If you ask a structural engineer, the answer to what is supporting a building is seldom the finishes, the glazing or the lobby. It’s the columns. The weight of each floor, the furniture on each floor, the weight of each person on each floor, the force of wind on the façade gradually sieve down through the columns to the foundation. If you get the calculation wrong, it doesn’t matter about anything else in the building.
That is the duty of precast concrete columns on many of the UAE’s buildings, and on nearly every project at the design and procurement phase, a question arises: what is the capacity of a particular column, and what regulates that capacity? This guide addresses this question in a straightforward manner based on its structural underpinnings and the practical considerations that impact on it on a UAE project.
Understanding what ‘Load Bearing Capacity’ Actually Means
Specificity is important before considering the factors that contribute to the number. Load-bearing capacity is the greatest axial load (nominal axial load) that a column can carry when applied parallel to its length, which is typically expressed in kilonewtons following metric conventions in the UAE.
In practice, it is nearly impossible to load a column in pure compression. Eccentric loading, wind pressure and frame action all create bending moments as well as axial loads, and structural engineers design the column for a combination of axial load and moment, rather than a single capacity value – this is referred to as a P-M interaction diagram.
Most reinforced concrete design in the UAE is based on ACI 318-19, and the maximum nominal axial compression capacity of a tied or spiral column is given as φPn(max) = φ × reduction factor × [0.85f’c(Ag − Ast) + fy × Ast]. The strength reduction factor φ is 0.65 for tied columns and 0.75 for spiral columns, and the additional reduction factor, due to the fact that no column is ever loaded concentrically, is 0.80 for tied columns and 0.85 for spiral columns. In the real world, capacity is simply the product of four factors: the compressive strength of the concrete, the gross cross sectional area of the column, the area and yield strength of the reinforcement used, and the conservative factor the code uses to reduce the capacity from the theoretical maximum to what is allowed for the real world. An engineer is able to provide much more control over all four these over a precast production than a site pour can offer.
Concrete is the Starting Point of the Book
Typically, the most important factor in column capacity is the concrete strength. In the United States and most other countries around the world, concrete with a strength of 4,000 to 5,000 psi (28 – 34 MPa) is used for the most part in building columns; and concrete of 6,000 to 10,000 psi (41 – 69 MPa) is typically specified for the most heavily loaded columns in high-rise buildings. A higher strength mix allows the same engineer to be used in a smaller cross section, a factor of direct importance in the market where every square metre is a factor of real value.
The structural columns are mostly in the C40 to C60 range, with C40 being used for low and mid-rise applications, and C50 being the industry standard for high-rise frames and industrial structures with heavy loading. KKK Precast does not necessarily use one concrete grade for all projects; the durability requirements are as important as the raw strength of concrete where the Gulf’s coastal exposure is a design consideration.
Steel Reinforcement Does the Work Precast Concrete Columns Can’t
Concrete is strong against compression but weak against tension—just where the steel reinforcement comes in. The overall capacity of the reinforced column is the sum of the capacity of the concrete and the capacity of the reinforcement, and as the capacity of steel is much greater than the capacity of the concrete, a small amount of reinforcement results in a significant overall capacity.
ACI 318-19 prescribes a minimum cover of 1 percent and maximum of 8 percent for the longitudinal reinforcement ratio (the area of the reinforcement divided by the gross cross-section area). The minimum 1 percent is only included to ensure that the section does not have too little steel to resist eccentricity effects, and the maximum 8 percent is primarily to prevent bars from being too close together for ease of concrete placement and proper concrete consolidation around the bars. Reinforcement ratios typically range from 1.5 to 4 percent, above which there is typically little value and increased detailing complexity with increased cost.
Capacity is Directly Proportional to Cross-Section Size Scales
The gross cross sectional area, the Ag term in the ACI formula, scales in directly with concrete strength, thus making dimension of the column one of the easiest levers an engineer has for scaling the capacity of the structure up or down. Typical minimum section for precast columns for UAE building projects is approximately 300mm x 300mm, and for the heaviest of applications, up to approximately 1200mm x 1200mm, with single piece column lengths up to approximately 24 metres depending on transport and handling considerations.
As a general guide of how these variables interact and is not an alternative to a calculation for a particular project, a 400mm x 400mm column cast in C40 concrete with an approximate reinforcement ratio of 2%, is in a safe range of axial loading in the low thousands of kilonewtons, whereas a column with the same reinforcement ratio, but cast in C50 concrete and measuring 600mm x 600mm, is in a very different range of axial loadings. The actual number for any real column would vary based upon slenderness, eccentricity, connection detailing and the load combinations that are relevant to this project, and that is why KKK Precast does not specify a capacity number for a particular size.
In designs where the building height is greater than the span of one precast column, column-to-column splices participate in the load path and are critical to the design as well as the column sections they join.
Slenderness Changes the Equation Precast Concrete Columns
A well-designed tall, slim column has less capacity than a short column of the same cross section and concrete strength, simply because the mechanics of structures makes this so – it is not a disadvantage of precast construction. This is the slenderness effect (also known as the P-Delta effect), which occurs when lateral deflection is induced by a load applied axially, and the resulting bending moment needs to be considered.
The ACI 318-19 prescribes a slenderness check to be used to assess if these second order effects need to be considered. Generally, short columns in non-sway braced frames with a slenderness ratio lower than about 34 – 12 times the ratio of end moments can be designed without taking into account moment due to lateral deflection. That is why multi-storey precast buildings in UAE mostly combine the precast columns with shear walls or a concrete core to ensure the columns’ lateral stability, maintaining their slenderness ratio within limits and allowing them to perform their gravity load transporting function effectively.
Why the Connection Matters as Much as the Precast Concrete Columns
A point that surprises developers and project managers is that it is sometimes not the weak link in the system. The connection is. A precast column designed for a certain capacity can only achieve the capacity when the base connection is precisely designed and installed, which can be a grouted sleeve splice, a bolted base plate, or a grouted pocket foundation. If the connection is under-designed relative to the column that it is attached to, then the system can only support as much as the connection can.
That’s why KKK Precast does not design connections solely to attach to a column, but part of the structural system as soon as the column dimensions are determined. A precast supplier’s shop drawings should also be as detailed as the column cross-section and reinforcement, otherwise you should ask a few questions about the supplier.
Prestressed vs Reinforced Columns
Reinforced precast and prestressed columns are used throughout the UAE and selection of the type is more related to the ability of the column to sustain the loads under service conditions than the axial capacity. A prestressed column is formed by prestressing the steel before casting the concrete, thereby putting the concrete into compression before the application of any loading, thus minimizing the likelihood of cracking and enhancing serviceability. That crack resistance is of significant importance in the UAE’s coastal setting because strict control of the cracks is fundamental to preventing chloride ingress, which is the reason for corrosion over time. With extreme loading, however, it is usually more significant the concrete grade and cross section than the distinction between the use of prestressed and conventional detailing.
The UAE Climate and Durability Precast Concrete Columns
Indirectly and significantly, the UAE climate impacts structural capacity as a result of its impact on long-term durability. Inadequate cover, incorrect mix specification or insufficient surface finishing can all expedite deterioration of concrete in the presence of marine air, sulphate bearing soils or repeated thermal cycling in the case of a column. Dubai Municipality’s building regulations have minimum requirements for concrete durability, and require that precast elements are tied at each storey to structural elements or supports in accordance with the relevant structural code. Contingent on these requirements, precast manufacturers must also adhere to minimum requirements for cover depth, chloride resistance and sulphate resistance; the big long term concern to the rated capacity of a column in a coastal environment is reinforcement corrosion.
The Following are Topics to Discuss with Your Structural Engineer
Prior to specifying column capacity, it is important to establish the required design axial load at each column location, the bending moments generated by lateral loads and frame action, the effective height to use when calculating slenderness, the concrete grade to meet structural and durability requirements, the type of column connection at the base and at any inter-storey splice, and the type of frame (braced or unbraced). Your engineer will have confirmed the axial load and moment for each of your columns, at this point a precast manufacturer can optimise the cross section of these columns, the concrete grade, and the ratio of reinforcement in each column to get that capacity in an efficient way.
Final Thoughts on Precast Concrete Columns
In the UAE, the load bearing capacity of a precast concrete column is not a fixed value but rather a design value that can be influenced by several factors such as the grade of concrete used, the cross-section of the column, the ratio of reinforcement, column slenderness and the quality of the connections that tie the system together. All of those factors are in the direct control of a capable structural engineer, and a precast manufacturer that applies them with the same rigour. In its Umm Al Quwain plant, KKK Precast produces its columns based on the same principles: that is, on the project-specific structural drawing, instead of a generic catalogue shape, and therefore the capacity mentioned on the drawing is the actual capacity on site.