Double Tees vs Hollow core
When it comes to long spanning capabilities for commercial projects, it’s very difficult to beat the economy and performance of double tees and hollow core planks, especially where strength and serviceability requirements are critical.
Although both products are very well understood from a design perspective; and decades of development and refinement have brought their production down to near-commodity standards, many precasters engage in debates of the pros and cons of both systems.
The truth is that while both products perform the same basic function, and have similar design methods, they are intrinsically very different from each other in terms of application, production method and detailing among other things.
Double tees are characterized by a very wide flange in standard widths of 8,10,12 and 15 feet, connected to a pair of vertical stems usually up to 3 feet in depth, spaced between 4 and 6 feet apart; whereas hollow core planks are typically either 4ft or 8 ft wide in depths up to 16 inches. Both products trace their roots back to the early 1960’s when there was a great surge in the development of early precast technology.
Production methods are very different – Hollow core is produced primarily using a dry-mix extrusion or slip forming process that doesn’t typically allow for the addition of reinforcement other than the main strand. Conversely, double tees are produced using a wet cast method in steel forms which allows for the addition of shear reinforcement, weld plates and other inserts. Both products are commonly topped in the field.
*Did you know that the first application of long-span double tees was on a two-story office building in Winter Haven, Florida, designed and built in 1961 by Gene Leedy? Leedy experimented when building his architectural office by using structural elements of prestressed concrete and designing the new “double-tee” structural elements. In their early days, the applications of double tees were limited to multi-story car park structures and roof structures of buildings, but they have now been used in highway structures as well, for example the recent development of the New England Extreme Tee.*
Perhaps the primary attributes to consider when comparing such elements is span-to-depth ratio and spanning capability. While both elements can operate in similar span ranges (up to 80 feet, typically), double Tee’s have the edge in terms of spanning capability. This is because of their expanded form factor which affords them much higher moment of inertia (stiffness) per unit cross sectional area compared to hollow core. The drawback is of course the exposed stems and increased member depth can be inconvenient in some structures. For instance, double tees are normally specified with span-to-depth ratios up to L/30 while hollow core is commonly up to L/50. This means that in the same span range, hollow core planks will span the same distance with a much shallower element; producing significant savings in structural depth.
For equal common service load comparisons of standard double tees and hollow core cross sections; an analysis of common key performance indicators (KPIs) related to production economy yield the following findings:
Production Labor Per SF
Due to its simpler operational cycle, it’s no surprise that hollow core has the edge when it comes to labor costs during manufacturing. Hollow core typically costs between 0.002 and 0.005 man-hours per square foot to produce compared to double tees at between 0.015 to 0.03 man-hours per square foot which is a staggering difference of about 500%.
Concrete Volume Per SF
Interestingly, hollow core and double tees consume a strikingly similar volume of concrete per unit plan area; typically, in the range of 0.015 to 0.020 cubic yards per square foot. Although if spans are equal, double tees may have a slight edge, especially in the wider size ranges.
Reinforcement Density Per SF
At first glance, hollow core seems to be very lean on reinforcement, only consuming the prestressing strand required for strength. In most common hollow core systems, there is no need/possibility to add top mats or shear reinforcement like what is used in the manufacturing of double tees. This can be seen as a drawback against hollow core in some cases where higher shear is needed. On the basis of strand alone, double tees might use about 50% less strand per cubic foot of concrete than hollow core, but the additional mild reinforcement in double tees offsets this advantage. The result is that hollow core will consume up to about 4 lbs of strand per cubic foot of concrete, whereas double tees can consume up to 5 lbs or more in total reinforcement per cubic foot, half of which is strand.
During transportation and erection, double tees certainly pose greater logistical challenges than hollow core. Double tees are mostly shipped as single elements with the wider and longer varieties requiring special permits. This means that basic transport costs can be easily between 2 to 5 times more for double tees per square foot.
When erecting. Larger cranes are needed for double tees with larger pieces reaching 50,000 lbs and over, while hollow core planks of the largest varieties rarely exceed 15,000 lbs for 4 foot wide planks. In such cases, the economies of crane cost and area per-pick need to be weighed.
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