Notes on Seismic design of buildings In Lebanon

BEIRUT FEBRUARY 22, 2010
ATTENTION: DR BILAL ALAYLI, PRESIDENT OF THE OEA- BEIRUT
SUBJECT: SEISMIC DESIGN OF BUILDINGS IN LEBANON

Notes on
Seismic design of buildings In Lebanon

Earthquake (EQ) resistant constructions involve the responsibility of all participants in the construction process and also the Governmental authorities from the standpoint of seismic regulations.
The objective of these notes is to explain why seismic designs of buildings in Lebanon, as performed and implemented, fall short of responding to the imperatives and goals of seismic resistant constructions. It also aims to address the requirements in low rise constructions and the regional seismicity with regards to the Lebanese regulations. In this perspective it will concentrate on the following main issues:
  1. Discuss briefly the seismic deign procedure used in building constructions in Lebanon, its accuracy and deficiencies.
  2. Discuss the Lebanese seismic regulations
1- Seismic design procedure

In Lebanon, the seismic design procedure generally used by Engineers in building constructions is the simplified Equivalent static Lateral Forces (ELF) approach and the designs are based on 3D linear analysis results. Usually Shearwall systems and cores constitute the Lateral Load Resisting System (LLRS) and the rest of the structure is considered as not participating to the lateral resistance.

1.1 Accuracy and deficiencies of the ELF Method

This static approach, based on ductile considerations and accepted by Codes provisions, is restricted by conditions regarding the regularity, continuity and redundancy of the structure. Although those conditions are not generally met in more than 70% of the newly constructed buildings, this approach in itself, cannot be considered as an accurate straightforward method to be used in seismic design if not accompanied by the required experience and Engineering judgement. The main reasons behind are the following:
a- The ELF method relies on Structural configurations that behave well under EQ excitation (Seismic Concept) and elements’ ductility. It requires an understanding of the dynamic behavior of the structure and the elastic and inelastic structural response among others, and implies to pay more attention to a conceptual and comprehensive design than numerical computations.
b- This method is characterized by its inability to identify or detect deficiencies existing in the structural system. Examples of major deficiencies include among others: Stress concentrations resulting in high ductility demands (overstrong elements), excessive deformations due to cracking and/or rocking of foundation resulting in collapse of the structure by pounding to adjacent constructions or by P-delta effect, torsional effects due to eccentrically located shearwalls, and brittle failure by shear of overstrong elements and/or resulting from higher modes effects not taken in consideration by the ELF method (especially in mid and high rise buildings). It should be emphasized that, in seimic considerations, the danger comes from both understrong and overstrong elements and also that a brittle failure by shear cannot be avoided by the only direct application of the static method.
c- Non structural elements (masonry infill) are not taken in consideration by this procedure. They not only alter the behavior of the structure (stiffening effects) but may also contribute to create a soft story mechanism.
d- The secondary members supposed not participating to the lateral resistance should be designed and their stability assessed as they undergo the large inelastic deformation to ensure their ability to maintain their gravity load carrying capacity (P-delta effect).

1.2 Observations

The evaluation and inspection of several newly constructed buildings show, in addition to some flaws related to modeling, detailing, and workmanship issues, the presence of conceptual flaws accompanied generally by inconsistencies and/or deficiencies in the structural system resulting in non conservative and unsafe designs and, in some cases, leading to the most notable and typical failures observed during earthquakes.
All of these deficiencies result from the general lack of experience in this field and from the direct application of the ELF method where the particularities of seismic designs and the dynamic nature of earthquake effects are generally overlooked and/or misunderstood. This raises the concern that these deficiencies may, most probably, also exist in many other constructions.

2- Seismic regulations

Seismic design of buildings is required by the CDR, since the early 90's for all public constructions and more recently by the Governmental decrees No 646 (2004) and 14293 (2005). Both regulations classify Lebanon as a moderate seismic area corresponding to the Zone 2B of the UBC code. The Gov. decree No 646 exempts buildings composed of 3 floors or less from seismic requirements.

2.1 Concerning Low rise buildings

The recent minor ground shakings in South Lebanon (Sarifa) and the extent of damages observed in this category of buildings show the vulnerability to ground shaking of this kind of constructions not only well documented theoretically but also extensively demonstrated by the recent earthquakes in Turkey, China and other countries (and more recently by the earthquake in Haiti).
This issue not only raises the problem of seismic requirements for this category of buildings (and consequently the validity of the exemption of buildings composed of 3 floors or less from these requirements as stipulated in the Governmental decree No 646), but also the danger of the state of mind considering earthquake effects as comparable to wind loads effects.
Therefore and with regards to the Gov. decree No 646, if exemption from seismic requirements could only be maintained for individual or residential flats, it is not recommended to exempt from these requirements all other low rise constructions, and more particularly school buildings, hospitals, or industrial buildings.

2.2 Regional seismicity

The regional seismicity issue is being raised to point out the importance of seismic considerations due to the real threat of earthquakes in Lebanon, and to consequently stress the need for more stringent requirements.
Lebanon, situated in a seismically active region, is crossed by the major fault of Yammouneh and by other known faults (Roum, Rachaya, and serrhaya, the newly found coastal fault) in addition of numerous cited secondary faults and of probably other unknown faults. The Yammouneh fault, with an identifiable total fault length of about 1000 km, could generate an earthquake of Magnitude 8+.
In the region, the USGS Earthquake information center data reports a count (since the year 33 BC) of 54 events of Magnitude 6 or greater having their Epicenter location inside or close to Lebanon, 20 of them were inside a radius of 100 km from the central region of Lebanon, and 8 out of these 20 had a magnitude greater than 7.
In view of the historic EQ records, Lebanon should be considered an area of high seismicity. The UBC 97 classify Lebanon in Zone 3 and the IBC 2000 approach based on the Major Credible Earthquake (MCE) provide ample support to this statement.
Therefore it is recommended to modify the classification of Lebanon from the zone 2B to the Zone 3 as stated in the UBC 97. The added cost resulting from this modification will only be in the order of 1% to 2% of the total cost of the building, yet it will help to safely cover the uncertainties regarding the expected intensity of earthquakes in Lebanon and also has the advantage to reduce the damages in the constructions during earthquakes of lesser intensities (damage control issue).

Note: Other important issues concerning EQ designs are not addressed herein such as the role of the Geotechnical Engineer, the major role of the Architect in building configurations, the role of the Contractor in construction aspects and the damage control issue of interest to the owner and/or the end user.

Bassim Saab, Civil and Structural Engineer