Architectural Considerations in Earthquake Engineering Part II. by: Mustafa Mahamid, PhD, SE, PE

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1 Architectural Considerations in Earthquake Engineering Part II by: Mustafa Mahamid, PhD, SE, PE In Part I of this article, an introduction to the different types of irregularities that affect the performance of the building during an earthquake was given. In this part and in the coming parts, a discussion of these irregularities will be presented, as well as, its Architectural implications, historical performance, and solutions to improve the performance during an earthquake. 1 In Plan Configuration Problems a. Re entrant Corners The inside corner of a building is the common characteristic of a building configuration that has the in plan shape of L, T, H, + or any combination of these shapes. From seismic perspective, there are two related problems of such building. The first is the variation of rigidity and differential motions, and the second is torsion. The variation of rigidity which results in differential motion between the different parts of the building results in local stress concentration at the notch of the corner. Consider the building shown in Figure 1(a) where two parts of a building are shown. Under an earthquake motion, the North South wing of the building tends to deflect less than the East West wing due to geometrical reasons. If these wings are tied together, the two wings attempt to move differentially at the corners as shown in Figure 1(b) that results in a stress concentration at the corners. North Figure 1(a)

2 North Figure 1(b) For the second problem, Torsion, it may occur due to the fact that the center of mass and the center of rigidity cannot geometrically coincide for all possible earthquake directions. Torsion results in rotation that tend to distort the building in various ways depending on the nature of the ground motion. This results in forces that are difficult to analyze. As the stress concentration at the notch and the rotational effects are interrelated, the seriousness of this problem and the magnitude of the forces depend on the following factors: The length of the wings and their aspect ratio The height of the wings and their height/depth ratios The mass of the building, and The structural system used This type of irregularity is may be found in older buildings built from unreinforced masonry. Due to the fact that it is hard to find seismic separation in these buildings, and the mass of these building is massive, prospects for stress concentration and torsion are high.

3 From architectural perspective, re entrant corners create a useful set of building shapes and provide a high percentage of perimeter rooms with access to light and air which makes these configurations common for high density housing and hotel projects where habitable rooms must be provided with windows. During the 1970 s, daylight and natural ventilation increased the need for narrow buildings and buildings re entrant corners. The historical performance of buildings with re entrant corners has seen in various during past earthquakes. This problem was identified by observers at the end of the 19 th century and was acknowledged by experts by the 1920 s. This phenomenon was reported after the 1925 Santa Barbara Earthquake, 1964 Alaska Earthquake, 1985 Mexico City Earthquake, and 1994 Northridge Earthquake. Figure 2 shows an example of such damage after the 1964 Alaska Earthquake in Anchorage Alaska. Figure 2: Damaged Building During 1964 Alaska Earthquake Solution to re entrant corner plan irregularity There are two solutions to this problem and they are: i. Separate the building structurally into simple shapes by providing seismic joints.

4 ii. Tie the building together strongly at lines of stress concentration and locate the elements to reduce torsion. If solution (i) is selected the individual parts of the building should be designed to resist gravity loads as well as lateral loads on their own; for the design of separation joints, the maximum drifts from each part of the building at the separation joints should be considered to calculate the separation required according the governing building codes. In tall buildings, the relative motion between the two parts of the joint becomes very large which may create major problems in Architectural detailing. If solution (ii) is selected, collectors at the intersection areas need to be used if the design allows. Since the free ends of the wings tend to distort the most under torsion, it is desirable to place resisting members at this location. Figure 3 shows all possible solutions mentioned above. Figure 3: Possible Solutions for Re entrant Corner Irregularity b. Variation in perimeter strength and stiffness This part discusses the effect of wide variations in strength and stiffness of seismic resistant elements located on the perimeter of the building. When the seismic resistant elements are arranged properly around the perimeter they reduce the torsional effects because of the lever arm with respect to the center of mass. If these elements are not arranged properly, the effect could be extreme and destructive.

5 This problem may occur in buildings that are geometrically symmetrical and simple as well as irregular buildings. If there is a wide variation in strength and stiffness around the perimeter, the building tends to rotate around the center of resistance as shown in Figure 4. Figure 4: Rotation of a building with variation in strength and stiffness at the perimeter A common example of this condition occurs in store front design, particularly in corner lots, and in free standing commercial and industrial buildings with wide openings around the perimeter. Buildings with this configuration have performed poorly during earthquakes; for example the J.C. Penny department store in Anchorage, Alaska in the 1964 earthquake. The store was fivestory building of reinforced concrete construction. As shown in Figure 5, the first story had shear walls on all four elevations. The upper stories had open north walls resulting in U shaped shear wall. When the building was subjected to east west lateral forces, the building resulted in large torsional forces that caused substantial damage as shown in Figure 6. The building was not repairable and had to be demolished after the earthquake.

6 Figure 5: Shear Wall arrangement of the J.C. Penny Department Store Demolished after 1964 Alaska Earthquake Figure 6: J.C. Penny Department Store after the 1964 Alaska Earthquake

7 Solutions to Variation in Perimeter Strength and Stiffness The purpose of the any solution to this problem is to reduce torsion and to balance the resistance around the perimeter. There are four alternative strategies to solve this problem. i. Design a frame with approximately equal strength and stiffness for the entire perimeter. For the perimeters that need to be covered, use light weight cladding or use concrete panels or masonry isolated from the frame of the structure. This is illustrated by the light frame figure in Figure 7. ii. Increase the stiffness of the open facades by adding shear walls at or near the open face if the design allows. This is illustrated by the stiff wall figure at open face in Figure 7. iii. Use very strong steel frame at the open façade with stiffness and strength approximately equal to the other perimeter elements. Although the steel frame cannot approach the shear wall stiffness, this solution could be useful in wood buildings where stiffness and strength of steel frame maybe equal to wood panel shear wall. This is illustrated by the Steel MR Frame in Figure 7. iv. The possibility of torsion maybe accepted and the structure designed to resist it. This can be employed by using a rigid and stiff diaphragm which can be designed to act as a unit. This could be useful in small buildings. Figure 7: Possible Solutions for Variation in Perimeter Strength and Stiffness Irregularity