stability rather than ductile failure introducing the slow penetration. Then, it is important to survey the potential risk of rapid penetration at ground investigations. A value of difference between pa and q_d is considered as the margin of safety that is also identified by a safety factor of bearing capacity F_s, which is defined as the ratio q_d divided by pa. Bearing safety of the supporting ground is ensured by verification of F_s by a threshold value of SR. As this type of machinery is usually placed on construction grounds for a short period, 1.5 of SR is referred for the temporary placements. However, a limiting depth of penetration decreases in the ground that composes the potential risk of rapid penetration. Therefore, authors propose to use the higher value of 3.0 for safety unless nothing of the potential risk of rapid penetration is confirmed. This paper describes safety requirement for prevention of the overturning for machinery.