How to Attach Legs to a Concrete Table

Aesthetics usually drive a design, so I suggest you start there.

Q: I am about to start building indoor/outdoor concrete tables with square tubular welded frames.  Can you recommend how best to fix the concrete top to the steel frame?  I’m not sure if it’s best to cast some type of flanged nut into the concrete and bolt the frame to it, or try something else.  If there is some hardware that would provide a simple solution, I’d love to know about it.

A: I like to use stainless steel tee nuts embedded in the concrete. Let’s dive in a little deeper as to why that’s the case.

how to attach legs concrete table

Where To Start With This Classic Challenge

Designing a table and legs involves a number of choices and decisions; those decisions and choices are both structural and aesthetic and are interdependent.

There are a lot of considerations when it comes to attaching legs to a concrete top. Connecting legs to a table is a classic challenge, and solutions to it are seen in all forms of table construction. Different materials require different solutions:

  • Wood legs often require an apron and a stretcher or trestle to keep the legs from wobbling or breaking off if the table is dragged across a floor.
  • Steel legs tend to be simpler, since steel can be bolted or welded to create a much more compact, strong and rigid connection.
  • Concrete is a relatively poor material to make legs out of, since the legs need to be fairly massive to provide the necessary bending stiffness that table legs require, and in order to make a strong and rigid connection to the top.

Factors that affect both include top and leg material (steel, wood, GFRC, etc.) are:

  • aesthetics
  • top thickness
  • leg location
  • leg shape
  • weight limits
  • handling characteristics (will this be moved once or many times)
  • seating location
  • etc.

The Specifics of Attaching

The table’s top thickness depends upon the span and what is providing the strength. This decision influences the structural design of the GFRC top itself, especially if the GFRC is expected to be self-supporting (meaning it’s not sitting on a structural frame). It also affects how the top and legs are connected. Thin tops can’t span long open spaces and don’t provide much strength for an embedded anchor, while thicker tops can (and do). A good compromise between strong legs and a thin top would be to weld the legs to horizontal steel beams that connect the legs together and keep them rigid and strong. The horizontal steel beams form a frame that also fully supports a thin GFRC slab. The slab can simply sit on top of the frame unbonded, or it could be glued to it using construction adhesive.

Alternatively, if the legs of the table are separate and bolted to the GFRC top, then the top must be made to be fully self-supporting. This forces the table top to be thicker, which in turn affects aesthetics. In addition, the top must have stout anchors cast into it where the legs bolt to the top (and the leg design must accommodate such a connection method). Stainless steel tee nuts can be used for this connection.

Tee nuts are usually used in furniture made of plywood or particle board. The tee nut forms a solid anchor within the concrete top where a machine bolt can thread into. Each tee nut should be buried at least 1″ into the concrete.

When embedding the tee nut, grease the nut and the bolt threads (I use Vaseline), then run the greased machine bolt into the nut to keep concrete out of the threads. A good tip is to run the bolt through the nut so about 1/4″ of bare thread extends beyond the base of the tee nut. This will create a pocket inside the concrete and will keep the bolt from bottoming out (and possibly creating a blowout). Use a jig, as shown, to keep the nut and the bolt hole perfectly vertical in the concrete.

Attaching legs to concrete table

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Q: Can I use monofilament fibers in GFRC?

A: No.

First, let’s understand what “monofilament” means. Fibers can be configured in single strands (monofilament) or bundles (fibrillated). Many types of fibers that are sold in fibrillated configuration have fiber bundles that are designed to disperse in the mix – break up into single fiber strands.

GFRC fibers are different. The fiber bundles are meant to stay bundled (about 200 filaments per bundle with 19mm “fibers”). Each bundle is bonded together so that the filaments remain grouped during mixing. Here’s why:

With monofilament fibers, each fiber is exposed to the concrete’s cement paste and is well bonded to the matrix. While this seems ideal, it creates a composite that results in stiffer, less elastic material.

monofilament-fibers-for-concrete

Monofilament Fibers (image courtesy of www.dmireadymix.com/products/view/reinforcing-fiber)

GFRC fibers, on the other hand, remain in a bundle. This results in a stiffer fiber grouping that maintains the fiber’s orientation even when the overall length gets very long compared to the diameter of the individual fiber filament. Long, stiff fibers (or fiber groupings) are far more effective than long, thin, floppy fibers that fold over and ball up.

fibrillated-fibers-for-concrete

Fibrillated Fibers (not GFRC fibers, image courtesy of www.dmireadymix.com/products/view/reinforcing-fiber)

Fiber Bundle Properties

In addition, the fiber bundles themselves are engineered to create a strong, yet flexible concrete. The spacing between each filament is about 0.1 microns, or 1/10,000,000 of a meter. That’s 1000 times smaller than the average diameter of a human hair! The spacing between the filaments is too small to let cement particles penetrate, but it is large enough to let the polymer enter.

Capillary action draws water and polymer into the dry fibers as they’re mixed into the concrete. By filling the spaces between the filaments with polymer, two things are achieved:

  1. The polymer blocks any mineral growth from entering the fiber bundle.
  2. The stretchy polymer bonds the inner filaments to the filaments at the outside of the bundle.

Only the outside filaments are in contact with the concrete matrix, but the inner filaments are not. This is important because:

  • If the spaces between the filaments were to become filled with rigid crystalline mineral deposits (which occurs during concrete hydration over time), all of the filaments would be rigidly bonded together and the fiber would act as a single, thick fiber bar.
  • Rather, the stretchy polymer allows the inner filaments to move relative to the outer filaments, yet still bear some of the load.

In a way, the polymer and glass fiber filaments create mini bungee cords within the concrete, giving GFRC great flexibility.

The Importance of Surface Area

As explained above, the surface area of fiber which is exposed to the cement paste is important. This is true for a number of concrete ingredients. For example, the surface area of pigment and how well it disperses in the mix are important factors in creating the final color. Surface area is an important concept that plays a role in mix design and adjustments.

Other Considerations about Fibers

Please note, this article discusses only the difference between monofilament and fibrillated fibers. There are many other important characteristics relevant to fibers. For example, depending on the material the fiber is made of, the fiber may be hydrophobic (repel water and therefore have no effect on mix water requirements) or hydrophilic (attract water and therefore affect mix water requirements).

Here are some good articles by Concrete Construction magazine about the broader topic of various fiber chemistries and configurations. While they don’t cover GFRC, they give a good overview of other types of fibers.

 

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