C-Store Design

C-Store Design:  A Blend of Structural Steel and CMU Construction

C-Store Structure:


The type of foundation utilized for C-Stores is dependent upon the results of the site-specific geotechnical report, however due to the light loading from the superstructure the foundation will not necessitate deep foundations (drilled piers, auger cast piles, etc.). For foundations on non-expansive soil, continuous spread footings with a non-structural slab are commong. For foundations on expansive soils, stiffened slab-on-ground foundations are typically utilized since the controlling loading will be the active soil and not the superstructure loading.


Typical C-Stores are constructed with CMU (concrete masonry unit) walls and structural steel open-web roof joists. The CMU walls serve as the load bearing element for both gravity loading (dead, live, snow, wind uplift, etc.) and as shear walls for lateral loading (wind, seismic). Open-web steel roof joists are a practical option for the roof framing due to their truss configuration which allows them to span large distances at a relatively low cost. The front of the C-Store is typically framed with structural steel columns and beams due to the large storefront windows that preclude the use of CMU walls.

Fuel Tanks:

Fuel tanks present a special condition for C-Store design as they can become buoyant during flooding events. For example, the self-weight of a 20,000 gallon fuel tank is approximately 10,000 pounds. Assuming that the soil has become saturated and the fuel tank is empty, the buoyant force will be approximately 165,000 pounds. To overcome this buoyant force, deadmen anchors are employed  to provide additional restraint.












Fuel Canopy

The fuel canopy structure typically consists of HSS (Hollow-Structural Section) steel columns with “carry” wide-flange steel  beams directly above spanning the short dimension of the canopy which then support the “purlin” wide-flange steel beam spanning the short direction. The metal roof deck is fastened to the soffit (bottom) of the steel beams. The HSS columns are typically founded on deep foundation elements.


Construction Photos

Beam Pockets for Open-Web Steel Roof Joists

C-Store Owners: Timewise-Landmark, Buc-ee’s, Stripes

Composite Structural Steel Beams and Deck

Structural Behavior:

Composite construction refers to two load-carrying structural members that are integrally connected and deflect as a single unit. For composite beams, the two load carrying members are the structural steel beam and the concrete on composite metal deck with the shear studs being the element that connects them.

Utilizing composite action creates a stiffer, lighter and less expensive structure than if the two elements were not integrally connected and makes this system one of the choice options for commercial construction.

Composite Deck:

Typically accompanying composite steel beams is composite deck. Composite deck utilizes the steel deck and the concrete slab to form an integral unit that plays upon the concretes compressive strength and the steel decks high tensile strength. The element that integrally connects these two components are the steel embossment in the metal deck.

Advantages of Composite Construction

  • Reduced structural steel frame cost compared to non-composite steel construction.
  • Reduction in time and labor cost due to composite deck serving as both the form deck (which in most cases does not require shoring) and the positive reinforcement in the final structure.
  • Compared to cast-in-place construction which requires shoring and re-shoring, composite construction can drastically reduce the construction schedule.
  • Reduction in weight of structural steel frame which also can lead to a less costly foundation.
  • Reduced live load deflection and improved vibration performance due the composite construction being stiffer than comparable systems.
  • Potential for shallower beams which can reduce building height.
  • Increased span lengths are possible.

Disadvantage of Composite Construction

  • Material cost typically higher compared to cast-in-place concrete systems.
  • Installation of shear connectors requires specialized equipment (automatic stud welders) which typically mean having to bring on a speciality sub-contractor.
  • Introduction of camber can create issues with concrete levelness and finishing.

Shear Stud Installation:

Limiting Concrete Cracks: Re-Entrant Corners

Whether you are a contractor, architect, engineer or simply a home owner with a concrete foundation you have most likely heard the adage that “all concrete cracks”. While this is true, there are measures that can be taken to greatly reduce the magnitude and frequency of cracks. To do this, one must have an understanding of the factors that lead to cracking. One such factor is stress concentrations in the concrete which are evident at re-entrant corners. Re-entrant corners are defined as any inside corner that forms an angle of 180° or less. In a solid object that is subjected to internal or external loads, re-entrant corners create high stress concentrations. If that solid object is concrete, which is strong in compression but weak in tension, then it will inevitably lead to a crack that will propagate at approximately 135° from the corner. Re-entrant corner cracks are especially prevalent in concrete slabs that are relatively thin in comparison to their plan size. In this article, I will focus on re-entrant corners in slabs-on-grade.

Examples of loads that can induce stress in concrete slabs include:

Shrinkage of the slab during the curing process when the concrete will shrink in volume as the chemical reaction between the cement and water takes place. Depending on the curing methods in place, the top and bottom surface of the slab will cure at different rates which induces stress in the slab.
Temperature changes. As with all materials, when concrete increases in temperature it will expand and when it decreases in temperature it will shrink. This expansion/shrinkage induces stress in the slab due to restraints such as friction with the bottom of the slab, stiffening ribs, piers etc.
External loads such as additional material or assemblies placed on top of the slab.

There are a number of measures that can be utilized to control re-entrant corner cracks including:

Contraction (Control) Joints: Place contraction joints at the re-entrant corner to create weak planes in the slab that will increase the possibility of cracks forming in the bottom of these contraction joints rather than at ~135° from the corner. Contraction joints can be formed by tooling the joints while the concrete is still plastic or with a saw after the concrete has set. It is important that contraction joints are placed as soon as possible before re-entrant corner cracks begin to form.
Construction Joints: Placing a construction joint 90° to the interior corner eliminates the re-entrant corner and thus the stress concentration.
Wet Curing. Wet curing of the slab will slow down the curing process and will create a more uniform cure rate between the top and bottom of the slab. This has the effect of reducing but not eliminating internal stresses. Wet curing can be accomplished by ponding the slab, utilizing foggers to maintain a humid environment on the slab, or by applying a chemical curing compound. In my experience, ponding of the slab is the most effective means of wet curing however it is typically the least practical.

Water to Cement Ratio: The primary ingredients in concrete are cement, water, fine aggregate and course aggregate. The water chemically reacts with the cement to bind the aggregate in a solid matrix. To fully hydrate cement, a water to cement ratio (w/c ) of 0.26 is required. Additional (free) water is added to the mix to increase the workability of the concrete. As more free water is added to the mix, it increases the shrinkage of the concrete because the free water will eventually evaporate out of the concrete. For our slab-on-grade design, Dudley Engineering typically specifies a maximum w/c ratio of 0.45. Additional workability can be achieved by adding water-reducing admixtures or superplasticizers to the mix.
Fly Ash: Fly ash is a recycled material that can be utilized in limited quantities to replace cement. Replacing a small portion of the cement with fly ash can have the benefit of reducing the expansion of the concrete during curing.
Concrete Additives: There are chemical admixtures which can be added to the concrete mix that reduce the shrinkage rate of the concrete. Recently, on a post-tensioned slab-on-grade foundation that was intended to remain exposed, Dudley Engineering specified a shrinkage-reducing admixture in the concrete. This, along with other measures listed above, has produced a slab that is showing no signs of visible cracks.
To have a slab-on-grade foundation that is relatively crack free even at re-entrant corners, a combination of the solutions addressed above should be utilized. In addition to having a more aesthetic slab, it will also exhibit better structural performance throughout the life of the structure.