Impact of the new codes for design of Cement silosKhaled Eid, PEM.Eng. Head of Steel Department RHI Egypt, Consultant Engineer.1 AbstractThe new Euro code BS EN 1991-4 actions on silos has significant impact when compared to othercommonly used codes such as the Din 1055 part 6 or ACI313-18 when considering the eccentricdischarge , previous codes states only there will be significant wall moment without stating how tocalculate it.This paper will highlight the impact of the asymmetric loading in designing silos with eccentric discharge 3by presenting a practical case study comparison for a 14000 m raw meal homo inverted cone silo 52meter height and 23 meter diameter designed by ACI and the new Euro code, keeping the same wallthickness to identify the effect on pre stressing, conventional steel reinforcement, cost and design time.2 IntroductionSilos is generally used in storing granular and powder material in industrial planet most of the large silosare in cement planet, silos either concrete or steel filled from top by either conveyors, bucket elevators orairlift and discharged from the bottom by either from the center or eccentric through discharge gates as ininverted cone silos which is our case studyThe designer should consider the construction method which is usually constructed by the slip formtechnologyDesign of silos in the past was based only on static pressure with no allowance to of pressure differencedue to material flow which resulting to a bending moment stress, there is no direct software for predictingthe walls stress on silos during filling and discharge that makes the silo design requires a very specializedmechanical/structural skills.Footing should be noticed for any differential settlement will cause significant redistribution of loads theremain 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 ≤1.0dc dimension of inside silo diameterh height of vertical walled from transition to the equivalent surface3 Type of material flowThe Euro code states 3 main types of material flow3.1 Mass flow
In this follow al the material flow in the same time atthe same speed.Pressures in a mass flow bins are relatively uniformacross any horizontal crosssection of the hopper.3.2 Pipe flowA funnel flow bin is a bin in which the flow channel donot intersect the silo wall, part of the stored material isin motion while the rest is stagnant. If not properlydesigned the non-flowing solids might consolidate anda pipe will form through which the material will flowwhile the rest will remain stagnant.Discharge pressure can be ignored, in case of squatsilos wit concentric gravity discharge and silo wit topmechanical discharge3.3 Mixed flowA funnel flow pattern in which the flow channel intersect the vertical wall of the silo at a point below thesilo surface the designer un that case should consider the unsymmetrical pressure.`Flow channels in an IBAU central cone silo
4 Pressure and loadsThe ACI design during filling and according to 4.4.1 provides calculation for wall friction , lateral load andvertical load with an increasing factor of 1.35 for concrete and 1.5 for steel discharge load pressure4.1.1 Filling pressureAll loading code use janssen formula to determine the filling pressure which the horizontal pressureincrease with height from top to bottom, based on an e-function and with the silo diameter, the wall frictioncoefficient, the material specific weight and the horizontal pressure ratio as the main parameters4.1.2 Discharge pressureSymmetrical discharge load equals the horizontal filling pressure, in case of circular silos with large outeccentricities 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 siloinner radius.4.1.3 Patch loadingPatch loading that represents the small eccentricity during filling and discharge by adding local load acting overspecified 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 apartabout two weeks after it was first filled to capacityJ. W. Carson and T. Holmes4.1.4 Eccentric discharge :
when discharge opening is not centered in the bottom bin the material flow during discharge will bethrough eccentric channel in this case the ratio of horizontal pressure in the follow channel to the rest ofthe bin is direct proportion to the radii of the follow channelP0/Pn=r/R (ref A.wjenike) 2M=KR P5 Case studyIn the following will introduce a case study fordesigning a new inverted cone Homogenizingcircular silo by both the ACI and the Euro code in a1.6 million ton/year, all wall above the cone level arepost tensioned supported on non pre-stressed wallresting on a ring footing.The silo covered with a horizontal steel metal deck5.1 General input dataFiling System multiple pointsDischarge System multiple pointsSilo bottom Inverted coneBulk Density for volume Calculation 1.300 t/m3Bulk Density for Load Calculation 1.500 t/m3MaterialsFcu (Non Pre stressed) 35.000 N/mm2Fcu (Pre stressed) 40.000 N/mm2Steel Grade 40/60DESIGN CODESLoad on Silos ACI-313-97 Diameter(DI) 22.500 mSILO GEOMETERYSTORAGE COMPATEMENT DIMENSIONS Storage Volume 14,072Height(H) 52.000 12.750 39.250 m3 m Wall Thickness 34.000 cmNON STORAGE DIMENSIONS
Height (H) 12.750 0.000 12.750 m Overall DIMENSIONSDiameter (DI) 21.780 m Height (H) 52.000 0.000 52.000 meterWall Thickness 70.000 cm Diameter (DO) 23.180 meter5.2 Output dataThe difference in the horizontal wall pressure is not significant but the effect of the shear force andbending resulting from Euro code is governingthe wall design reinforcement. The maximummoment usually occurs with case of eccentricflow channel while the ACI produces zeromoment.The amount of pres stressed cable increases incase by around of 38% , for conventionalreinforcement an increases by 42% and theoverall coast increase by nearly 20% butdesign time jumps to 3 times more.
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 376 446.85 700.356 ConclusionThe designer must have a full prediction of load combination and load path especial consideration andcare to be taken in eccentric discharge flow channel, possibility of unexpected loading cases such asnon-uniform and thermal effects. Special attention must be given to how the most critical details in the siloand 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 andmonitored during and after construction.The new Euro code method is time consuming than previous design codes but recommendation for newsilos to be designed with the Euro code as the ACI will lead to underestimate the wall loads, thedischarge from single outlet to be minimized to the shortest period. High quality control on site duringconstruction is a must. Previous silos also may be reassert by the Euro code especially for eccentricdischarge silos7 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?”