Digging deeper into thermal expansion

Recently, I wrote about plumbing pipe thermal expansion and expansion fittings. Based upon feedback from some in the design community, there seems to be a feeling that more design professionals within the consulting industry need to have a better foundation to enhance their understanding of the topic.  

As undergraduate engineers, this topic is very briefly discussed and only from a materials standpoint. However, very few — if any — practical applications are presented or discussed that would suggest types of mitigation to address this physical phenomenon within the various systems.

As engineers, we are all exposed to the concept that all materials undergo dimensional changes due to temperature variations. The amount of change depends on the material characteristics (linear coefficient of thermal expansion or contraction) and thermal (temperature) changes. In other words, when the temperature increases, the material grows or expands; and as the temperature decreases, the material shrinks or contracts.

Provided the thermal properties of the various materials within or surrounding the systems are relatively the same; there is little concern or consideration regarding any substantial impact on the overall building or its internal systems. But every material has a different rate of thermal expansion. Additionally, the temperature gradient between the exterior of the building and the internal systems that may be enclosed by the building envelope can differ greatly. This results in the mixture of materials expanding or contracting differently than other materials in close proximity; resulting in the need to compensate for these differences.

As today’s building materials may have greater linear coefficients than the “natural” materials used in the past, we need to be aware of the potential issues that these differing coefficients may cause. The issue documented in the previous article was magnified by the installation conditions being in the “coldest” period of the year and the conditioning of the interior space once the building envelope was enclosed. Hence, the piping systems were not simply dealing with operational thermal changes, but had the added stress of adjusting to the changed environmental conditions. The designer would normally have been safe using PVC for a one- or two-family DWV (Drainage, Waste, & Vent) installation. But as the photographs showed, that was not the case for a four-story structure.

When piping is restrained between two fixed points (anchor points) the piping material will be subjected to compressive stress upon a thermal rise or tensile stress on a thermal decrease. Generally, the piping material can withstand these stresses, at least within reason. But failures frequently occur at pipe joints or fittings when the piping cannot move freely. It is between these retraining points that one must provide expansion loops (a specific type of offset) and expansion joints.

Expansion loops are the preferred means of addressing these stresses as the piping material is considered a continuous and integral single piece of material. However, loops and offset require “space” to accommodate their placement. The other means is to provide a manufactured expansion joint; the packed slip type depends on slipping or sliding to accommodate movement and requires an elastomeric seal with packing and lubrication. The other is the packless bellows-type which consists of a tin-wall convoluted section that allows movement by bending or flexing. Regardless of the type of expansion joint, adequate access for service and maintenance is required. There is a greater potential for leakage or failure with these joints than with loops or offsets.

Piping systems are contained within the building structure, which acts as a constraint. The building components “restrict” the linear changes of the piping material between fixed points. As one should have noticed in the previous article, vertical stacks had branch lines, these branch lines had their movement restricted by “holes” drilled through wall studs. Once the linear expansion grew sufficiently to “push” the branch lines pipe wall tight to the stud material, the resulting strain at the fittings or joints resulted in damage to the system.

Generally speaking, polymer or thermoplastic piping expands and contracts at a much greater rate than metallic pipe materials. The changes with these materials can be up to 10 times faster than the more conventional metallic piping   

The DWV systems generally represent the greatest concern as they are rigid and larger. These systems have many restraint points; fixture connections, branch lines from the vertical to the horizontal with minimal allowance for movement. While a typical floor height can generally handle the linear changes within that floor, it is the sum of the floors that can create the greater problem as the expansion/contraction becomes additive. While water piping is generally smaller and less constrained as the thermal growth can be compensated within the piping runs as they have many offsets or sufficient flexibility in the piping runs to “snake” the runs by flexing the material. But one needs to be aware and have concerns for the larger main risers with their associated floor branches. One does not want to transfer the stress from the risers thermal change to the piping locked into position at the floor level. Remember, frequent failures occur at joints or fittings, Hence offsets or changes in direction need to be utilized to minimize the transfer of these stresses to the fitting or joint and the riser/floor branch intersection.

So how does one go about compensating for these differences in thermal expansion and contraction in order to keep everything in the same relative position with its surroundings? Experienced and seasoned engineers have learned how to apply offsets, expansion loops, expansion joints, anchor points, guides, thrust blocks, etc. The idea is to contain and control the thermal changes between fixed points. Based on engineering studies, restraining piping every 30-feet to prevent movement provides for a satisfactory installation. It is between these “fixed” or restrain points that the thermal change now needs to be addressed by piping layout (loops or offsets) or by manufactured expansion joints.

As noted, the loops or offsets are the preferred means of control and are generally limited to absorbing a maximum of 1 1/2 inches of movement for metallic piping materials. Thus, by anchoring at points on the length of run that produces 1 1/2 inches of movement and placing the expansion loop or joint midway between these anchors, the maximum movement that must be accommodated is limited to 3/4 of an inch. The piping configuration used can be in the form of a “U” bend, a single-elbow offset, a two-elbow offset or a three-, five-, or six-elbow swing loop. In many, if not must piping systems, the loop or joint can be eliminated by taking advantage of the changes in direction typically required by the piping layout.

This all goes back to having an understanding of your material selection along with an understanding of the characteristics of those selected materials. If your system layout is such that thermal movement can be absorbed within that layout by offsets, changes in direction, etc., you have a good design. However, one must work within the limits and constraints established by the structure and in coordination with the other trades, so we may have to use loops or expansion joints. We, as design professionals, must realize the potential for thermal growth and address it before it becomes a problem in the field.

The thermal expansion has pushed up the fire caulk that had been place around the stack to fill in the floor assembly penetration. And as a side note, the fire stopping would appear to not meet the minimum requirements of the code. This, in my judgment, indicates a lack of craftsmanship and quality control.

As one can see from this single example, designers and engineers must consider thermal expansion and contraction during the design process. Failure to do our due diligence can have costly effects. Not only do we need to consider the operational changes that result from thermal expansion and contraction; we must also be aware of those same characteristics as they relate to the changes that occur between the installation and the building operational temperatures. The materials do not know the difference between the installation or operational phases nor do they care.

So as designers and engineers, we need to do our due diligence as it relates to thermal expansion and contraction effect on our designs. Regardless of the materials that we use for the “Basis-of-Design,” thermal characteristics need to be evaluated. And where necessary means must be incorporated into the design to compensate for those thermal changes, both installation and operation. And when a contractor decides to use an alternate or optional material, require that they make the engineering design changes necessary to assure proper operation. They may determine that the cost savings they think they have is nonexistent, once the redesign costs are included in the cost evaluation.

It is the responsibility of the Engineer-Of-Record (EOR) to protect the public health, safety and welfare of the building while looking out for the owners’ and/or firm’s interests. A design and/or installation as shown in the above photographs fail to meet those responsibilities. The designers and engineers that work under a professional engineer’s responsible charge need to understand the material characteristics and environmental conditions that impact those materials. They also need to understand the environmental conditions that can impact the construction process.

The construction team is responsible for the “means and methods” of the construction process. While the project specifications may allow the contractor some latitude in material selection, the “Basis-of-Design” is the standard that any options or alternates the contractor proposes must meet. It is the contractors’ responsibility to make any and all changes to the designs that are necessary should they deviate from the “Basis-of-Design.” And those proposed design changes must be accepted and approved by the EOR, who is responsible for the design that contains their signature. If the contractor is not capable or qualified to alter the design, then they must obtain those services at their cost.

It is not the owners’ nor the design team who is responsible for costs associated with the contractors’ decision to utilize other than “Basis-of-Design” materials or proposed alternates; the contractor must take that into consideration when offering the change. When a contractor proposes to go outside of the “Basis-of-Design,” they need to understand the full implications of that decision and adequately consider all of the costs associated with the decision.