Enter your email:

Construction Topics




















Become a FB fan

Construction Network

Trades Hub





1. What Should I Know about Concrete Slabs?
2. What Should I Know about Concrete Walls?
3. How do Tilt-up Walls Work?
4. What Should I Know about Concrete Columns, Piers and Beams?
5. What Should I Know about Precast Concrete?
6. How to Plan for Concrete Stairs?
7. Tricks of the Trade & Rules of Thumb for Concrete Structures:

What Should I Know About Concrete Slabs?

Reinforced concrete slabs are floors that are supported by walls, beams or columns. Other than the tolerance for the floor surfaces, these slabs are quite different than the slab-on-grade floors discussed earlier. Reinforced concrete slabs are structural elements that require great care in design, detailing, and installation. Consider the story about the early years of reinforced concrete, in which the workmen weren’t willing to remove the shores on their first slab. It simply didn’t seem natural that concrete could span that far. The workman thought that they’d remove the shoring and the slab would come crashing done on them. Of course, due to the beauty of reinforced concrete, it didn’t.
A brief study of the types of slabs will help you understand how the slabs actually carry the load. The figure below illustrates five types of slabs. The one way slab is unsupported on two sides by walls or beams. The distance “L” is important as the length of the span. The main reinforcing steel in a one-way slab spans from support to support.

The two-way slab is more complex. The slab is supported on all four (4) sides so part of the load is carried in the “L” direction and part in the “W” direction. If the “L” length is greater that two (2) times the “W” length (long, skinny slab) the slab functions like a one-way slab. The reinforcing in a two-way slab spans in both directions.
This information about different types of slabs can be helpful to a construction supervisor if an Owner is considering a field change (as they so often do). For example, if an Owner requests an additional rectangular opening through a slab for process piping, the Construction Supervisor must usually give an opinion on the spot. The opinion can be much more accurate when understanding if the slab is one-way or two-way and which way the opening should be turned. By the way, always check with the Structural Engineer before making changes in structural reinforced concrete work.
The remainder of the slabs in the figure use columns, rather than beams or walls, for support. The flat slab uses drop panels and column capitals to reduce the stresses at the column-slab intersection. Where spans are not too large, or loads to heavy, a flat pate is used. Its simplicity makes forming more economical. Finally the grid slab (or waffle plate) uses special forms to reduce the amount of concrete required.
Whichever type slab is used, great care must be taken in the forming and shoring considerations. Many construction fatalities have occurred in this area. The process must be thoroughly considered and planned, checked by capable reviewers and carefully executed. With the huge construction loads involved, everything must be done correctly. The forming and shoring of slabs must be completed carefully and correctly.

What Should I Know about Concrete Walls?

Concrete walls have many uses including retaining earth walls, tank walls, shear walls in multi-story buildings, etc. A properly installed cast-in-place concrete wall achieves excellent structural strength, fire resistance and an attractive finish.
The forming of concrete walls is typical labor intensive, always time consuming, and sometimes downright unsafe. Too many accidents happen each year when forming concrete walls. The two most important items in wall forming are the panel form itself and the wall tie.
Panel forms can be built out of plywood and lumber on the job or manufactured panel forms (such as Symons forms) can be used. On large concrete walls, gang forming adds additional efficiency. Large sections of wall forms are put together, lifted in place by the crane, and after the pour the crane moves the gang form to the next pour location.
Wall ties hold the forms on either side of the new concrete wall together. Conventional types of wall ties are shown in Symons website. The Design Professional may approve the type of tie used, but it will often be the Contractor’s responsibility to determine wall tie placement.
Wall forms fail far too often and it is a lack of understanding that usually causes the failure. The pressure on a wall form from a fluid (wet concrete) is simply the product of the depth of the wall and the density of the concrete. Therefore, if a 10 foot high concrete wall were placed very quickly, the pressure at the bottom of the wall form will be
10’ x 150 pounds/cubic foot = 1500 pounds/square foot

And the pressure midway down the wall form will be 5’ x 150 pounds/cubic foot = 750 psf
If the wall was formed with snap ties with a load limit of 2000 pounds, the bottom of the wall would have a snap tie at one foot on center horizontally and vertically. This solution doesn’t seem very practical. What has developed over the years, though, is a method of placing a concrete wall in lifts, so the concrete lift previously placed takes a set (no longer acts like a fluid) before the next lift is placed.
With this method, the concrete vibrator penetrates a maximum of about one foot into the previous concrete lift to establish a bond, but not to the bottom of the form to keep all the concrete plastic (another work for fluid). Considering the ten foot high wall, if one chooses to use three (3) lifts the maximum for pressure will be 3.33’ x 150 lbs/cf = 500 psf, then a wall tie that carries 2000 pounds could be placed every 4 square feet.
You must be careful when forming concrete walls. Tremendous amounts of pressure exist and must be reckoned with. Far too many accidents occur because of shoddy workmanship and not taking the time to think about how the fluid concrete will act. Always consider the concrete a fluid and try to determine which part of the forming system is most likely to fail. Learn to think about failures, and how they may occur, in order to avoid them.

How Do Tilt-up Walls Work?

Tilt-up concrete involves placing concrete as a slab-on-grade, (either on the building floor or a temporary casting slab) and lifting the panel to become a section of wall. Expensive wall forms are replaced with inexpensive slab-on-grade forms. The process has been gaining popularity since World War II and been aided by the development of heavy duty cranes.
Lifting a concrete slab from a horizontal position to a vertical position causes significant additional stresses in the concrete. These considerations must be analyzed by a Structural Engineer and reinforcement and lifting inserts located. Specifically who provides this information varies among projects, but it definitely is not a case where the Construction Supervisor can “wing it”.
In order to get a better understanding of tilt-up concrete go to the Dayton Superior Corporation website to learn about the various products used in the process.

What Should I Know about Concrete Columns, Piers & Beams?

Concrete beams and columns are typically detailed reasonably clearly on the structural drawings. While the Design Professional reviews the reinforcing steel shop drawings for conformance to design, the Construction Supervisor should review the shop drawings to verify that the dimensions are correct and that everything will fit. Reviewing for fit includes plenty of things and the Construction Supervisor may wonder why this checking shouldn’t be someone else’s responsibility. Perhaps it should be. The reality of the industry, though, is that the Construction Supervisor has to put the project together and make it work.

For example, a concrete beam and column intersection may appear quite straight forward on the contract drawings, but the reinforcing steel shop drawings show the bar sizes and splices which at this location allow no room for the concrete between bars. Probably other people should have caught this problem previously, but the Construction Supervisor has to deal, immediately when discovered in the field, with the cost, schedule and performance implications of this problem. In many cases, the Construction Supervisor’s experience allows him to guess at the potential problem areas and review those areas carefully. Incredible amounts of time, money and frustration can be saved by catching the problem on paper rather than in the field. So the Construction Supervisor should carefully review rebar shop drawings for concrete columns, piers and beams, paying special attention to intersecting areas.

Years ago it was common practice for the Structural Engineer to show the reinforcing steel only where it was actually needed in the concrete beam. This practice meant less reinforcing steel used, but more exacting detail of the shapes and placement of the bars that were used. With the rise of labor costs, it became more economical to use more, simpler reinforcement to improve steel tying production. This change means there is more reinforcing steel in splice areas and intersection areas, making the coordination more important.

What Should I Know about Precast Concrete?

Using precast concrete often improves cost and efficiency. With skilled construction labor more scarce, and thus more expensive, the use of factory production measures to build structures becomes more practical. A precast plant can produce column, beams, wall panels, floor units and roof units in standardized shapes and large quantities. The members are then shipped to the project and erected. Specifically, some of the advantages of precast concrete are as follows:

  1. Less skilled labor requirements.
  2. Less overall labor cost per member installed due to mass production techniques.
  3. Controlled production conditions allow maximum use of available construction material (i.e. higher strength concrete and steel, less reinforcing steel clearance requirements, etc.)
  4. Improved construction schedules because the concrete work can be performed off site prior to when it could start on site.
  5. Final quality control less dependent on weather and season.

Of course, these are some disadvantages to precast concrete which include:

  1. Difficulties and costs involved with transportation
  2. Technical and cost aspect of site precast connections 

All things considered, though, precast concrete has some very strong advantages for certain construction projects.

With this in mind, the Construction Supervisor should understand what typical precast products are available. Some people mistakenly believe that a precast plant makes a totally new set of members for each project. Actually the precast industry has developed many standard members, with variations, which are normally specified. All the special requirements and conditions of a particular project are then integrated into those standard members.

One of the most common uses of precast concrete is for floor decks. The hollow core precast decks use high strength prestressed reinforcing and have open cores to lower the weight. To find specific load tables and information about hollow core precast decks, go to:

For other structural elements, including precast Tee decks, precast columns, and precast beams, go to:

As in many areas of construction, the shop drawings are produced by the fabricator and show all the necessary details to fabricate and install the work. The Construction Supervisor must pay particular attention to the precast shop drawings because of the nature of precast (meaning precast members are more difficult to change, cut or add to in the field than steel or wood). To review the precast show drawings with a strong consideration given to the work of the other trades can greatly simplify a job. While it may be easy to place some conduits in the precast form in the casting yard, drilling for that bent conduit after erection can be quite a challenge. It is prudent to give precast shop drawings to other trades to review and make suggestions for working together.

After the shop drawings are approved, fabrication can begin. If the precast is an important element in the construction project, it is worthwhile for the Construction Supervisor to continue to track it through the fabrication stages. This is an area where many people relax and subsequently find themselves in trouble. Typically, fabrication begins with an acceptable agreement between the Precast Supplier and the Construction Supervisor to ship at a certain time. It is easy to allow the “out of sight, out of mind” syndrome to take over (and often it works) but it is too important to leave to chance. Talk with the fabricator as the work progresses, if possible, talk to the actual supervisor of the work. Perhaps visit the plant. In general, make the commitment from the Precast Supplier be understood by the people doing the work. A word of caution here: This type of activity should foster teamwork not create enemies. As in many management tasks, the way it’s done is as important as what is done.

Prior to when the precast members arrive on the job, a plan for erection should be clearly agreed upon. This plan includes the following:

  • The starting crane location
  • The plan for crane moves
  • The ending crane location
  • Frequency of member shipment
  • Truck access to site and traffic control (where trucks go if setting slows)
  • Other trades not working under crane
  • Special setting tolerances or guaranteed openings.

In short, as many details and considerations as possible should be planned. Many over-worked Construction Supervisors will say, “I couldn’t possibly find the time to spend that much time on one item.” My response to that thought is that you must pick the few items that are most critical to the project and make the time to assure their correct completion. I know that every detail on the project is important, but the practice of this technique makes one realize that most of the smaller details get done anyway. This idea is the time management concept of prioritizing

Make sure to have other trades, particularly the Plumbing Contractor, HVAC Contractor, Electrical Contractor, etc. review the precast shop drawings prior to approval.

How to Plan for Concrete Stairs?

Reinforced concrete stairs have a clear use, i.e. carry pedestrian traffic from one level to another. Simple sets of stairs are designed as a one-way slab that is inclined. More complex stairs may have multiple, unsupported landings, radiuses walls, etc.

The first important decision regarding stairs for the Construction Supervisor is when to start the work. Some successful projects start the stairs during the structure erection while others leave the stairs until later in the project. Think carefully how you need to get workers and materials from floor to floor during the various construction phases.

 When thinking about when to schedule the stairs, consider the following:

  1. Flow of people from floor to floor during construction.
  2. Parts of stairs that are a finished product (exposed) that may be damaged in early phases of construction.
  3. Tie-in and scheduling of immediately adjacent areas.

As a final thought on stairs: a reinforced concrete, cantilevered or spiral stairs can be a beautiful piece of craftsmanship. The opportunity to work on such projects with master carpenters is a true privilege. What other business allows such one-time creativity and lasting results?

Tricks of the Trade & Rules of Thumb for Concrete Structures:

  1. Think about concrete fluid pressure (150 pounds/cubic foot) when forming and placing walls.
  2. Review rebar shop drawings for concrete columns, piers and beams, paying special attention to intersecting areas.
  3. Have the Plumbing Contractor, HVAC Contractor, Electrical Contractor, etc. review precast shop drawings prior to approval.