Construction plans:

A full example set of construction plans were developed based on the

model results and code detailing criteria

Seismic design of RC water tanks in AfghanistanStewart Mcilwraith

MSc EEDM

Supervisors: Carmine Galasso, Yasmine Didem, Ioanna Ioannou

1 Preliminary Earthquake Hazard Map of Afghanistan; Oliver S Boyd, Charles S Mueller and Kenneth S Rukstales for USAID and USGS2 ASCE-7 10 Table 15.4.2

Aim:

To provide the ICRC with a design model using capacity design for

elevated RC potable water tanks in Afghanistan, a highly seismic,

mountainous country that is prone to earthquakes up to Mw=8.01.

The model must reflect local construction conditions: low strength

concrete, simplicity of design, a minimal number of reinforcing bar

sizes and as easy to build as possible. It must be applicable to the

following variations:

• Heights: 10m and 20m

• Volumes: 20m3, 50m3 and 100m3

• Soils: Hard rock – soft clay

• Seismic accelerations up to 1.2g

Procedure:

As there are no

construction codes in

Afghanistan, a seismic

hazard map (fig.1) was

first established by

breaking down existing

acceleration data into 4

seismic zones. Zone 4 is

not considered in this

study as the seismicity is

too high for the

application of a standard

design model. Fig. 1: Seismic hazard classification

Given the lack of local codes, the ICRC wanted the design process to

follow the IBC 2012 where possible, an American code that is the

most recognised in Afghanistan. Site classes, modified accelerations

and seismic design categories were defined in accordance with this

standard and ASCE-7, where the IBC makes reference.

Bar one exception, all possible combinations of seismic zone and site

class fall within the ‘severe’ seismic design categories D or above.

For seismic zone 3, site class E and all of seismic zone 4, 1s

acceleration is greater than short period. The design model is not

applicable to these extreme seismicity cases.

Modal mass participation was

above 90% in both horizontal

directions (table 1).

Drift was within reduced limits for

structures in seismic design

categories D, E or F (table 2).

Moment, shear and axial force

results were obtained (fig.3).

Each element was dimensioned

and reinforced to meet the criteria

of ACI-318 special moment

frames for non-building

structures. This includes elevated

shear capacity, moment-axial

force interaction (fig.4) and the

assurance that column moment

capacity is greater than beam

moment capacity at joints (fig.5)

The pad foundation was

dimensioned using reactions from

strength design. Bearing capacity

was defined using allowable

stress reactions.

Table 2: Modal analysis results for case 5 - Seismic loads

Case LocationHeight

(m)

UX (mm) UY (mm) UZ (mm) Total storey drift

(mm)ρ

Drift limit (mm)

Total Storey Total Storey Total Storey

30 ground 0 0 0 0 0.00 1.3 0

30 1st beam level 4 21.8 21.8 6.3 6.3 4.1 4.1 23.01 1.3 46.2

30 2nd beam level 8 57.3 35.6 17.0 10.7 7.2 3.2 37.29 1.3 46.2

30 3rd beam level 12 95.3 37.9 28.4 11.4 9.5 2.2 39.67 1.3 46.2

30 4th beam level 16 131.5 36.3 39.3 10.8 10.9 1.4 37.89 1.3 46.2

30 Base of tank 20 161.3 29.8 48.4 9.1 11.6 0.7 31.15 1.3 46.2

30 Top of tank 24.2 177.6 16.3 53.1 4.8 11.6 0.0 16.94 1.3 48.5

Table 3: Drift at each level of structure

Finite element modelling was used to analyse

the structures behaviour under seismic and

other load combinations. The water was

modelled as a rigid mass located at the

centre of gravity. The first structure analysed

was a severe case: 20m high, 50m3 volume,

in seismic zone 3 and on site class D.Figure 2: Structural model with rigidly connected

fluid mass (left) and true sections (right)

Behaviour modification factor, over strength factor and deflection

amplification factor were defined based on structural system (special

RC moment frame for elevated tanks) as per ASCE-7.

Table 1: Design coefficients and factors for seismic force-resisting system2

Figures 3a-3e FEM results from left to right: drift, primary bending moment,

primary shear

Modified accelerations were

used to define the

acceleration spectra (right).

Fig. 4: Moment-axial force interaction of columns

Conclusions:

The model uses locally available materials and is geometrically simple to realise with a minimum of different cross sections and reinforcing sizes

The model is applicable to many different combinations of seismicity zones, site classes and size parameters

The model can be said to respond to seismic design criteria set forth in IBC 2012 for highly seismic zones

Geotechnical studies are very important to the design of the model as lower limit bearing capacity assumptions are difficult to respect

Short beam spans tend towards shear dominated elements and as such are heavily overdesigned in capacity design

Seismic zone 3 appears to be close to the limit of applicability of the model

Fig.5a and 5b: Exerts from reinforcing plans developed for the first case studied

Results:

42

5.6.5 Seismic detailing for earthquake-resistant structures (ACI-3187)

! ": "! "$ G&0&/( *,

This section is taken from ACI-318 11, the chapter references preceding detailing

requirements are references from ACI-318.

The articles included here reflect the major differences between designing special

moment frames and ordinary structures. For a full list of detailing requirements

please see APPENDIX III.

Strength reduction factors shall be applied to all members depending on the actions

the member is subject to.

Type of failure / element Tension

member

Compression

member

Shear

member

Joints

Strength reduction factor (# ) 0.9 0.65 0.6 0.85

Table 5-24 Strength reduction factors for reinforced concrete elements7

bh

ss

sb

d

sc0

5cm sb0

2bh

l0

0.5sc0

ch

sc

Fig. 5: Reinforcing detailing

for special RC moment frames