Seismic Response of a Historical Mamluk Style Minaret

Seismic response of a historical Mamluk style minaret

A.G. El-Attar, A.M. Saleh, A. Osman Structural Engineering Department, Cairo University, Giza, Egypt

Abstract

The dynamic behavior of Mamluk style minarets, which were reported to experience maximum damage during the Dahshur-Egypt 1992 earthquake, is investigated. A representative minaret (Manjaq AI-Yusufi, 1349 A.D.) was modeled using the finite element technique and analyzed to assess its dynamic characteristics and its response under different input ground motions. Results indicated that this type of minarets is prone to experience large damage during earthquakes with medium to high a/v ratios (0.8 < a/v 1.2). Results also indicated that the irregular distribution of mass and stiffness, typically found in this type of minarets, forms a potential source of damage during earthquakes.

1 Introduction

Historically, minarets were initially constructed as elevated structures attached to Islamic mosques to be used by the Mu'adhdin to summon people for the prayers [l]. In modern times, minarets are only used as an architectural feature to enhance the external appearance of Muslim mosques.

Cairo City is considered one of the oldest cities in the world that possesses a large inventory of historical minarets. Some of these minarets, such as Amr Ibn al'As minaret constructed 641A.D. date back to the early Islamic period. Chronologically, historical Cairene minarets can be categorized into five groups, namely, Tulunid (827 to 904 A.D), Fatimid (969 to 1171A.D), Ayyubid (l171 to

1250 A.D), Mamluk (1250 to 15 l7 A.D) and Ottoman Turk (15 17 to 1848 A.D). Among these groups, Mamluk minarets are considered to be the most deficient from the seismic point of view. Irregular mass and stiffness distribution along their heights made them more vulnerable to damage during earthquakes as

compared to other minaret styles. This was evident during the Dahshur 1992, where damage concentrated mainly in minarets belonging to the Mamluk era [2]. To assess the seismic response and performance of this type of structures, the Manjaq Al-Yusufi minaret (1349 A.D) was selected to represent the Mamluk style minarets.

2 Historical background and present situation survey

Manjaq Al-Yusufi minaret was built in 1349 A.D and represents the medial Mamluk period style (Figure 1). It is located on the northern side of the Cairo Citadel. It consists of a 3.8~3.8meters square base, carrying a vertical shaft that changes its cross section from a square to an octagon to a smaller octagon and finally a top cap (Mabkhara) supported on eight columns at the top. From inside, a cylindrical shaft of a constant inner diameter extends from the bottom square base up to the level of the upper balcony. A helical staircase, constructed from stones interlocked with the stones of the inner cylindrical shaft and resting at its center on a common stone column, exists at the center of the minaret. The minaret's total height is about 26 meters, above the existing ground level. According to Roberts [l], the upper part of the minaret (the "Mabkhara" cap with its supporting columns) is newly introduced and is not a part of the original construction. It is likely that the Arabic Committee for Restoring Islamic Heritage that was formed at the beginning of the twentieth century restored this part of the minaret after its collapse, probably by an earthquake. The actual dimensions of the minaret were obtained using modem surveying equipment. The minaret was checked for verticality and was found to be perfectly vertical. The minaret was visually inspected to assess its current structural conditions. It was found that the minaret at present is in a sound condition with no visible cracks detectable through its entire body. Slight deterioration of stones comprising the minaret base was detected. Such stone disintegration is attributed mainly to environmental conditions and aging process.

2.1 Construction material

Examining the minaret walls, it was clear that the bottom 20 meters of the minaret body were made of two layers of flat-faced limestone blocks, namely, external and internal. Each layer was built using blocks of different dimensions connected together by mortar. Between the external and the internal layers, a filling material consisting of cohesive and cohesionless materials highly chaotic with pieces of limestone was placed. For the remaining 6 meters, only solid limestone walls were used for the columns and the top cap (Mabkhara).

2.2 Local soil conditions

Boreholes carried out very close to the site indicated that the soil stratification at the site consists of a fill layer followed by sandy layers over which the minaret is

founded up to the rock level. Also. it indicated that no high ground water table is present on site.

3 Numerical model

Program COSMOSIM [3] was used for the finite element modeling of the minaret using the surveyed geometrical data. Eight-noded solid elements were used for modeling both the minaret body and the internal helical stair. Multi- layered walls were modeled using large number of elements to allow for modeling both the external and the internal limestone walls in addition to the filling materials in between. Figure 2 shows the finite element model of the minaret. All openings, recesses in the walls, helical stair and changes in the minaret cross-section were faithfully simulated to create a realistic model. The 1. mechanical properties used in the analysis are given in Table These parameters were extracted from laboratory tests of limestone blocks of similar minarets constructed during the same period as Manjaq AI-Yusfi [4].

Table 1 : The Mechanical characteristics of the construction material

Property Elastic Specific Poisson's Tensile Compressive Modulus Gravity ratio strength strength (MPa) (~lm~) (MPa) (MPa) Limestone 32650 23000 0.20 0.28 2.80 Filling 5000 20000 0.20 0.05 0.50

A damping ratio of 5% was used in the dynamic analysis to extract the natural modes and in the subsequent modal analyses. The first fifteen modes were utilized in the modal analysis. At this stage of work, linear elastic analysis was judged to be sufficient for the purpose of investigating the main dynamic characteristics of the minaret and to provide basic deformation modes as well as the distribution of stresses within minaret body.

4 Analysis of the minaret

4.1 Response to gravity loads

As a first step towards examining the level of stress within the minaret body, the considered minaret was analyzed for gravity loads. The load in this case is the minaret self-weight. Results indicate that the compressive stresses do not exceed

0.6 MPa in the minaret body, which are within the allowable stress limits for the used limestone blocks.

4.2 Dynamic characteristics

Table (2) lists the calculated natural periods for the first six modes for the minaret. It can be seen that, the minaret is highly rigid and posses low natural periods, leading to a high seismic risk during earthquakes with high frequency contents. Figure 4 shows the first three mode shapes of the minaret. It can be seen that they are all bending modes. Due to the nearly symmetrical geometry, modes 1 and 2 describing bending about axis X and Z almost have the same period. The same observation is true for modes 3 and 4 (Table 2).

Table 2: Natural periods for the first six modes

Mode Number Period (sec.) Description 1 mode 0.2013 First bending (lStdirection) 2ndmode 0.1989 First bending (2" direction) 31~mode 0.0799 Second bending (l" direction) 4thmode 0.0796 Second bending (2nddirection)

5thmode 0.0736 Torsional mode 6" mode 0.03 1 1 Bearthing

4.3 Seismic response The minaret was analyzed for three different earthquake records. In selecting these records, two main aspects were considered, namely, the frequency content, and the ratio of peak ground acceleration to the peak ground velocity (dv). As such. the selected records were picked to cover a wide spectrum of frequencies and (a%) ratios. The selected records were; (a) N-S component of 1940 Imperial Valley earthquake recorded at El-Centro (dv >l .2), (b) N-S component of 1966

Parkfield earthquake recorded at Temblor (1.2 > alv > 0.8), and (c) N-S component of 1985 Mexico earthquake recorded at Zihuatenejo (alv < 0.8) (Figure 4). All three records were scaled to a peak ground acceleration (PGA) of 0.12g, corresponding to the maximum PGA expected in Cairo City. The analysis was performed using standard modal analysis numerical techniques. The dynamic response of the minaret was determined using step-by-step integration for the first fifteen modes. In each analysis, the earthquake record was applied in each of the two principal directions of minaret (X and Z, in Figure 2).

Figures 5.a through 5.c show the envelopes of lateral displacements of the minaret along its height in both directions. These plots reveal one of the main dynamic characteristics of the minaret, namely, the relatively flexible upper part and the stiff lower body. The figures indicate a larger tendency of the upper part to sway relative to its stiff support. As can be seen, the largest minaret deformation is estimated due to El-Centro earthquake. This is attributed to the fact that the fundamental period of this earthquake is close to the fundamental period of the studied minaret. On the other hand, lowest lateral deformations are

obtained due to Mexico earthquake with a fundamental period of approximately one-second.

In addition to deformations, the stresses within the minaret body are calculated and presented in Table 3. The table summarizes the maximum vertical stresses within minaret body due to different earthquakes combined with self- weight stresses. As can be noted, analyses indicated that stresses exceeding both the tensile and compressive strengths of the construction materials can be expected under such ground motions. These stresses are all found to be located at the connection between the minaret cap and the minaret body (at the bottom of the columns carrying the cap) except for the maximum compressive stresses due to Parkfield earthquake that is located the lowest section of the lower octagonal part. The weakest point in the minaret body was identified as the columns supporting the top cap " Mabkharah". This part is also vulnerable damage due to earthquakes with low (alv) ratios due to its relative flexibility. This can be observed by comparing the stresses due Mexico earthquake to those due to Parkfield earthquake in Table 3.

Table 3: Stresses within minaret body due to different earthquakes

Stresses (MPa) El Centro Parkfield Mexico

Compression 3.43 2.28 2.74

Tensile 2.39 1.67 2.35

5 Vulnerability assessment

Seismic vulnerability assessment has become one of the most essential steps in the preservation studies of historical buildings. It basically represents the degree of loss that will be sustained by the structure for an earthquake of given intensity. For the considered minaret a vulnerability study is carried out using the strong motion records utilized in the dynamic analysis. The study is conducted to capture the threshold peak ground acceleration and consequently the intensity levels which causes the initiation of damage to the minaret. Analytical results indicated that the first initiation of tension cracks will take place at earthquakes with PGA ranging from 0.03g to 0.04g, while compression failure leading to a full collapse of the minaret will be initiated at earthquakes with PGA range of 0.09g to 0.12g.

6 Conclusions

The work presented in this paper provides a general overview of the dynamic characteristics of Mamluk style minarets and their response during earthquakes. The seismic vulnerability of such minarets is assessed and the expected modes of

failure are detected. Examining the analytical results, the following conclusions were drawn:

Mamluk style minarets are susceptible to damage by moderate earthquakes due to their particular geometry and height, in addition to the deterioration of their construction materials as a result of aging process and environmental

conditions.

Due to the typically high stiffness and low natural period of Mamluk minarets, they are prone to damage by earthquakes having medium to high a/v ratios (0.8 < dv < 1.2).

By assessing the seismic vulnerability of the minaret, it was found that the threshold PGA required for damage initiation of these minaret are ranging from 0.03g to 0.04g in tension and 0.09g to 0.12g in compression.

The weakest point in a Mamluk minaret body is identified as the columns supporting the heavy top cap (Mabkhara). This part of the minaret is also < vulnerable to damage due to earthquakes with low a/v ratios (dv 0.8) due to its relative flexibility.

Acknowledgement

The authors acknowledge the financial contribution of European Commission through the project CHIME (Conservation of astorical mditerranean Sites by Innovative Seismic-Protection Techniques).

References

[l] Abouseif, D.B., The Minarets of Cairo, The American University in Cairo Press, 1987.

[2] Sykora, D., Look, D., Croci, G., Karaesmen, E. & Kraraesmen, E., Reconnaissance report of damage to historical monuments in Cairo, Egypt following the October 12, 1992 Dahshur Earthquake. Technical Report NCEER93-0016,State University of New York at Buffalo, U.S.A., 1993.

[3] COSMOSiM, User's Guide Manual, Version 2.0, Structural Research and Analysis Corporation (SRCA), California, USA, 1997. [4] Aly, A.S., Kordi. E. & Elwan, S., Seismic Response of AI-Ghury Minaret. Proc. Of the Int. Collocuium on Str. And Geotechnical Eng., Ain Shams University, Cairo, Egypt, 1996.

[S] Naumoski N., Tso W.K., and Heidebrecht, A.C., A Selection of Representative Strong Motion Earthquake Records Having Different alv Ratios, McMaster University, Earthquake Engineering Research Group, EERG Report 88-01, Canada, 1988.

Figure 1 : Mmjaq AI-Yusufi Minaret

Figure 2: Finite element model

mG- 'c'

0 3 6 9 12 l5 18 21 24 27 30

(a) El-Centro Earthquake, 1940

(b) Parkfield Earthquake. 1966

0 3 6 9 12

Figure 4: Input ground acceleration

754 Earthquake Res~stat?tEnglneerzrzg Srrzlcfzwes III

30 1

-20 -15 -10 -5 0 5 10 15 20

(a) EL-CENTRO EARTHQUAKE, 1940

(b) PARKFIELD EARTHQUAKE, 1966

30 1

-20 -15 -10 -5 0 5 10 15 20

Figure 5: Lateral drift during different earthquakes.