Cladding University
Founded 1997
Engineering 401 - Insert Design - Why the Additional ALT 1.3 Factor
Instructor: Steve Strebel, ALT Cladding and Design
ALT Cladding and Design regularly specifies that an additional factor of 1.3
be applied when designing the insert portion of the anchor. This course
explains why we feel it is prudent to apply this factor.
Class Syllabus
Introduction
Detailed Explanation
Waiving the Factor
Summary
Introduction
ALT regularly specifies that an additional load factor of 1.3 be applied to concrete inserts. This is in addition to any load factors required by design codes (i.e., the 1.3 ACI factor for wind load and the PCI factor of 1.33 for connections). This factor is to be added to the applied loads no matter what standard is used to design the insert. It applies to both ultimate load and allowable stress design.
We have recently also added this requirement to all concrete anchorage elements to include not only inserts, but also expansion bolts and chemical bolts. Many of the reasons stated below apply to these conditions as well.
Detail Explanation
The insert / anchor is the most important structural element in the entire cladding system. Given the factors stated below, we feel it simply makes sense to require an additional factor of safety on the insert design.
- The insert is the only typical structural element that is not tested on the mockup. Although the mockup is not a substitution for calculations, occasionally, a structural problem will be exposed during the mockup structural test. Because there is never a poured floor slab at the mockup, we do not have the opportunity to catch something unforeseen with the insert and concrete slab connection during this test.
- The insert is the only structural element that is installed outside of the contractor's control. Every other structural element in the cladding system will be assembled or installed by the cladding manufacture. Even parts that are subcontracted out, eventually get sent to the contractor and get inspected (or at least seen) and installed by the cladding subcontractor. The exception to this is the insert. Usually, the general contractor for the building sets the inserts for the cladding contractor. The cladding contractor has little, if any, input into the actual setting of the units in the slab. While most GCs do a fine job setting the inserts, the issue is that this is the only element the cladding contractor does not install himself and therefore this presents an opportunity for something to get lost in the process.
- The edge of the slab is where the poorest quality concrete is likely to be. If there are going to be voids in the slab, improper mix, or other poor concrete, the likelihood of this occurring near the slab edge is higher than most of the rest of the slab.
- Concrete embedments are subject to brittle failure modes. If there is a design problem, the insert will probably fail rapidly with little warning. Unlike, the rest of the wall, where the ductility of the metal might allow someone to catch a problem before the actual failure, the insert will probably fail rapidly without warning. This is due to the brittle nature of concrete. (There is no yield stress for concrete like there is in metals.)
- The insert is the only element in the cladding that is often installed before the final design of the wall system is finished. It is not uncommon to have inserts being cast in-place while design discussions are in progress for the actual wall the insert will support. In these cases, if the design of the wall or anchor ends up placing additional load on the insert, we have the ability to examine each of the issues herein and possibly waive a portion of the extra load factor. Without this option, we probably have a big problem.
- Insert design rules are based on US or other British standards. These standards are based on a certain level of quality being achieved in the concrete construction. Unfortunately, the quality of concrete construction generally seen in Asia, is not quite at the same level of quality seen in, for example, Chicago. While this is not always the case, there is no question, it does occur and there is generally no way to tell what the concrete quality will be until well past the point the inserts are designed, fabricated, and sometimes even installed.
- Unforeseen field conditions do occur. When there is a field deviation from the drawings, this almost always impacts the anchorage. (The rest of the wall may be impacted too.) Having a little capacity to accommodate extra loading that may occur because the slab was poured too far in, is often a welcome bonus.
- Remedial work on inserts is VERY difficult. If other areas of the cladding need reworked, it will be inconvenient, but can generally be accomplished without a big deal. If the insert needs reworked, there are few options available because it is embedded in the concrete and cannot be accessed. For example, if a mullion is found to not not work in a certain location after it is extruded, steel can be added to the inside before the unit is erected. If an insert is found to be inadequate in the same location, the rework options are far more difficult. Reinforcing the insert with supplemental concrete bolts is very difficult. Load distribution problems, edge distance concerns, and the rigidity of the insert verses the slop in bolted connections, makes these conditions very hard to do.
- The cost to incorporate the additional factor is generally not significant. All to often, contractors will argue over this factor when the cost to them to comply is practically nothing. How much does an extra 25 mm of stud length cost? Often, the extra cost of complying with the additional load factor, is more than saved by just the rework that would be required for just a few anchors where tolerance or other field conditions cause a problem. Once the insert is in and the slab is finished, we can evaluate the situation and possibly waive a portion of the extra factor when such conditions arise.
- Because the specification says so. The additional load factor is clearly stated in our project specification. This is what we want. This is what we bought. This is what the cladding contractor agreed, in writing, to provide.
Waiving the Factor
Since the factor is not required by code, we can evaluate the condition and possibly waive a portion of the factor if it is prudent to do so. This is not to say that if the condition doesn't work, we will waive the factor. In general, we will only consider waiving the factor AFTER the insert is cast and all relevant contributing factors can be assessed. Also note that we do NOT waive the factor to save the contractor money. Only technical considerations are considered on the actual field condition in question.
Summary
There are several good reasons to design a little extra capacity into the insert. In practice, it has proved to be the prudent thing to do many times.