22 / JOURNAL OF PERFORMANCE OF CONSTRUCTED FACILITIES / FEBRUARY 1999 IMPACT OF SHELLING ON RC FRAMES WITH AND WITHOUT INFILL WALLS By H. Al-Khaiat, 1 S. Fereig, 2 J. Al-Duaij, 3 and T. A. Awida 4 ABSTRACT: The traditional building practices used for the construction of residential buildings in Kuwait proved to be extremely effective in resisting fire and enduring severe structural damage from shelling during the 1990 Iraqi invasion of Kuwait. This paper describes and illustrates how infill walls acted as wall-bearing elements to keep a building standing, even after the loss of supporting members. Two reinforced concrete (RC) frame structures, both of which were subjected to direct shelling and lost main supporting elements, are pre- sented. The first frame does not have infill walls, while the second has them. The difference in behavior between the two cases shows the effectiveness of infill walls in preventing progressive collapse under such abnormal loading. A finite-element technique using the modified interface friction element, which represents the interface between the frame and the infill, is presented herein to explain the behavior of frames with infill walls before and after shelling. The finite-element method gave a good correlation with the actual behavior of the cases investigated. INTRODUCTION In August 1990, Iraq invaded Kuwait and occupied the country for seven months until its liberation after the Gulf War. During these seven months, various buildings were subjected to shelling and fire. The extent of shelling and types of bombs used are difficult to document. In any case, considerable dam- age was done in terms of fire and shelling. Two reinforced concrete structures, both subjected to direct shelling and which lost main supporting elements, are presented. The first struc- ture does not have infill walls, while the second one does. The difference in behavior between the two shows the effectiveness of the infill walls in preventing progressive collapse under such abnormal loading. Both structures have been analyzed to check the adequacy of the original design with respect to the specified design loads and have been investigated after eliminating all the damaged elements in order to understand the postdamage behavior. A finite-element technique is used to represent the infilled frames. A modified interface friction element is used to rep- resent the interface condition between the frame and the infill wall. FINITE-ELEMENT TECHNIQUE Analyses of infilled frames using the finite-element method have been reported in many previous investigations. In some of these investigations, linear elastic finite-element models were employed without particular regard to the changing con- ditions at the interface between the frame and the infill panel. These models suffered from the obvious inability to fully rec- ognize the conditions at the interface. To include the interface conditions, a more accurate approach was proposed by King and Pandley (1978) using different elements for the frame, the infill, and the interface [Figs. 1(a and b)]. The modified inter- 1 Assoc. Prof., Civ. Engrg. Dept., Kuwait Univ., P.O. Box 5969, Safat 13060, Kuwait. 2 Prof., Civ. Engrg. Dept., Kuwait Univ., P.O. Box 5969, Safat 13060, Kuwait. 3 Assoc. Prof., Civ. Engrg. Dept., Kuwait Univ., P.O. Box 5969, Safat 13060, Kuwait. 4 Asst. Prof., Struct. Engrg. Dept., Facu. of Engrg., Ain Shams Univ., Cairo, Egypt. Note. Discussion open until July 1, 1999. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on January 26, 1998. This paper is part of the Jour- nal of Performance of Constructed Facilities, Vol. 13, No. 1, February, 1999. ASCE, ISSN 0887-3828/99/0001-0022 – 0028/$8.00 + $.50 per page. Paper No. 17454. face friction element is used to simulate the infill/frame inter- face. Loads are applied in increments and initial lack of fit, gap formation, or slip at the interface are readily allowed by suitable initial or subsequent automatic adjustment of the fric- tion element properties. Description of Finite-Element Model The infilled frame is represented by three different types of finite elements as follows: 1. The frame element. One-dimensional finite elements are used to represent the reinforced concrete frame as shown in the first part of Fig. 1(c). Each element has two rigid joints, each having three degrees of freedom. 2. The infill element. The infill panel is idealized by basic four noded rectangular isotropic plane stress elements having two degrees of freedom at each node [third part of Fig. 1(c)]. 3. The interface element. The interface between the frame and the infill panel is represented by modified rectangular friction elements [second part of Fig. 1(c)]. The friction element has three degrees of freedom at its node that is connected to the frame element and two degrees of free- dom at the other node, which is connected to the infill element. This takes account of the moments produced at the neutral axis of the frame element by friction at the interface. It is assumed that both normal and tangential displacements vary linearly along the length of the ele- ment. The properties of the interface element are as fol- lows. The stresses developed in the friction element can be de- fined by  = k (1) n n n  = k (2) s  s where  n and  = normal and tangential stresses devel- oped in the interface element, respectively; and  n and  s = relative normal and tangential displacement of the interface element, respectively; and k n and k s = normal and tangential stiffness of the interface element, respec- tively. In the case of an infilled frame: 1. The stresses developed in the interface element are checked, the properties of this element (k n , k s ) are ad- Downloaded 01 Jun 2009 to 84.246.73.14. Redistribution subject to ASCE license or copyright; see http://pubs.asce.org/copyright