A composite action beam may have a concrete section the top, carrying the compression load, and steel at the bottom, carrying tension.

But I have always thought that concrete had lower strength for a given mass then steel, compressive or otherwise. So why do engineers build steel concrete composite action floor systems at all? How could it possibly save weight?

To clarify; What are the practically significant advantages of s composite beam of steel and concrete over one made of purr steel?

  • 3
    $\begingroup$ Have you done any research on cost? Also define “better”... $\endgroup$
    – Solar Mike
    Commented Apr 15, 2021 at 8:46
  • $\begingroup$ @SolarMike yes. I keep getting directed to pages on how composite is stronger than concrete. Waste of time $\endgroup$ Commented Apr 15, 2021 at 9:29

5 Answers 5


Some other answer have touched upon this, but I think it needs to be made explicit:

Your mistake is in thinking that civil engineering is about making the lightest structure possible. It's not. Instead, it's about making the most cost-effective structure possible.

Give me a material that weighs a billion tons per cubic centimeter but is cheap enough to let me build my bridge for half the cost of concrete and/or steel, and I'll use that other material instead.

Sure, it's heavier, but it's just strong enough to overcome that disadvantage (i.e. the increase in deadweight) while making the bridge cheaper to build (and maintain), and that's all I care about.

So, is concrete weaker than steel, and does it therefore require much more mass to resist the same load? Yes, absolutely.

But concrete is also drastically cheaper than steel, so who cares about how much it weighs? Concrete is really good in compression (even if not as good as steel), so if we put the concrete there, we can use less steel and therefore get a cheaper bridge.


The most important advantage of reinforced concrete over pure steel structure is availability. Many countries/regions on earth rely on imports for general goods production needs, the cost of steel can run prohibitively high compared to concrete.

The next advantage is weight. In certain types of structure weight counts, such as underwater powerhouse. However, for most buildings, weight is seen as a disadvantage. The last I can think of is better fire protection, without needing separate fireproofing material.

BlackSmith has raised an interesting point - shapes. The many different shapes of steel have their disadvantages:

  1. Weaker in compression for buckling concerns.

  2. Weaker in resisting torsion.

  3. Escalate construction costs for fabrication, transporting (extra-long, wide elements navigating through crowded city centers), and erection (especially connections). Compared to reinforced concrete structures, the construction of steel structures often involves highly skilled laborers, and/or speciality trades. On the other hand, the reinforced concrete most often can settle on semi-skilled laborers.

Durability, longevity, and low maintenance - in certain environments such as water or oceanfront, corrosive environment, concrete structures are often outperformed steel structures. It is easier to maintain and repair, or restore to its original conditions.

Although the list of reinforced concrete advantages may get longer, we have to recongnize its main disadvantages that have greatly limited its usefulness in certain types of structures - self-weight, inflexibility (compared to structural steel), and its brittle nature. But it will always be here to stay for situations that can be benefitted from its advantages.

  • $\begingroup$ Weight? I fail to understand how concrete would reduce weight, unless you are somehow limited by density rather than strength $\endgroup$ Commented Apr 15, 2021 at 12:55
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    $\begingroup$ @Abdullah Read carefully, I stated that self-weight is important for hydraulic structures, and structures subject to buoyancy. Not reduce. $\endgroup$
    – r13
    Commented Apr 15, 2021 at 13:05
  • $\begingroup$ Also mass can haver other advantage IE. vibration dampening. Also concrete can have different adhesion properties and friction properties. Not too mention ease of transportability. There is a time and place where different materials will out perform each other. $\endgroup$
    – Forward Ed
    Commented Jun 22 at 20:40

I will make a slight add to the answers given here already, as follows.

Take the case of a bridge. When loaded, some parts of it will be in compression, other parts will be in tension. The designer's job is to manage the stress levels in the most cost-effective manner. If a certain amount of concrete is capable of handling the compressive stresses less expensively than using steel, then that portion of the structure will be made of concrete. If a certain amount of steel is capable of handling the tensile stresses less expensively than concrete, then that portion of the structure will be made of steel.

The natural outcome of experience building a lot of steel-and-concrete bridges will be mostly concrete in some parts of the structure and mostly steel in others.


A partial listing ( in addition to the previous answer) ; for reinforced concrete only simple generic shapes are needed ( bars) while steel only needs "I" , "H" beams, gussets, fasteners, welding . Careful inventory control of the previous steel items. Coating of these shapes to reduce rust during construction. Inspection to verify the correct shapes are installed properly. Most sections will be prefabricated at a remote location. And other factors.


I think I have figured it out.

The transfer of loads in a floor system has 2 main stages.

  1. Transfer of load from any random point on the slab to the beams. This has to be accompished by bending and shear in the slab.
  2. Transfer of loads down the beams to the columns. This has to be accomplished through bending and shear in the beams.
  • Concrete, due to it's low density, creates a thick slab for a given weight, making the slab more efficient at absorbing the bending moments at stage 1.

  • Rigidly attaching the concrete to the beams with shear studs synergetically works to

    • absorb some of the compression load due to beam bending, that would otherwise have to be carried by the beam's top flange and
    • prevent any part of the concrete slab from being in tension due to bending, thus increasing the bending strength of the slab.

The above combines with the advantages stated in other answers to make concrete-steel composite floors a cheap, practical, lightweight solution.


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