A veteran structural engineer offers expert advice on how to avoid problems with stone cladding and glass/aluminum cladding systems.
A veteran structural engineer offers expert advice on how to avoid problems with stone cladding and glass/aluminum cladding systems.
Over the last 12 years or so, I’ve devoted the majority of my professional work to designing stone cladding and glass/aluminum cladding systems, and I’ve seen plenty of errors that could have been avoided. Here are my recommendations to help make your next cladding project one you—and your client—will be proud of.
DO hire a façade consultant.
While this recommendation may sound self-serving, there is much value that a consultant can add during the design process. Façades have become a specialty system. It is also true that many structural engineers and architects are too busy to devote the time to get into the crucial details of façades. Façade consultants can provide useful feedback to the design team—reviewing the design team’s plans, recommending more efficient and effective products, and assisting in writing the project specifications.
If you’re doing a large project where you’re putting millions of dollars into the façade, hiring a façade consultant for a tiny fraction of the total cost is well worth it. I also recommend getting the façade consultant in at the design phase, to provide input on the buildability of the façade before the job goes out to bid.
A façade consultant can be particularly helpful when you have multiple systems—for example, brick, window, stone, and maybe EIFS. You’ve got four different materials, four different subcontractors, so the complexity goes up exponentially to oversee all these trades and make sure all the critical items are being addressed, especially where they interface.
DO require sole-source façade design (i. e., a cooperating architect and engineer).
Too often cladding systems are designed by several different parties, who pass the responsibility for coordination back to the engineer of record (EOR) or the general contractor. Requiring at least an informal relationship between the firms that perform the exterior design ensures that all the design and coordination responsibility lies with them. They can’t pass off the liability to other cladding firms.
Be sure to include the design of everything from the interior coat of paint outward. I’m beginning to see this requirement more often on large or significant projects, where the architect has written the specs to say that one source is responsible from the paint outward. There are still instances where each company does its own shop drawings, but it is preferable to have all shop drawings done by one firm.
DO employ the cladding engineer and cladding architect as consultants to the design team.
They can review the details of the architect to ensure that the system is buildable and functional. They can ensure that the design-build contracts and specifications have reasonable and necessary provisions that the cladding contractors can achieve.
It’s best to have both an architect and engineer involved. Usually the project architect is interested in thermal and moisture factors and aesthetic issues. The project engineer is trying to coordinate many complex code requirements with the form and appearance that the project architect wishes to achieve. The façade consultants can assist by advising on proven system types, realistic mullion sizes. stone panel sizes and thicknesses, among other issues.
DO expect the engineer of record to detail how the cladding will be attached to the structure.
In the U.S., it is common practice for structural engineers to be more involved in the design of the building’s wind and seismic systems than in that of the cladding system. But the cladding system imposes concentrated loads on the structure, and some EORs are not familiar with the special cladding loads on the structure, and consequently fail to provide the attachment points needed. Here again, early involvement of the façade consultant can provide load information to the EOR to assist in the structural design. Zones of high wind pressure require special consideration, in order to ensure that the cladding system remains intact and safe.
What sometimes happens is that the EOR decides to put a steel angle along the edges of the slabs, without checking how the cladding loads on that edge are going to be handled. We always recommend that the EOR add a few schematic details to show how the cladding should be attached, while performing an analysis showing that this part of the structure will hold the cladding.
I remember a case where vertical channels were being used to support stone cladding. The channels were designed to run from roof to grade, just beyond the edge of the floor slabs, so the space between the face of the building and the edge of the slab was critical. The EOR sized the channels based on the code building wind load, not the code cladding wind load, resulting in the provided space being insufficient. When we designed the cladding we had to add large HSS braces at mid-span of the channel to stiffen up the channels.
DO require that all expansion joints be specified and located on the design drawings.
I’ve seen solid pieces of stone span over one-inch expansion joints in a steel stud system. Stone is impossible to stretch, so it will probably break at those joints when any movement occurs. If the architect and engineer detail the location of the expansion joints, the problem can often be avoided.
My advice: The expansion joint should be specified and designed in at a specific location, so that all the exterior contractors know precisely where to install it.
DO provide adequate space for vertical deflection movement.
Some design drawings show a half-inch for vertical deflection joints, usually the same amount as specified by the engineer for the building structure. This would require the sealant to expand to one inch or compress to zero, but performance like that is unlikely to happen. Provide a location where the deflection joint can be double the design deflection shown by the EOR. In this example, provide a one-inch gap, which would allow the sealant to compress to 50% of the preferred size or expand by 200% of the preferred size, thus meeting manufacturer’s specifications for high-quality sealants. This might require overlapping the window system and the face of the remainder of the cladding, so that the window system can move behind the remainder of the cladding. This would conceal the larger than desired sealant joint.
DO provide enough setting space between the stone and the building structure.
Setting space is the empty space between the back of the stone and the building structure. Sheet metal anchors take more setting space than wire anchors. If sheet metal anchors are preferred, plan for the space that they need.
Here’s why this is important: In one project I worked on, the stone was supposed to attach to the CMU with a sheet metal stone anchor. To do that, you need at least a quarter-inch tolerance for shim anchors, another quarter-inch for the screw head, and an eighth-inch for the sheet metal anchor, so that’s at least five-eighths of an inch, and in some places I had only a quarter-inch of space. As a result, we had to use a less desirable type of anchorage.
Rule of thumb: I like to see one to two inches between the CMU or concrete and the stone. For steel stud systems, provide at least 1½ inches of setting space.
DON’T vary the setting space on building elevations.
Varying the setting space around the building increases the number of anchors required, thus increasing the complexity of the job. Each anchor type is engineered, so this adds to the engineering cost and increases the risk of errors.
As noted above, the setting space should be 1-2 inches. If the building design changes during development, the designers should be aware that varying that distance will mean the cladding contractor will have to use many different type of anchors. Finding good skilled labor is often difficult, so we want to keep the complexity of the installation as low as possible to help the contractor to assemble the system properly. If you keep it simple, you’re more likely to get it done right.
DON’T go overboard with the number of stone types and thicknesses you spec.
The more varieties of stone you use and the greater the number of thicknesses, the more complicated the job becomes, thus adding to engineering and installation costs. Of course, the architect may be using a mix of stones to create a specific effect. Just be aware that you’re adding complexity and multiplying the types of anchors, so it’s more likely that some inexperienced installer will pick up the wrong anchor.
DO keep in mind that drawings are 2-D but cladding is 3-D.
I just received a beautifully detailed design development sketch. All the necessary components were show in the section except the in-plane stiffeners. An isometric sketch or a 3-D model would immediately have shown that the system was stiff and well designed for wind and seismic loads perpendicular to the wall plane, but that there was little strength from side to side. A crucial factor almost got missed!
A BIM or 3D CAD model would have been helpful here in enabling the installers, who might not be good at visualizing in 3D, to understand the design more clearly.
DO ask the façade consultant to review shop drawings and structural cladding calculations.
Catching errors and misunderstandings while they are still on paper is much more cost-effective than changing them in the field. Cladding is usually performed as a design-build function: The architect will draw what he or she wants to see visually, but may leave a lot of details to the stone or glass contractor to figure out. So, just as it is important to get the façade consultant involved in design, the construction phase is also a good time to have the consultant take another look, if only to get another pair of eyes on the drawings and calculations to make sure everything’s workable. Catch the misunderstandings and miscalculations while they’re still on paper, because they are harder to fix after being installed out in the field.
DO conduct a ‘special inspection.’
The International Building Code requires a “special inspection” for certain types of construction, the cost of which falls to the building owner. However, many owners just don’t understand why they need another inspection on top of the ones that the local jurisdiction is already doing. The difference is that the local inspector may walk through the site for 20 minutes perhaps three or four times during construction, while the special inspector is on site more frequently—even, in the case of critical components, during the actual installation.
For example, with some epoxy anchor bolts, you have to make sure the hole has been cleared of dust or you’ll just glue the dust to the bolt. A special inspector will make sure the bolt holes are clean, that the right-size drill has been used, as well as the right epoxy for temperature and moisture conditions. The special inspector might even pull on the fasteners with a testing device. Omitting special inspection reduces the number of people checking the contractor’s work, and may make it easier for mistakes to slip through.
DO have the façade consultant check for common mistakes during the installation process.
Having reviewed the plans and calculations, the façade consultant should be brought in to review the installation to be certain the owner is getting what was contracted for.
What are the common mistakes? The tolerance for cladding may be very tight, maybe a quarter or eighth of an inch, but the tolerance for concrete or CMU may be three-quarters of an inch. The difference is often made up with shims exceeding the thickness specified by the cladding engineer. It’s useful to have a facade consultant review the installation to catch these “field fixes” and make sure they’re still correct.
I saw a project where the stud wall supporting a multi-story window system was cut short by 2-3 inches and shims were used to make up the difference to reach the anchors. However, the installer was not paying attention to the quarter-inch maximum stated by the window system engineer, and filled in with three inches of shims. When this change was caught, the window engineer had to design new anchors to span the distance with the extra shims that were added. Adding the extra shims could have caused the system to fail in high winds.
DO develop a maintenance and repair plan.
The façade consultant or a knowledgeable contractor should create a document that tells the O&M staff how often to inspect various cladding items and how much wear and tear is acceptable. The plan should include checking fasteners and wall anchors for corrosion and inspecting for any loose or missing fasteners. Fasteners are subject to moisture damage. Wind can cause them to oscillate and can work bolts loose. Facades can last a lifetime if they are well maintained, but they will be junk in a few years if regular maintenance is ignored.
DO regularly inspect the cladding sealants.
It’s in the owner’s interest to have the cladding system examined as a routine maintenance item. Sealants lose flexibility and can break, letting moisture in to corrode critical parts. A good silicone sealant may be warranted for 20 years, but it’s still important to examine that sealant periodically to make sure it hasn’t cracked.
DON’T throw out the shop drawings!
Shop drawings add design and engineering information to the design team’s plans. Since cladding is often a design-build item, the plans of the design team are more schematic and appearance-oriented than a true representation of the cladding system design and attachment. I visited one college campus to do a seismic evaluation of the cladding system, but the building engineers and facility managers kept the plumbing and mechanical systems shop drawings in their office, and sent the shop drawings to a storage facility, never to be seen again. I couldn’t finish my evaluation because they didn’t keep their documentation. The shop drawings should be kept readily available with the building structural and architectural drawings. BD+C
William Kirkham, PE, SE, is a structural engineer with APT Engineering, Portland, Ore. He is licensed in 12 states.
Have your stone tested by a reputable laboratory
Stone is a natural material, and its structural properties can vary from one quarry to another, which is why it’s important to test it. Though a good structural engineer can develop a workable design for any situation, the cost of paying an engineer to work with untested stone likely will exceed the cost of a basic stone testing program. Moreover, the test values are usually better than the publicly posted data, so stone anchor design is more cost effective with testing.
Here is a basic list of tests for natural stone:
• ASTM C-97 Water Absorption Test
• ASTM C-99 Modulus of Rupture Test
• ASTM C-170 Compressive Strength Tests
• ASTM C-880 Flexural Strength Test
Usually published stone values are more conservative than the results of actual testing, so performing these tests may allow you to reduce the size of the stone anchors. More information about the tests is available from the Building Stone Institute .
In some cases, ASTM C1354, Standard Test Method for Strength of Individual Stone Anchorages in Dimension Stone, is called for. This provides for testing of specific or proprietary anchors that are being used in the project, such as an epoxied dowel in the stone, or a headed stud that slips into a machined slot in the stone. In these cases, simple mathematical analysis is often overly conservative due to the complex nature of the anchor design, so it’s important to conduct the test.
—William Kirkham, PE, SE