design of silo a paper to the cement conferance

1. Impact of the new codes for design of
Cement silos
Khaled Eid, PE
M.Eng. Head of Steel Department RHI Egypt, Consultant Engineer.
1 Abstract
The new Euro code BS EN 1991-4 actions on silos has significant impact when compared to other
commonly used codes such as the Din 1055 part 6 or ACI313-18 when considering the eccentric
discharge , previous codes states only there will be significant wall moment without stating how to
calculate it.
This paper will highlight the impact of the asymmetric loading in designing silos with eccentric discharge
3
by presenting a practical case study comparison for a 14000 m raw meal homo inverted cone silo 52
meter height and 23 meter diameter designed by ACI and the new Euro code, keeping the same wall
thickness to identify the effect on pre stressing, conventional steel reinforcement, cost and design time.
2 Introduction
Silos is generally used in storing granular and powder material in industrial planet most of the large silos
are in cement planet, silos either concrete or steel filled from top by either conveyors, bucket elevators or
airlift and discharged from the bottom by either from the center or eccentric through discharge gates as in
inverted cone silos which is our case study
The designer should consider the construction method which is usually constructed by the slip form
technology
Design of silos in the past was based only on static pressure with no allowance to of pressure difference
due to material flow which resulting to a bending moment stress, there is no direct software for predicting
the walls stress on silos during filling and discharge that makes the silo design requires a very specialized
mechanical/structural skills.
Footing should be noticed for any differential settlement will cause significant redistribution of loads the
remain supports, there were many reported case of silo collapse plus many other unreported.
According to silo geometry there are 4 types classification types.
1. Slender silo: where 2≤hc/dc
2. Intermediate slenderness silos where 1.0<hc/dc
3. Squat silo where 0.4<hc/dc ≤1.0
4. Retiling silo where bottom is flat and hc/dc ≤1.0
dc dimension of inside silo diameter
h height of vertical walled from transition to the equivalent surface
3 Type of material flow
The Euro code states 3 main types of material flow
3.1 Mass flow

2. In this follow al the material flow in the same time at
the same speed.
Pressures in a mass flow bins are relatively uniform
across any horizontal cross
section of the hopper.
3.2 Pipe flow
A funnel flow bin is a bin in which the flow channel do
not intersect the silo wall, part of the stored material is
in motion while the rest is stagnant. If not properly
designed the non-flowing solids might consolidate and
a pipe will form through which the material will flow
while the rest will remain stagnant.
Discharge pressure can be ignored, in case of squat
silos wit concentric gravity discharge and silo wit top
mechanical discharge
3.3 Mixed flow
A funnel flow pattern in which the flow channel intersect the vertical wall of the silo at a point below the
silo surface the designer un that case should consider the unsymmetrical pressure.
`
Flow channels in an IBAU central cone silo

3. 4 Pressure and loads
The ACI design during filling and according to 4.4.1 provides calculation for wall friction , lateral load and
vertical load with an increasing factor of 1.35 for concrete and 1.5 for steel discharge load pressure
4.1.1 Filling pressure
All loading code use janssen formula to determine the filling pressure which the horizontal pressure
increase with height from top to bottom, based on an e-function and with the silo diameter, the wall friction
coefficient, the material specific weight and the horizontal pressure ratio as the main parameters
4.1.2 Discharge pressure
Symmetrical discharge load equals the horizontal filling pressure, in case of circular silos with large out
eccentricities the Euro code gives 3 values of radius of the flow channel rc as 0.25r , 0.4r and 0.6r where r is the silo
inner radius.
4.1.3 Patch loading
Patch loading that represents the small eccentricity during filling and discharge by adding local load acting over
specified zone on any part of the vertical wall of a silo considering only the case that produce the great effect.
The brand new 9 000ton bolted steel silo split apart
about two weeks after it was first filled to capacity
J. W. Carson and T. Holmes
4.1.4 Eccentric discharge :

4. when discharge opening is not centered in the bottom bin the material flow during discharge will be
through eccentric channel in this case the ratio of horizontal pressure in the follow channel to the rest of
the bin is direct proportion to the radii of the follow channel
P0/Pn=r/R (ref A.wjenike)
2
M=KR P
5 Case study
In the following will introduce a case study for
designing a new inverted cone Homogenizing
circular silo by both the ACI and the Euro code in a
1.6 million ton/year, all wall above the cone level are
post tensioned supported on non pre-stressed wall
resting on a ring footing.
The silo covered with a horizontal steel metal deck
5.1 General input data
Filing System multiple points
Discharge System multiple points
Silo bottom Inverted cone
Bulk Density for volume Calculation 1.300
t/m3
Bulk Density for Load Calculation 1.500
t/m3
Materials
Fcu (Non Pre stressed) 35.000 N/mm2
Fcu (Pre stressed) 40.000 N/mm2
Steel Grade 40/60
DESIGN CODES
Load on Silos ACI-313-97 Diameter(DI) 22.500
m
SILO GEOMETERY
STORAGE COMPATEMENT DIMENSIONS Storage Volume 14,072
Height(H) 52.000 12.750 39.250 m3
m
Wall Thickness 34.000
cm
NON STORAGE DIMENSIONS

5. Height (H) 12.750 0.000 12.750 m Overall DIMENSIONS
Diameter (DI) 21.780 m Height (H) 52.000 0.000 52.000 meter
Wall Thickness 70.000 cm Diameter (DO) 23.180 meter
5.2 Output data
The difference in the horizontal wall pressure is not significant but the effect of the shear force and
bending resulting from Euro code is governing
the wall design reinforcement. The maximum
moment usually occurs with case of eccentric
flow channel while the ACI produces zero
moment.
The amount of pres stressed cable increases in
case by around of 38% , for conventional
reinforcement an increases by 42% and the
overall coast increase by nearly 20% but
design time jumps to 3 times more.

6. 800
700
600
500
400
300 ACI
200
Euro Code
100
0
Total Reinforce Cost per Total cost
cables ment ton Storage factor
ACI 268.4016 263 337.03 585.50
Euro Code 371.9516 376 446.85 700.35
6 Conclusion
The designer must have a full prediction of load combination and load path especial consideration and
care to be taken in eccentric discharge flow channel, possibility of unexpected loading cases such as
non-uniform and thermal effects. Special attention must be given to how the most critical details in the silo
and its supporting structure will be constructed and fabricated, always try to use standard method else all

7. details to be clearly provided. Extreme care to the foundation settlement which must be avoided and
monitored during and after construction.
The new Euro code method is time consuming than previous design codes but recommendation for new
silos to be designed with the Euro code as the ACI will lead to underestimate the wall loads, the
discharge from single outlet to be minimized to the shortest period. High quality control on site during
construction is a must. Previous silos also may be reassert by the Euro code especially for eccentric
discharge silos
7 References
1. Hug Mckay “Implication of the new Euro code on the design of cement raw meal silo”
2. ACI Standards 313-97 standard practice for design and construction of concrete silos and
stacking tubes for storing granular materials.
3. Din 1055-6 “loads in silo bin”
4. IBAU HAMBURG “Central Cone Silos from the structural point of view silos.
5. John W. Carson and Tracy Holmes “Silo failure: why do they happen?”