# Concrete Silos: Failures, Design Issues and Repair/Strengthening Methods
## 3 Design Issues in Reinforced Concrete Silos
### Design Factors
#### 3.1.1 Silo Design Codes  
Silo designs are fundamentally dependent on models and building codes such as the British Standard BS EN 1991-4:2006, Australian Standard AS 3774-1996, and American Society of Agricultural Engineers ANSI/ASAE EP433 DEC1988 (R2011), and American Concrete Institute ACI 313-97 [24]. On the downside, the models are based on a simplified characterization of the actual loading and unloading pressures at the bottom and on the wall of the silo hoppers. In brief, there is a marked difference between modeled data and the real pressures observed during real loading and unloading of grains. Even though the oversimplification is unjustified because it increases the risk of failure, [PERSON], [PERSON], and [PERSON] argue that the exclusion of rigorous deductive processes is validated because it helps to maintain conservatism and the maintenance of safe margins of error [57]. Figure 2: Distribution of the axial (**a**) and hoop (**b**) forces on the foundation of the silo [39]. The ability to maintain safe margins of errors proposed by [PERSON], [PERSON], and [PERSON] argue [57] could be a challenge because different countries conform to different standards. Western countries such as the US, UK, Canada, and Australia have adopted the American Society of Agricultural Engineers ANSI/ASAE EP433 DEC1988 (R2011), American Concrete Institute ACI 313-97 [24], EN 1991-4:2006, Australian Standard AS 3774-1996. In contrast, Asian countries have developed their own silo design codes such as GB50077-2003 [56]. The latter silo code design is inherently limited compared to Western standards because of the following reasons: One, it provides an inaccurate assessment of the Rankine active earth pressure, and the general calculation of the wall pressure in silo structures. The GB50077-2003 [56] formula for Rankine active earth pressure on the silo wall is inaccurate if the bulk silos assume a conical pile shape. In general, the Rankine formal is based on the assumption that the interface between the hopper walls and the bulk solids is smooth and erect. Various designs are used in the construction of silo hoppers such as multi-hopper, asymmetric cone, non-symmetric cone, wedge with converging end walls, chisel, and transition hoppers as shown in Figure 3[1]. Apart from the shape of the geometry of the hopper, the shape of the floor and ceiling (cylindrical/dome, hemispherical, truncated conical roof, conical and false ceiling) influences structural stability. Non-cylindrical floor plans are associated with the accumulation of unwanted bulk solid materials [58]. Each hopper design form and geometry influences the risk of design-induced structural failure. Even though the design risks should be addressed in the silo design codes, available codes are limited [1]. The British Standard BS EN 1991-4:2006 and Australian Standard AS 3774-1996 are most comprehensive because they address loading/unloading factors for different silo geometries. In contrast, the American Society of Agricultural Engineers ANSI/ASAE EP433 DEC1988 (R2011), and American Concrete Institute ACI 313-97 only focus on the symmetric single cone silo hopper geometry but exclude wedge with vertical end walls and square pyramids. Apart from the exclusion of critical  
Figure 3: (**a–f**) Standard silo hopper designs [1]. information associated with hopper geometries, the silo design codes have wide variability in the column load transfer coefficients. In particular, the load transfer coefficients increase by 75%, 36%, and 7% in the Australian Standard AS 3774-1996, British Standard BS EN 1991-4:2006, and American Society of Agricultural Engineers ANSI/ASAE EP433 DEC1988 (R2011), respectively [24]. Another key limitation is the inability to predict the unloading dynamic effect and overestimation of the vertical silo pressures in the British Standard BS EN 1991-4:200 and underestimation of the vertical silo pressures in the American Society of Agricultural Engineers ANSI/ASAE EP433 DEC1988 (R2011) [24].