If you ask any structural engineer which thing is crucial in making a building stand, they will not answer the lobby, the windows or the finishes. They will declare the columns. They’re responsible for supporting all the weight of all the floors stacked on top of them, all the live loads, all the dead loads and wind.
Today, precast concrete columns are used in some of the world’s toughest construction projects in the UAE. However, when it comes to design and procurement, a common question that always arises is what is the actual load bearing capability of these columns, and what controls that capability?
This guide provides an answer that is easy to understand – based on the actual principles of structure, verified data from industry and context that is relevant to the UAE.
Understanding the Meaning of “Load-Bearing Capacity”
It is good to be precise before jumping to numbers. Load-bearing capacity means the maximum force, parallel to the axis of the column, that it will support without failure applied to a precast concrete column. This is known as the nominal axial load capacity (also known as the load capacity) in metric design, which is widely used in the UAE, and is expressed in kilonewtons (kN).
The columns in practice are seldom designed to support only vertical loads. They are also subjected to bending moments caused by eccentric loading, wind pressure and frame action. Therefore, structural engineers always design the columns for a combined interaction of axial force and bending moment, which is called the P-M interaction diagram.
In ACI 318-19, the maximum nominal axial compression capacity of a reinforced concrete column is determined by:
φPn(max) = φ × reduction × [0.85f’c(Ag − Ast) + fy × Ast]
where φ is the strength reduction factor: 0.65 for tied columns, 0.75 for spiral columns; reduction is the reduction factor; Ag is the gross cross-sectional area of the column; Ast is the area of the smallest cross-sectional area of the column.
Capacity in simple terms is dependent on the cross sectional area of the concrete, the cross sectional area of the reinforcement, the yield strength of the steel, and the total cross sectional area. Precast columns can be better made because all four factors are controlled in a precast factory.
The Starting Point for Capacity is the Concrete
The largest contributor to the column capacity is concrete grade. Precast manufacturers in the UAE employ higher strength concrete mixes than those normally used on site, in part for structural performance, but also because the high water-cement ratio and density of the concrete is critical to the durability of structures in the coastal environment of the Gulf.
The typical range of concrete strengths used in most building columns is 4,000–5,000 psi (28–34 MPa). Strengths between 6,000–10,000 psi (31–70 MPa) are used for high-rise columns with heavy loads. A more expensive concrete with higher strength can be used resulting in smaller cross-sections in the columns, thus restoring floor space.
Precast manufacturers in UAE usually employ concrete grade of C40 to C60 (40-60 MPa) when it comes to making structural columns. C40 is used for low, medium rise structures. For high-rise frames and industrial structures with heavy loads, C50 becomes the standard. A small number of specialist manufacturers have developed high performance concrete mixes for particular purposes in which space is tight and loading very heavy.
This Course Covers the Various Components of a Steel Reinforcement and Its Functions in a Structure
Concrete is strong in compression, and weak in tension. That’s where steel reinforcement comes in to make up for that shortcoming and make the column’s load-carrying capacity vastly greater.
Reinforced concrete column: The total nominal load is the sum of the load that the concrete bears and the load that the steel bears. The yield strength of steel is much higher than concrete, therefore a small amount of steel will significantly boost the strength of a column.
ACI 318-19 specifies the minimum and maximum longitudinal reinforcement ratios, which are the ratio of the steel area to gross cross sectional area, as 1% and 8% respectively. The 1% is a minimum provision to ensure that the column contains sufficient steel to prevent bending in the event of eccentric loading and to limit creep and shrinkage. A maximum of 8% will prevent reinforcement congestion and make it impossible to place and consolidate a concrete properly.
In the industrial field of precast concrete design, the engineer from UAE usually has a target ratio of reinforcement from 1.5 % to 4 %. In most applications a ratio greater than 4% will need careful detailing, and will give an additional material cost without the same proportionate structural advantage.
The Range of Capacities is Determined by the Size of the Column Cross Section
The gross cross sectional area (Ag in the ACI formula) is the actual area that is directly multiplied by the concrete strength to calculate the portion of load sharing for the concrete. This means that, in addition to being one of the simplest, it is also one of the most flexible tools an engineer can use to scale capacity up or down.
The precast concrete columns are used in buildings, single/ multi-storey. The standard minimum section size begins at 300 x 300 mm, while the maximum sections reach 1200 x 1200 mm, and columns can be 24 metres in height, as one piece.
For a sense of the scale, a 400 x 400 mm precast column in C40 concrete, with 2% steel reinforcement, will have a design axial load of between 2,500 and 3,000 kN. The same reinforcement ratio column of C50 concrete moves to the range of 6,000 to 7,500 kN when it is enlarged to 600×600 mm. Indicative design capacities are based on the various factors such as slenderness, eccentricity, connection details and load combinations that are specific to each project.
Gulf Precast produces columns of different shapes and of different requirements such as rectangular, circular and even irregular. Precast columns can be produced more than 5 levels (up to 18 metres) in height in one run.
Column-to-column splices are used when the height of a building exceeds the height of a single precast column to continue the structural system in an upward direction and to join the precast columns properly.
The Effects of Slenderness on Load Capacity
A tall column with a small cross-sectional area (in engineering terms, a slender column) will support a least amount of load as compared to a short column with a large cross section and same grade of concrete. This is not a fault of the precast design, but a structural mechanics principle – the slenderness effect or P-delta effect.
Slenderness check is required in ACI 318-19 to check if second order P delta effects are to be considered. For non-sway (braced) frames, short columns – with a slenderness ratio of less than 34 – 12(M1/M2) – can be designed without magnification. In addition to that, beyond that point the design should consider extra moment due to lateral deflection when subjected to an axial load.
This is why in UAE multi-storey precast buildings structural engineers join precast columns with shear walls or concrete cores. The bracing system provides lateral stability, maintains acceptable slenderness ratio, and enables efficient action of precast columns as gravity load-carrying members instead of lateral force resisting members.
The Importance of Connections in Real Capacity
One of the things that often baffles developers and project managers: the column is usually not the weak link. The connection is.
Studies have shown that joints typically are the weakest parts of prefabricated concrete columns. The weakest link in the structural system and its bearing capacity is the mortar joint, whose bearing capacity limits the bearing capacity of the precast concrete elements. The experimental results indicate that the thicker the joint, the smaller is the bearing capacity of the system.
This discovery could have significant implications for the UAE’s precast sector. A well designed connection (grouted sleeve splices, bolted base plates, or grouted pocket foundations) should have the same design capacity as the column itself. If the joint fails at 2,200 kN, then this is its strength, not the column’s, since it is rated at 3,000 kN.
The equation given in ACI 318 can be used to make safe estimates of the load bearing capacity of a wet connection between reinforced precast columns, in any quantity of recycled aggregate in the concrete mix.
This is understood by leading manufacturers in UAE. Their engineering teams do not make connections as an add-on to the structural system, but as part of it. If the shop drawing is coming from a reliable precast supplier, you should be as detailed as you can be with the details of the connections, as well as the column dimensions.
Prestressed vs Reinforced Precast Columns – Does It Change Capacity?
Reinforced and prestressed are both used in the UAE precast column supply. It’s about behaviour rather than capacity.
Prestressed concrete load bearing column is a column that is prestressed with tensioned steel strands or tendons to place the concrete in compression prior to loading. This will help to prevent cracking and enhance serviceability when loaded. At most precast plants, the prestressing is done as a pre-tensioning of the steel before concrete is cast.
Prestressed columns are more resistant to cracking under service loads as compared to the conventionally reinforced columns of equal dimensions. This is of relevance in the UAE’s aggressive environment, as concrete crack control has a direct impact on the long-term chloride resistance and structural durability. The difference between the two approaches, however, is not as important as concrete grade and cross section under extreme loading for axial load capacity.
The Following Factors May Affect the Long-Term Capacity of the UAE
The climate in the UAE has a further impact on durability, which is an indirect effect on the structural capacity over the life of the building. Inadequate mix design, cover depth and surface finishing may result in deterioration of concrete when exposed to marine air, sulphate-rich soils or temperature cycling.
The Building Code Regulations and the Technical Guide TG-02 of Dubai Municipality prescribe minimum requirements for durability of concrete in construction works in UAE. Dubai Municipality’s Building Code Regulations specify that in the case of precast concrete or other heavy systems, elements shall be firmly tied at their levels by tying them together at the support or directly to the supports as recommended by the applicable code.
In addition to the connecting requirement, precast concrete manufacturers in UAE must meet minimum specifications for the concrete mix, which include the minimum concrete cover to reinforcement, chloride resistance and sulphate resistance. These requirements help ensure long-term load-bearing capacity by avoiding corrosion that causes a loss in strength, a “silent killer” of reinforced concrete structures in coastal areas.
The Following is a List of Key Points to Discuss with Your Structural Engineer
With the structural engineer, the following factors need to be discussed when specifying the capacity:
- The required design axial load for each column location.
- The bending moments due to lateral loads and frame action.
- The effective height for the column to obtain slenderness.
- Required concrete grade (for both design and durability considerations).
- The type of connection at the base and at inter-storey splices.
- Whether the building is a braced or unbraced frame system.
When durability, fire resistance, and load carrying capacity are required for the project, reinforced concrete columns are often chosen. These can be cast in the factory and be produced as precast members depending on the project design and construction schedule, they have consistent quality control that can not be accurately matched in the construction site.
Your engineer will be able to confirm the design axial load and moment at each column, after which a DM-certified precast manufacturer can then carry out cross section, concrete grade and reinforcement ratio optimisation to achieve that capacity, efficiently, economically, and with a guaranteed quality to a documented standard.
Final Thoughts on Precast Concrete Columns
The load bearing capacity of precast concrete columns in UAE does not have a fixed value it is a design value which is dependent on concrete grade, dimension of the cross section, reinforcement ratio, slenderness and the quality of connections. All of these factors are under exact control of a good structural engineer and a competent precast manufacturer.
The UAE’s factory environment lends consistency to that. All columns are produced at a controlled environment at a precast facility that has been certified by DM, with proven concrete mixes, and with production quality control records. This uniformity is the reason why the term load-bearing capacity is a verifiable and accurate figure and not an estimate.
It is not a luxury in a nation developing as rapidly and audaciously as the UAE. It’s the building block of everything else.
