It is nearly 20 years after publication of the first edition of Iran’s Seismic Design Standard. Iran’s Concrete Standard is also 15 years old. Seismic designing of concrete structures which are performed in Iran based on the two aforementioned standards have undergone variations with each edition of the respective Standards. Yet, there are some items in the documents which still require revision and modification. Although experience has proved that the structures designed based on these Standards exhibit acceptable behavior in the event of earthquake but there remain still some serious questions about the primary assumption of the Standards.
Many structure designers are unfamiliar with the term “unsuitable initial guess”. Many of us have the experience that the indeterminate structure might exhibit a response other than what expected in routine engineering calculations if suitable structural system is not chosen for the structure and/or if our initial guess about the structural elements is not correct,. In simple words, it can be stated that if our initial guess about dimensions of an element is improperly larger than its requirement, the force might be transferred toward that element due to extra stiffness provided for it, and in the subsequent step of trial and error in designing, even larger dimensions might be considered for the same element. Junior engineers have occasionally accepted such a design with emphasis on ratio of the stresses acquired from computer software packages whereas an experienced engineer is able to rapidly distinguish unsuitability of designing under such conditions.
The mechanism of changes in seismic design regulations sometime reminds the abovementioned example. We are dealing with standards in seismic design that add a value to seismic forces in each edition and assume more restrictive rules for this purpose. The question remains for structure designers whether they shall revise the fundamental philosophy of seismic design instead of adding coefficients or not.
Additionally, it must be noted that except for conventional and ordinary buildings, the structure designer in many projects need to know the supporting philosophy behind the rule mentioned in the standard in order to appropriately apply it under complicated conditions in the plan. It is customary in accredited international standards to provide the main documents together with their interpretation. Furthermore, the engineers have the possibility to access the details of negotiations of the committee in charge of codifying the standard. In some cases like Concrete Standard ACI318, the standard text is provided to the engineering society one year before its official publication. And, the collection of questions and obscurities posed by engineers and researchers are published in a journal together with the responses made by the codifying committees. These supporting journals help the engineers to both understand the philosophy of the regulations stated in the standard and also more properly comprehend the limitations and capabilities of rules and relationships stipulated in the standard.
Unfortunately, Iran’s Seismic Design Standard lacks an official and itemized interpretation by the committee in charge of its codification. The author can only mention two books that properly interpret the respective Standard. The first one is the book “A Philosophical Approach to Computational Rules of Buildings against Earthquake” by AliAsghar Taheri Behbahani and the second one is “An Explanation to the 2nd Edition of the Standard of Building Design against Earthquake” by Dr. Aliakbar Aghakouchak. Concerning Iran’s Concrete Standard, only interpretation of the first part is attached and the part devoted to designing lacked any official interpretation until 2005. In this year, the interpretation of the second part was published (two years after publication of its first revision and around 13 years after first publication of the Standard).
Seismic Design in Iran’s Standard No. 2800
The first standard of building design against earthquake was published with the number 2800 in January 1988. Before that date, seismic design in Iran used to follow a chapter of Iran’s Standard No. 519 and had a tendency toward foreign standards.
The major philosophy of seismic design in the respective Standard – selected in accordance with UBC Standard – is based on applying a lateral force as a coefficient of building mass. The coefficient introduced in this Standard consisted of 4 components: peak ground acceleration (PGA) of design, building reflection coefficient, building significance coefficient, and building behavior coefficient.
The peak ground acceleration had been determined based on iso-acceleration maps prepared for Iran in the study conducted by Barbarian et al where Iran was divided into three zones with high, moderate, and low PGA.
Building reflection coefficient was defined as a function of period of the structure and type of soil in the site. The respective function determines the spectrum form of design based on Iran’s Earthquake Standard.
Building significance coefficient increases or decreases the value of earthquake coefficient based on significance of the building and the level of probable damages resulting from its destruction.
Building behavior coefficient takes into account the effect of non-linear factors in reduction of the force exerted to the structure.
In the second and third editions of the Standard provided to engineers in the years 1999 and 2005 respectively, this designing philosophy was preserved and merely the values of coefficients varied. For instance, in the second edition of Iran’s Standard No. 2800, the zonal divisions of country were increased from 3 to 4 zones based on peak ground acceleration and the building reflection coefficient increased around 25%. The same coefficient again increased for an additional 10-30% in the third edition such that it maximum value was 2.0 in the first edition of Standard, increased to 2.5 in the second edition, then to 2.75 and in some cases up to 3.25 in the third edition (Figure 1).
Changes in the standard in successive editions were not, of course, limited to these items but also entail numerous other issues including rules regarding secondary displacements, permissible displacements, structure behavior coefficient, and vertical forces of earthquake.
Despite these modifications, the basic philosophy of Iran’s Earthquake Standard has remained the same. Seismic design based in the respective Standard depends on using an elastic reflection spectrum. This spectrum is acquired based on maximal base shear response of a structure with one degree of freedom in elastic behavior state. The spectrum is then moothed using statistical methods.