All structures above ground are supported by that which is underground. In the UAE, that foundation is typically a precast reinforced isolated footing for light industrial buildings, warehouses, solar farms, transformers and light pole installations.
Let’s dive into the details of what these footings are, how they are engineered and why they are now the preferred means of delivery for precast footings at job sites throughout the UAE.
Why Precast Concrete Reinforced Isolated Footings Leads in the First Place in This Category
Isolated footings support a single column load down to the soil and obtaining the correct load transfer is crucial.
Cast in place footings rely on conditions on the day of the pour — accuracy of the formwork, curing temperature, consolidation of the concrete around the rebar cage. Precast isolated footings eliminate that variation by moving the pour to a manufacturing plant.
Improved dimensional tolerances, equal compressive strength (typically in the 4,000 to 5,000 psi range), and a footing that is ready to set versus days of curing prior to load bearing.
That “advantage” in schedule reliability is important in the UAE in summer, when conditions for cast-in-place curing are far from ideal, for a large portion of the year.
What an Isolated Footing Does
An isolated footing (also known as a spread or pad footing) distributes the point load of one column across a larger area of soil so that the bearing pressure stays below a safe limit.
Precast versions are cast in one reinforced concrete pad, either square or rectangular, and the column connection is formed during casting, no additional pour is needed on site.
1. Reinforcement Detailing
It is the steel cage inside an isolated footing that bears the column load bending and punching shear.
The steel cage inside of isolated footings resist the bending and punching shear created by the column load. For precast production, reinforcement mesh, bar spacing, and cover are determined and verified before the pour, not in a trench.
This is important since defects in the footing, such as cracking or settling, often are not detected until years after the construction has been completed — after the footing is buried and inaccessible.
2. Column-to-Footing Connections
It’s at the point where the column actually connects to the footing that most isolated footing designs make or break.
There are usually three types of precast suppliers’ connections:
- Embedded base plates with anchor bolts for steel columns.
- Pocket connections – where the precast or steel column is placed into a pre-formed socket and grouted.
- Protruding starter bars for cast in place columns above.
One of the most frequent coordination issues that occur on UAE sites is selecting the wrong connection type for the superstructure.
3. How Soils React and Affect Footing Size
The dimensions of an isolated footing cannot be given independently of the ground on which it is placed.
They all relate to the geotechnical report; the allowable bearing capacity, groundwater level and soil type all affect the design of the footing.
The high water tables and variable sabkha soils in many places in the coastal regions of the UAE require footing designs to be increased in the bearing area and/or to be deeper than would be required in more stable soils.
4. Precast vs. Cast-in-Place Applications
Even in cases of irregular column layouts or one-off large footings where transport size become impractical, cast-in-place isolated footings still makes sense.
Precast turns into a valuable asset when the same footing size is required dozens or hundreds of times as in warehouses, solar farm racking, transmission and light pole lines.
Precast’s speed advantage continues to multiply on the basis of the standardization of design and repetition of the casting.
5. Where They’re Used
Precast reinforced isolated footings come into view in:
- Pre-engineered steel building columns
- Solar PV racking and tracker posts
- Transformer and switchgear support columns
- Light poles and signage bases
- Perimeter fencing or gate posts
When it comes to a single, reliable point load that must be down fast, isolated footings are more likely to be the first option over a continuous strip or raft foundation.
6. Handling, Transport & Installation
The best footing in the yard is not the best on the site.
In production, lifting points are cast in and are rated according to the weight of the unit and not improvising a lift on the wrong lifting point with chains or straps may cause the pad to crack before it is even loaded.
On-site, the footing should have a thin layer of blinding concrete or granular fill, which is compacted and leveled, so that the footing bottom surface contacts the soil evenly and completely.
The Standards That Tie This All Together Precast Concrete Reinforced Isolated Footings
In the United Arab Emirates precast reinforced isolated footings are mostly designed according to ACI 318 for structural concrete design and ACI 336 contains specific guidance on shallow foundations.
Reinforcement detailing is according to ASTM A615 or ASTM A706 for rebar and the final products are verified against the geotechnical report and building code of the concerned municipality, which could be Dubai Municipality, Department of Municipalities and Transport, Abu Dhabi or other municipality.
UAE suppliers, like most precast underground and foundation products, do not always use one code, but instead incorporate American and local municipal codes.
The Concrete is the Easy Part, Coordination Is the Real Challenge
The footing is a standard element that is well known and well documented.
Co-ordination, namely matching the footing size and type to the structural engineers’ column design, ensuring that geotechnical assumptions regarding bearing capacity are correct in the field and ensuring that the lifting and transport arrangements are planned before the units are cast, is what really throws a wrench into schedules.
Let’s say you have dozens or hundreds of repetitive footings on a site and a design error is discovered in early stages of construction, you will save a revision; if it is discovered later in construction, you will save a re-pour.
The Bottom Line on Precast Concrete Reinforced Isolated Footings
Precast reinforced isolated footings have both the quality control of the factory and the speed that a repetitive column grid requires.
The trick to getting the right reinforcement, connection type and sizing against the geotechnical report is to get them right—right the first time.
If a project is designed with repeated columns, it’s a good idea to ask a supplier early in the process how they size the columns and detail the connections for each of the different types of soils, before the first unit is cast.
