HHA values the talent and expertise of our staff, and are dedicated to sharing our knowledge with our colleagues, clients, and our community. Here you’ll find research, case studies, opinions that drive our daily design decisions and what we have been up to lately.
BY JUSTIN PECHAR, P.E., S.E.,
HHA Director of Structural Engineering
Concrete slab on metal deck floor systems is one of the most popular floor systems in the country, especially here in the northeast where structural steel buildings tend to be more prevalent than reinforced concrete. When combined with composite beams and girders, these floor systems are economical, strong, and stiff, eliminating most walking induced vibration issues.
Almost all these floor systems are constructed unshored, meaning the floor beams and girders are not shored from below during the concrete placement. Most flatwork contractors and erectors are not set up to provide this extensive amount of shoring. Furthermore, the shoring adds additional cost and interferes with the construction activities below. Therefore, the steel beams and girders are usually designed to support the weight of the wet concrete and remain stiff enough, so they do not deflect excessively during the concrete placement. If the beams and girders are not stiff enough, this can lead to problems during the concrete placement such as concrete ponding in the bay, excessive concrete usage to compensate for the deflected structure, or possible collapse from overload.
So how stiff should the members be?
A great rule of thumb for the beams and girders is to limit their deflection alone during concrete placement to ¾ of an inch maximum. This is a deflection limit that is checked under what is known as the precomposite loading condition and includes the weight of the plastic concrete (i.e. wet concrete weight), the beam or girder self-weight, and a 20 psf live load to account for the construction activities on the deck during the slab on deck placement. Holding this deflection limit will usually lead to beams and girders that are stiff enough to avoid deflection problems during concrete placement.
Another check that should be done in conjunction with this is the deflection across the diagonal of the bay. This check takes into account the composite or combined deflection of the beams and joists acting together as a system. This deflection should be held to less than the diagonal length divided by 360. It is important to make sure the computer software you are using takes into account the combined deflection of the girder and beam to make this check accurate. If it becomes too difficult to maintain these limits and keep the beam depth within what is allowed for the project, then the designer can look to cambered beams to offset the deflections.
However, our experience with cambered beams is they can lead to a host of other problems such as being over cambered or under cambered. Cambering is not an exact science, and the beams will tend to lose some of their camber or relax from the time they leave the fabricator’s shop to the time they arrive on-site, so cambering should be used as a last resort.
The nice part that can come with all of this is that when a beam or girder is sized for stiffness following these precomposite deflection criteria (and also verify the beams or girders are also properly braced against lateral torsional buckling), the designer will often find that they get the strength for free. In other words, the beam or girder depth and weight selected to provide the required stiffness will usually lead to a beam that is plenty strong to support the floor (or roof) dead and live loads. Most importantly, however, when coupled with the proper metal deck selection and design, the design will result in safer more efficient slab on deck placement and a final concrete volume that is very close to what the flatwork contractor estimated in his bid.
Justin Pechar, P.E., S.E., is the Director of Structural Engineering for Hyman Hayes Associates. With over 20 years of experience in the field of structural engineering, Justin has expertise in the design and detailing of steel and masonry buildings, forensic structural investigation, and troubleshooting existing structures. He has diverse project experience including work in healthcare, higher ed, K-12, technology and municipal projects. Justin is passionate about promoting the field of structural engineering and the innovation thereof, through practice, mentoring, and presentation opportunities.