Impact of the new codes for design of Cement silos Khaled Eid, PE M.Eng. Head of Steel Department RHI Egypt, Consultant Engineer.
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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 by presenting a practical case study comparison for a 14000 m3 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.
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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
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Type of material flow
The Euro code states 3 main types of material flow
3.1 Mass flow
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.
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Flow channels in an IBAU central cone silo
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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 :
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) M=KR2P
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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 t/m3 Bulk Density for Load Calculation t/m3
1.300 1.500
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
SILO GEOMETERY STORAGE COMPATEMENT DIMENSIONS Height(H) 52.000 12.750 39.250 m
NON STORAGE DIMENSIONS
Diameter(DI) m
22.500
Storage Volume m3
14,072
Wall Thickness cm
34.000
Height (H)
12.750 0.000
Diameter (DI)
21.780 m
12.750 m
Wall Thickness 70.000 cm
Overall DIMENSIONS Height (H)
52.000 0.000
52.000 meter
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.
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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
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
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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?”
