seismic performance of elevated steel silos during van earthquake, october 23, 2011

ORIGINAL PAPER

Seismic performance of elevated steel silos during Van

earthquake, October 23, 2011

Eren Uckan Bulent Akbas Jay Shen Rou Wen Kenan Turandar

Mustafa Erdik

Received: 23 December 2013 / Accepted: 4 July 2014

Springer Science+Business Media Dordrecht 2014

Abstract Silos are commonly used industrial structures to store dry/granular materials

like cement or wheat. A typical silo consists of a vertical steel tank supported by a braced

steel frame which rests on concrete support. Due to unloading purposes, the tank is gen-

erally located at an elevated position. This makes the structure vulnerable to axial loads in

columns due to excessive overturning moments generated at the base of the structure.

During the October 23, 2011 Van earthquake in Turkey, many silos collapsed either due to

column buckling or foundation problems. In this paper, the field observations regarding the

seismic performance of silos after the Van earthquake are first summarized. Then, the

seismic performances of two steel-elevated silos located in the earthquake region are

studied. One of the silos survived the earthquake by some minor damages in the form of

buckling (at bottom horizontal brace) and spalling of concrete support, while the other silo

remained undamaged. Nonlinear dynamic time history analyses are performed to evaluate

the seismic performances of both silos. As the input ground motion, the recorded ground

motion from a temporary aftershock station (about 2 km away from the silos) in the second

earthquake is used. Analyses indicate that design and construction quality of elevated silos

determine the seismic performance. Finally, recommendations are given to improve the

seismic performance of new constructions.

E. Uckan (&) K. Turandar M. Erdik

Kandilli Observatory and Earthquake Research Institute (KOERI), Bogazici University, Cengelkoy,

Istanbul 34684, Turkey

e-mail: [email protected]

B. Akbas

Department of Earthquake and Structural Engineering, Gebze Institute of Technology, Kocaeli, Turkey

J. Shen

Department of Civil, Construction and Environmental Engineering, Iowa State University, Ames, IA,

USA

R. Wen

Sharma & Associates, Inc., Countryside, IL, USA

123

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DOI 10.1007/s11069-014-1319-9

Author's personal copy

Keywords Elevated silos Non-building structures Van earthquake

1 Introduction

Elevated silos are one of the most widely used non-building structures in industry to store

dry materials such as wheat or cement. The center of gravity of these structures is generally

high which makes the structure extremely vulnerable to ground shaking due to the

increased bending moments and axial loads. Collapse of a silo may cause loss of material,

cleaning up of the contamination, replacement costs, and most importantly possible injury

or loss of life. Therefore, they need to be designed to redistribute the forces within the

structure when a local failure occurs. This will also help decrease the insurance premiums

of the companies.

The studies on the seismic performance of silos have gained great attention in recent

years. Silvestri et al. (2012) investigated the lateral loads induced by effective mass of the

grain on ground-supported (flat-bottom) silos in detail. Silo failures were widely observed in

the past earthquakes. In a more recent study, they also carried out shaking table tests of

ground-supported (flat-bottom) silos (Silvestri et al. 2014). In the December 7, 1988 Spitak,

Armenia earthquake, a part of the granary in the flour mill complex was damaged (Dogangun

et al. 2009). In the August 17, 1999 and November 12, 1999 Duzce, Turkey earthquakes at

the SEKA Paper Factory, three reinforced concrete silos containing wastewater completely

collapsed (Erdik and Uckan 2013). In the September 21, M = 7.6, 1999 ChiChi, Taiwan

earthquake, a concrete factory silo collapsed. In May 21, 2003 Zemmouri,M = 6.8, Algeria

earthquake, some storage facilities and equipment were severely damaged. In the recent

M = 7.2 Van Earthquake, 2011, many small-to-medium size companies suffered from

similar problems and also continuing legal issues between the silo manufacturers and plant

owners (Akbas and Uckan 2012; Uckan 2013). In addition to such studies, vibration control

methods in industrial facilities were also studied by Paolacci et al. (2013). This paper deals

with the performance of the silo structures in the Van earthquake.

On October 23, 2011, 1:41:21 p.m. local time, the city of Van in eastern Turkey was hit

by a major earthquake causing heavy loss of human lives and properties. The epicenter of

the earthquake, Tabanli Village, was 30 km north of Van (Fig. 1) in the eastern region of

Turkey by Lake Van (Erdik et al. 2012).

The earthquake is considered to be a very strong damaging earthquake with a magnitude

of Mw = 7.2 and a depth of 19.2 km. It was associated with a reverse faulting mechanism

dipping toward north (Erdik et al. 2012; Atzori et al. 2011; METU/EERC 2011; EERI

2012), caused significant damage in the cities of Van, Ercis as well as in many villages,

caused deaths of nearly 600 people, and injured 2,000, and about 15,000 buildings were

damaged in the region (Bayrak et al. 2013; Taskin et al. 2013). About 58 buildings are

reported to have totally collapsed, 52 of them are located in Ercis, while 6 of them are

located in Van. Earthquake-affected areas are mainly located within a circle of 30 km

radius and around the epicenter (Fig. 1). Peak ground acceleration (PGA) of the main

shock, measured at the Muradiye station, was 0.18 and 0.17 g in NS and EW directions,

respectively, and 0.08 g in vertical direction (Table 1) (AFAD 2011). The number of

aftershocks with M = 4.04.9 reached up to 114, the biggest number of aftershocks ever

recorded after any major earthquake in Turkey (Fig. 1) (Bayrak et al. 2013). Significant

aftershock has been associated with this earthquake.

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Two weeks after the main shock, another earthquake with a moderate magnitude

(M = 5.6) hit the town of Edremit, located 10 km south of Van. It was possibly triggered

by the first event and caused by a strike-slip fault. About 30 buildings already damaged in

the main shock collapsed. The water and electric networks were damaged again. No

damage was observed in Ercis.

Many reconnaissance reports and papers have been published after the earthquake, but

none of them specifically investigated the damage to industrial facilities, especially to silos

Location of the silo (red) and the aftershock station of KOERI (white)

Strong motion station (white) (AFAD)

Fig. 1 Main shock, big aftershocks, and the second (Edremit) earthquake [red stars: M C 5.0, pink circles:

4.0 B MB5.0, yellow stars: the main shock (M = 7.2) and Edremit Earthquake (M = 5.6)] (Kalafat et al.

2012)

Table 1 Characteristics of the Muradiye and Semsibey earthquake records

Date Record name Location Depth

(km)

Magnitude

(Mw)

PGA

(g)

23 October 2011 Muradiye NS

(main shock)

Van Muradiye

Meteorology Directorate

19.02 7.2 0.179

9 November 2011 Semsibey (second

earthquake)

Semsibey Primary

School

5.6 0.09

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(Bayrak et al. 2013; Ozden et al. 2011; Taskin et al. 2013; Onen et al. 2012; Guney 2012).

This paper aims at investigating the damage at silos which collapsed during the main

shock. The seismic performances of two steel-elevated silos located in the earthquake

region with concentrically braced frames are studied. One of them survived the earthquake

by some minor damages in the form of buckling (at bracings) or spalling of concrete due to

insufficient seating widths of the supporting concrete, while the other silo remained

undamaged. Nonlinear dynamic time history analyses are performed to evaluate the

seismic performances of the silos. As the input ground motion, the acceleration records

from the nearest station (Semsibey) are used. The ground motion is scaled by a factor of 2

in order to match the calculated bias adjusted PGA value of PGA = 0.2 g.

2 Seismicity, site observations, and strong motion properties of the Van earthquake

Seismicity of the region mainly consists of continent-to-continent collision of Anatolian

and Arabian plates (Fig. 2). The earthquake is reported to have occurred on a blind oblique

trust fault, namely Van fault, oriented toward the NESW direction (Erdik et al. 2012;

KOERI 2011; Bayrak et al. 2013) and is not marked on the active fault map of Turkey

(Emre et al. 2012).

2.1 Seismicity and site observations

Part of the convergence between these two plates takes place along the BitlisZagros fold

and thrust belt at a rate of approximately 2.4 cm/year (Erdik et al. 2012). The focal region

Fig. 2 Active tectonic plates, major faults, and their movement directions (Gulen et al. 2002) [from KOERI

Report (KOERI 2011)]

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of the earthquake and much of easternmost Turkey lie toward the southern boundary of the

complex zone of continental collision between the Arabian Plate and the Eurasian Plate.

The geology of the surrounding area is composed of lake, river, and land sediments having

layers of loose sand, gravel, and clay (Ozden et al. 2011). Groundwater level is known to

be high especially for the areas close to Lake Van.

Geotechnical observations after the earthquake indicated that there were mainly liq-

uefaction and liquefaction-based lateral spreading and displacement, landslides, slope

failures, and rockfalls. Water transmission/distribution systems suffered from ground

shaking and soil failure problems. In Ercis, water supply system has been interrupted for a

couple of days due to pipe breaks in the main water supply system. Local authorities

indicated that as a result of pressure reduction in the water supply system, it was likely that

it be polluted by the neighboring sewage system due to potential fluid flow. Due to tensile

and compressive soil deformations, pipe crashing and pullout damages were also observed.

However, after minor repairs and reconfigurations, all services were fully functional

(Akbas and Uckan 2012). The heavy precast concrete frames with precast roof beams

suffered from connection problems. Noticeable displacements and deformations were

observed at the beamcolumn connections and footings of the structures with precast

concrete frame systems located around the VanErcis highway. More severe cases were

observed at the industrial zone of Van. The precast concrete beams slipped off from their

seats and collapsed because of inadequate steelconcrete bondage.

2.2 Strong ground motion

In the main shock, the strong motion stations in Van were not operating. Nearest station

was the one in Muradiye (Fig. 1). Following the preliminary field investigation conducted

by KOERI (2011) on October 24, 2011, the deployment of 10 additional seismic instru-

mentation began on October 25, 2011. Buildings with minor damage seemed to have

experienced only non-structural damage due to mostly separation of infill walls from the

structural frame. This type of damage was also a manifestation of long duration and rather

low amplitude (believed to be\0.2 g).

Distribution of PGA associated with the main shock was obtained by KOERI (2011) by

utilizing the software ELER. The unadjusted estimates of a PGA on the order of 0.6 g and

peak ground velocity (PGV) of 50 cm/s may have been experienced in the epicentral

region. However, there was no evidence from the field to support these levels of ground

motion.

The bias adjusted ground hazard in Van was estimated by (Erdik et al. 2012) from the

nearest stations in Muradiye and Bitlis. The distribution of revised PGAs is shown in

Fig. 3 where the PGA is only about 0.2 g.

The damage observations in Van and Ercis relate to a building stock that has been

exposed only to about 50 % of the reference (or design) acceleration of 0.30.4 g level. As

such, any nearby aftershock (M C 5.5) that can create ground motion above 0.150.2 g

level can cause additional damage and collapses as evidenced in Van city by the November

9, 2011, M5.7 Edremit earthquake (Erdik et al. 2012).

3 Earthquake damage

Almost all elevated cement and wheat silos that were fully loaded during the earthquake

collapsed or seriously damaged. Some suffered from rupture at their base due to excessive

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bending and bearing stresses. Local buckling and anchorage failures were also observed.

The tall twin silos collided with each other. Figs. 4 through 11 show some of the heavily

damaged or fully collapsed silos during the earthquake. Figs 4 and 5a show collapsed

cement silos in Van Industrial Zone due to supporting concrete failure.

These silos were immediately retrofitted with tie beams after the earthquake. The

buckling load for the gusset plate at the connection (Fig. 5b) is estimated as 190.0 kN for

the two gusset plates with dimensions of 20 mm 9 50 mm 9 200 mm and with yield

stress, Fy, of 250 MPa. A simple analytical study has shown that the axial load to be

transferred by the connection was on the order of 210 kN during the earthquake. Thus,

buckling is the expected behavior for this connection. The deformed shape in Fig. 5b

indicates buckling due to uniaxial compression at the two parallel vertical gusset plates and

bending deformation at the base plate (horizontal) due to axial load in the silo leg. The

cement silos near Ercis in Figs. 6 and 7 are other examples of supporting concrete failure.

A number of elevated wheat silos collapsed due to inadequate design in Van Industrial

Fig. 3 Distribution of the bias adjusted PGA (%) associated with the main shock obtained by the ELER

software (PGA = 0.2 g) (Erdik et al. 2012)

Fig. 4 Construction of a tie beam for the collapsed cement silo in Van Industrial Zone due to supporting

concrete failure

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Zone (Fig. 8). Brittle welded connection failure caused failure of some silos in Van

(Fig. 9). A minor shell buckling is observed at an elevated gravel silo in Van (Fig. 10).

Fig. 11 shows an undamaged ground-supported and a collapsed elevated wheat silo in the

industrial zone of Van.

a b

Silo leg

Silo body

Fig. 5 A collapsed cement silo in Van: a construction of tie beams for the damaged supporting concrete

foundation, b gusset plate buckling due to inadequate design at the leg-to-silo body connection

Fig. 6 A collapsed cement silo near Ercis due to failure of the supporting concrete

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personal copy

Fig. 7 A collapsed silo due to supporting concrete failure in Ercis

Fig. 8 Collapsed elevated wheat silos due to inadequate design in Van Industrial Zone

Fig. 9 A collapsed silo due to brittle welded connection failure in Van

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4 Structural framing and design of non-building structures in ASCE 7-10 (2010)

For non-building structures not similar to buildings (elevated tanks, vessels, bins, flat-

bottom ground-supported tanks, concrete chimneys, cooling towers, etc.), a structural

framing system is selected and assigned appropriate values of seismic load reduction

coefficient (R), over strength factor (X0), and displacement amplification factor (Cd)

(Table 15.42 in ASCE 7-10 2010). Seismic response of a building-like or a non-building

structure is most significantly affected by the following parameters:

a. Earthquake input: location and soil condition dependent. No difference for any

structure.

b. Initial structural dynamic properties: initial stiffness and damping.

a b

Fig. 10 A minor shell buckling at an elevated gravel silo in Van

Fig. 11 An undamaged ground-supported (left) and collapsed elevated wheat silo in Van Industrial Zone

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c. Structural configurations: plan and elevation configurations.

d. Seismic load resisting system (SLRS).

e. Interaction between structural and non-structural components.

Concentrically braced frames (CBFs) are probably one of the most efficient seismic load

resisting systems (SLRSs) for building-like industrial structures. It has certainly become

the most popular SLRS in buildings after 1994 Northridge earthquake. The seismic per-

formance of CBFs in past earthquakes has been good to excellent, but some brittle damage

patterns were frequently observed including fracture of brace members due to excessive

buckling deformation and fracture of connections. Buckling deformation is desirable;

however, fracture under cyclic load is not. Seismically compact section must be used in

cases where inelastic deformation is expected. Fracture of a connection that is a brittle limit

state has to be prevented by imposing the maximum force possible for inelastic design

(capacity design approach). Special CBF is expected to have significant inelastic defor-

mation supply, and thus, designed with relatively lower strength than otherwise elastic

response would require (on the order of three times). Ductile behavior and corresponding

reduction in design lateral load come from the yielding of the tension brace (diagonal). The

resulting frame deformation necessitates buckling of the compression brace (diagonal).

ASCE 7-10 (2010) gives the design requirements of CBFs. However, ASCE 7-10

(2010) has been primarily developed on seismic performance of building-like structures.

Building is defined as any structure whose intended use includes shelter of human occu-

pants (ASCE 7-10 2010). This definition is in terms of function, but does not mention

anything related to structural characteristics, which include the following:

a. Diaphragms or other horizontal elements as integral parts of lateral load resisting

system.

b. Non-structural components consisting of significant portion of total seismic weight,

affecting actual stiffness and damping.

The requirements given for building-like structures in ASCE 7-10 (2010) might be

applicable to industrial structures with non-essential modifications. Structures not similar

to buildings have fundamentally different response and need special attentions by indus-

trial standards in addition to the requirements in ASCE 7-10 (2010). Non-building struc-

tures similar to a building have the following features:

a. Non-buildings structures are designed and constructed in a manner similar to

buildings.

b. They will respond to strong ground motions in a fashion similar to buildings.

c. They have basic load carrying systems similar to those used for buildings.

d. Lateral forces are usually transferred with diaphragms or other elements.

The similarities between the requirements for building-like and non-building structures

similar to buildings allow applying building code provisions directly whenever applicable.

Differences between the building and industrial structures similar to buildings are also

listed as follows (ASCE 7-10 2010):

a. Non-structural components in buildings mainly serve the human occupants, whereas

they serve mainly specific functions of the structures of particular industry in non-

building structures that might have substantial impact on actual damping, stiffness, and

strength. For example, the presence of partition walls and exterior enclosure walls in

conventional buildings may result in large damping and initial stiffness than in non-

building structures.

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b. Building-like structures might have irregularities in mass (affecting distribution of

lateral forces if equivalent lateral force procedure is used and might require response

spectrum procedure) and stiffness (which might cause significant torsional

irregularity).

c. Differences are reflected in the design process. For instance, the fundamental period of

an industrial structure shall be evaluated by a rational method other than the

approximate period for building-like structures, whereas for conventional buildings,

the fundamental period can be obtained through empirical relations.

d. Drift limitation for building-like structures (generally 2 % 9 story height) does not

apply to non-building structures, and if analysis indicates, it is not critical for structural

stability.

5 Analytical study

Two silos are used for an analytical study (Figs. 12, 13). Silo-1 in Fig. 12 is a typical silo

in the region. These silos did not experience any significant damage during the earthquake.

Silo-2 experienced some damage at horizontal braces and supporting concrete footing

(Fig. 13). The bottom horizontal brace member has been buckled, and the bearing strength

of the supporting concrete was exceeded and the base plate had some damage. Both silos

are located in northern Van, 10 km to the city center (Fig. 14).

The steel grade for the silos is classified as Fe37 with Fy = 235 MPa for the braces and

rings, whereas FE52 steel grade with Fy = 345 MPa is used for the legs. Modulus of

elasticity is taken as 200,000 MPa. The legs, rings, and braces (Figs. 15a, 16a) are modeled

as steel frame elements, and body of the silo is modeled as thin plate shell. Cross-sectional

properties are given in Table 2. Structural framing of the silos consisted of horizontal legs,

braces, horizontal braces, silo body, and rings inside the body. The total height of the Silo-

1 and Silo-2 is 17.884 and 14.398 m, respectively. The rings inside Silo-1 are located at the

levels of 8.134, 9.634, 11.134, 12.634, 14.134, 15.724, and 16.384 m, whereas the rings

inside Silo-2 are located at the levels of 4.730, 6.230, 7.730, 9.230, 10.730, 12.259, and

12.919 m. For both silos, 11.0 tons of mass was assigned on center of each ring level

except the upper ring, where 6.0 tons of mass is assigned on the center of upper ring levels

to represent the mass and weight of the dry materials inside the silo, i.e., silo is assumed to

be fully loaded. Thus, the total weight of each silo is assumed to be fully loaded with

(7 9 11.0 tons ? 6.0 tons) 9 9.81 m/sec2 = 830 kN of dry material. The masses are

connected to the rings with infinitely rigid, weightless, and mass-less fictitious frame

members. The silo is assumed to be simply supported to the ground. Moment releases are

assigned on both ends of braces and horizontal braces.

Dynamic properties of the silos are given in Figs. 15b, c and Figs. 16b, c. The first

mode of vibrations of Silo-1 and Silo-2 are found to be 1.09 and 0.63 s, respectively.

Modal mass participating ratio for both silos is about 86 % for the first mode, where it adds

up to 99.5 % for the first two modes.

There was only one ground motion record for the Van earthquake recorded at Muradiye

(Fig. 14). There was another ground motion station in the city center of Van, but the night

watch shut down the electricity the night of the earthquake for some unknown reason

according to local authorities. The distance between the location of the recorded ground

motion station (Muradiye) and the silo location is about 70 km (Fig. 14). On November 9,

2011, a second earthquake hit the region again with a magnitude of 5.6. Fortunately, this

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earthquake was recorded at many station around the earthquake region, and one of the

stations, Semsibey, was within a few km of the Silos (Fig. 14). Characteristics and

acceleration time histories of the Muradiye and Semsibey ground motions are given in

Table 1 and Fig. 17, respectively. For comparison, uniform hazard spectra for 5 % critical

damping are constructed for three earthquake levels, namely EQ-1, EQ-2, and EQ-3, based

on RHA (2007). Short-period spectral acceleration, Ss, and 1-sec period spectral acceler-

ation, S1, values corresponding to these earthquake levels are given in Table 3. Effective

PGAs corresponding to these earthquake levels are determined as 0.24, 0.42, and 0.62 for

EQ-1, EQ-2, and EQ-3, respectively. Fig. 18 shows the response spectra corresponding to

Muradiye and Semsibey ground motions. For comparison, design spectra corresponding to

10 % probability of exceedance in 50 years as described in Turkish Earthquake Code (TEC

2,165 mm

3@1,

952

mm

2,27

8 m

m9,

750

mm

17,8

84

mm

Fig. 12 A typical undamaged

silo in Van (Silo-1)

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2007) are also given in Fig. 18 for different site conditions. Response spectrum definition

in TEC (2007) is based on effective peak ground acceleration which is given as 0.4 g for

the earthquake site.

a b

Fig. 13 Framing of a minor damaged silo due to horizontal brace buckling and concrete support failure in

Van (Silo-2)

Lake Van

Turkey

Fig. 14 Muradiye (AFAD) and Semsibey Primary School (KOERI) stations and silo locations (source:

Google Earth)

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To determine the seismic response of the silos, nonlinear dynamic time history analyses

are carried out. Damping ratio is assumed to be 2 % for dynamic analyses. P-Delta effect is

always included in the analyses. Axial plastic hinges are assigned to the middle of the

braces. Post-buckling behavior of the braces under compression is accounted for (Fig. 19).

In Fig. 19, Ry is ratio of the expected yield stress to the specified minimum yield stress, Fy;

Fcr is the critical stress; Ag is gross area; Pcr is maximum axial buckling load; dyc is the

yield displacement under compression; dyt is the yield displacement under tension; Py is

the maximum tension force; and Presidual is the residual load after buckling of the member.

Dead and live loads are assumed as the initial conditions of the nonlinear dynamic time

history analysis. Semsibey record is used for the nonlinear dynamic time history analyses

and scaled to 0.20 g as justified in Sect. 2.2. To further investigate the nonlinear behavior

of the silos, Semsibey ground motion was scaled to 0.40 g as defined in TEC (2007) for the

site.

Peak axial compression forces and peak drift ratios are summarized in Table 4. Both

silos did not experience any inelastic action when subject to Semsibey ground motion

scaled to 0.2 g, i.e., they remained elastic (Figs. 20, 21). Drift ratios were about 0.57 and

0.41 % for Silo-1 and Silo-2, respectively (Table 4). Axial force in the most critical brace

in Silo-1 was about 180 kN corresponding to a demand/capacity ratio of 0.6. This silo did

not experience any damage during the earthquake, and the analyses indicated the same

way. Axial force in the most critical brace in Silo-2 was about 150 kN corresponding to a

demand/capacity ratio of 0.5 (Table 4). This silo suffered minor damage during the

earthquake in the form of bottom horizontal brace buckling and minor damage in

a b c

First ring

Upperrings

Brace

Leg

Horizontalbrace

Fig. 15 Model details and dynamic properties of Silo-1: a structural model, b Mode 1 (T = 1.09 s),

c Mode 2 (T = 0.20 s)

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supporting concrete. However, dynamic analyses indicated that axial force level for the

horizontal braces in Silo-2 was way lower than their capacity and axial load demand/

capacity ratio was about 0.10 (Fig. 21). This indicates that damage occurred in the hori-

zontal braces due to some other reason. It should be pointed out that Silo-2 had no tie

beams connecting the supporting concrete.

When subjected to Semsibey ground motion scaled to 0.4 g, the seismic response of

both silos was moderate to heavy (Figs. 22, 23). Drift ratios increased to 1.01 and 0.71 %

a b c

First ring

Upperrings

Brace

Leg

Horizontalbrace50

0 m

m1,

952

mm

9,66

8 m

m

14,3

98

mm

2,22

78

mm

Fig. 16 Model details and dynamic properties of Silo-2: a structural model, b Mode 1 (T = 0.63 s),

c Mode 2 (T = 0.13 s)

Table 2 Silo properties

Component Cross-sectional

Silo-1 Silo-2

Legs /300 9 6 mm hollow pipe section TS178 9 178 9 6.4 mm

Braces 80 9 80 9 5 mm angle 80 9 80 9 5 mm angle

First ring UPN120 UPN120

Upper rings UPN80 UPN80

Horizontal braces 80 9 80 9 5 mm angle 80 9 80 9 5 mm angle

Body of silo 5 mm thin plate 5 mm thin plate

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for Silo-1 and Silo-2, respectively (Table 4). Braces in Silo-1 yielded, whereas only bottom

short legs in Silo-2 experienced inelastic action, but not considered to be significant

(Fig. 23).

6 Comments

This study investigated the effect of Van earthquake, 2011 (M = 7.2) in Turkey on

industrial structures, especially on elevated silos. During the earthquake, some industrial

structures with precast concrete framing systems, water and electric utilities, and elevated

silos in concrete plants were either damaged or fully collapsed. Moreover, most of the silos

were not insured. Therefore, legal issues also arised between the silo providers and owners.

The equipment in these structures moved and/or collapsed, causing major business inter-

ruption, loss of market, and stock damage. The loss of capacity in small- and

a Muradiye

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0 10 20 30 40 50 60Ac

cele

ratio

n (g)

Time (sec)

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0 10 20 30 40 50 60

Acce

lera

tion

(g)

Time (sec)

EW - November 9, 2011

Semsibeyb

Fig. 17 Acceleration time history of the ground motions: a Muradiye Station NS component, b Semsibey

Primary School EW component

Table 3 Ssand S

1values (RHA

2007)Earthquake level Ss (g) S1 (g) PGA (g)

EQ-1 (50 % PE in 50 years) 0.59 0.16 0.24

EQ-2 (10 % PE in 50 years) 1.05 0.31 0.42

EQ-3 (2 % PE in 50 years) 1.57 0.52 0.62

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00.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3

Sae

(g)

T (sec)

Stiff Soil (TEC)Stiff Clay (TEC)Soft Clay (TEC)Soft Soil (TEC)Muradiye NSSemsibey EW

Fig. 18 Response spectra corresponding to Muradiye and Semsibey stations and TEC (2007) (5 % critical

damping)

a b

P

PCR=Ry FcrAg

Py =Ry Fy Ag

Presidual 0.3 Pcr

P

PCR

Py

0.3 Pcr

ytyc

3yc

Fig. 19 Force-deformation behavior of a typical brace member: a hysteresis with post-buckling behavior,

b idealized behavior

Table 4 Peak axial compression

force and drift ratios

Pmax Axial compression strength

Silo no. PGA Peak axial force (P) P/Pmax Peak drift ratio

(g) (kN) (%)

Silo-1 0.2 180 0.60 0.57

0.4 300 1.00 1.01

Silo-2 0.2 149 0.50 0.41

0.4 235 0.78 0.71

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microenterprises had additional adverse socioeconomic effects due to loss of employment

and production. The earthquake is estimated to have caused a total of 12 billion USD in

total economic losses (Erdik et al. 2012). All these facts pointed out the need for con-

structing safer new building and non-building structures and retrofitting thousands of

Fig. 20 Seismic response of Silo-1 subject to Semsibey EW components scaled to 0.2 g

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similar existing vulnerable ones. The uneven damage distribution on silos in the earthquake

posed a new question why not this one?. Damage to infrastructure and utility brought

more reasons for erecting safer non-building structures in industrial facilities.


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7 Conclusions

In this study, seismic performances of the silos in the earthquake based on site observations

are first investigated in detail. Seismic analyses on silos are also carried out to determine

the seismic response of typical silos in the region through nonlinear dynamic time history

analyses. The analytical studies are carried out on two elevated silos, and the results are


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discussed. The nonlinear behaviors of the members are modeled by considering the tension

(yielding) and compression (buckling) hysteresis curves. As the input ground motion, the

acceleration records from the nearest station (Semsibey) are used. The response spectrum

of the ground motion is far below the TEC (2007) design spectrum. Therefore, inelastic

behavior is not observed during the analysis. In general, the numerical results are observed

to be consistent with the site observations. This is probably due to the low level of ground

motion. However, when the input motion is scaled up to match the design earthquake,

significant inelastic behavior is observed in one of the silos.

The main outcomes of this study are summarized as follows:


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personal copy

1. The failure of silos were brittle, sudden and generally due to weak detailing,

inadequate design of supporting concrete footings.

2. Field observations revealed only one cement silo collapsed due to inadequate design of

braces and legs, whereas others collapsed because of supporting concrete failure due to

lack of tie beams between the supporting concrete footings.

3. Demand to capacity ratios remained well below 1.0 for the silos; that is, Van

earthquake was not considered to be a strong earthquake for the non-building

structures in the region.

4. Overturning is a major problem for silos. Aspect ratio for the silos in this study was 8.3

for Silo-1 and 6.65 for Silo-2.

5. In current practice, supporting concrete area is generally equal to the base plate area.

In these cases, tie beams connecting the supporting concrete footings should be used to

prevent failure of the supporting concrete.

6. Bearing strength of the supporting concrete is exceeded in many cases, and supporting

concrete has no confinement effect when tie beams are not used.

7. Construction quality and detailing are key issues in enhancing the seismic performance

of the silos.

8. The silos that survived the earthquake without any damage were designed, detailed,

and constructed well.

9. The scope of the future studies should be to investigate simple, practical, and efficient

retrofitting methods. This is also expected to have an influence on the earthquake

insurance premiums of likely facilities.

Acknowledgments This research was supported by the Directory of Scientific Research Project of Bos-

phorus University (Grant # BAP5715) and Directorate of Kandilli Observatory and Earthquake Research

Institute, Bosphorus University (KOERI).

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seismic performance of elevated steel silos during van earthquake, october 23, 2011

Documents

steelelevated silos

abstract silos

akbasdepartment of earthquake

earthquake region

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industrial structures

theseismic performances

structure vulnerable