4.3.1 FEMA-310
FEMA-310 [20] is a guiding document for the codes like IBC-2006 [29], ACI [1], Corps. of Engineers [3], FEMA-356 [21] and ATC-40 [2] for the determination of the irregularities and design requirements of the building structures having such irregularities. According to this code, if the stiffness of a story is less than 70% of adjacent stories below or above, or less than 80% of the average stiffness of the three stories above or below, the “Life Safety” and “Immediate Occupancy” performance levels, which are explained in the code in details, can not be met. For such structures, dynamic analysis should be performed and the strong column weak beam behaviour should be checked.
4.3.2 EUROCODE-8
In the current EUROCODE-8 [18], it is stated that the soft story mechanisms must be prevented, otherwise the local ductility demands in the soft story columns may be exceeded. As a counter measure, this code requires the following provision in all of the main beams and columns:
(4.1)
Here, ∑MRc is the sum of the columns that are connected to the considered joint and
∑
MRc ≥1.3∑
MRbis also stated that, in the calculation of MRc, the minimum column moment values within the range of column axial forces produced by seismic design procedure should be used.
EUROCODE-8 requires the provision given in Eq.(4.1) to be verified for both orthogonal earthquake directions and for their reversals. In addition, similar to FEMA-310 [20], dynamic analysis procedure is recommended in the design of such structures.
4.3.3 Indian Earthquake Code
With some modifications and additions made, Indian Earthquake Code [47] is based on FEMA-310 [20]. According to this code, the stiffness of a story should not be less than 60% of the adjacent story above or should not be less than 70% of the average stiffness of the three stories above. On the other hand, the Indian Earthquake Code requires the relative displacements in adjacent stories to be bigger than 1.3, in order to define the irregularity as a soft story.
The Indian Earthquake Code also requires pushover analysis by referring ATC-40 [2]
for the determination of the ductility demands. However, accepting that this method may not be very applicable, it suggests an amplification factor of 2.5 to be used for amplifying the member forces to be used for the design of the soft story’s columns and beams. Alternatively, an amplification factor of 1.5 to be used for the same purpose is suggested if symmetric shear walls are arranged in plan of such buildings.
4.3.4 Turkish Earthquake Code-1998
Turkish Earthquake Code-1998 [38] contains vertical and plan (horizontal) irregularity definitions and provisions as counter measures against these irregularities. Concepts like strong column-weak beam and shear safety of beam and column joints are introduced for the first time by this code in Turkey.
In the Turkish Earthquake Code-1998 [38], it is suggested that the vertical irregularities like weak and soft story should be avoided in the design of structures.
Due to this, in this version of the Turkish Earthquake Code, soft story irregularity is defined to be existing in the structure, if the stiffness irregularity factor, ηki, which is the ratio of the relative average lateral drift at any story ((∆i)av) to the relative average lateral drift at the adjacent story above ((∆i+1)av), is greater than 1.5 for any orthogonal earthquake direction. This condition can be expressed as
(4.2) above-mentioned irregularities, Equivalent Earthquake Method should not be used due to this code. For such structures, the code recommends performing dynamic analysis. In the dynamic analysis of these structures, the coefficient β, which is used to scale the total base shear obtained from dynamic analysis to the total base shear obtained from the Equivalent Earthquake Method for any of the orthogonal earthquake direction, should be taken as 1. The coefficient β is taken as 0.9 for the buildings regular in plan or in vertical. Although this coefficient has no physical meaning, it surely increases the capacity of the building structure by increasing the local ductility demands.
4.3.5 Turkish Earthquake Code-2007
The current Turkish Earthquake Code [39] that became active in our country in 2007 also contains vertical and plan irregularity definitions and provisions as counter
5
[38]. Similar to the previous one, this code prevents the vertical irregularities like weak and soft story. On the other hand, the soft story irregularity is redefined in this code. Different from the previous code, there is a soft story irregularity in the building structure, if the stiffness irregularity factor, ηki, which is redefined as
(4.4) or
(4.5)
is greater than 2 for any orthogonal earthquake direction. The code limits the use of Equivalent Earthquake Method in the buildings with a soft story higher than 25 m for the fist and second earthquake zones similar the Turkish Earthquake Code-1998 [38].
Additionally, the building height limit of 75 m is dropped to 40 m for the building structures for which dynamics analysis is to be performed. In this code, the recommended value of the coefficient β is lowered to 0.9 in the dynamic analysis of such structures. For the buildings regular in plan or in vertical, β is recommended to be taken as 0.8.
Other requirements such as strong column-weak beam; shear safety of beam and column joints, and limiting the lateral displacements with little definition changes are kept the same as the previous code. [38].
av