TIPS ON CUTTING HOLES IN STRUCTURAL MEMBERS
By: David Simpson, P.E., SECB
If done properly, cutting of holes in existing structural support systems can be accommodated. For the Engineer this requires a knowledge of the hole size and hole location. With this information, the beam is analyzed for this condition. In some cases, the opening may have no detrimental effect. In other cases, reinforcing will be required around the opening or the aspect ratio of the hole will need adjusted.
Unfortunately, openings are made years later by contractor’s intent on installing their systems in the most direct fashion possible. Sometimes the path from Point A to Point B has a beam in the way. The solution is to cut out anything in the way. There have been many occasions where our office is involved in a renovation and discovers these conditions. It is incumbent on us as professionals to notify the owner and recommend a repair. This invariably adds an unforeseen cost to the renovation project.
How do we avoid these problems? Coordination and communication is the answer.
Scenario # 1:
The structural engineer and MEP engineer are working closely together. Accommodations are made to allow unimpeded routing of duct work. Beam depths are reduced or reoriented. If beam openings are required, the beam depth is increased to minimize the impact of the beam opening.
Scenario # 2:
The reality is, on many occasions, the HVAC contractor will change the routing, duct size or equipment in their low bid, thus making the pre-cut openings useless. In this case, the MEP engineer or contractor must notify the structural engineer where these new openings will occur. It is possible that reinforcing will make it work but there may be situations where the new routing will not work, especially if the structural steel is in place.
Scenario # 3:
In the course of a building’s life, many renovations have been made to plumbing or duct work systems. Structural elements were indiscriminately cut or removed. These changes have severely compromised the structural integrity of the building. It may be years before this is discovered unless a failure occurs before then. The following are general guidelines and are shown here to offer initial understanding when planning cutting of structural openings. The actual execution of these procedures should be directed by a qualified professional engineer.
Truss members indiscriminately cut for duct work installation
Here are some basic guidelines when planning openings in steel beams.
- The hole should be centrally placed in the web and eccentricity of the opening is avoided as far as possible
- Unstiffened openings are not always appropriate, unless they are located in low shear and low bending moment regions
- Web openings should be away from the support by at least twice the beam depth, D or 10% of the span,
( ), whichever is greater
- The best location for the opening is within the middle third of the span
- Clear spacing between the openings should not be less than beam depth (D)
- It is always preferred if the openings can be predetermined and made in the shop and not in the field. The cost is negligible in the shop as opposed to expensive in the field
Concrete Floors on Metal Deck (In Existing Buildings):
These guidelines will assume the deck is composite with shear studs on the beams. For openings at right angles to the deck span across the ribs:
- For openings up to 10”, round or square, require no special treatment
- For larger openings, supplementary framing may need to be added to carry the opening edges
- For openings less than 24”, the effect of the transferred loads to the adjacent beams will have minimal impact on the adjacent beams
- For openings greater than 24”, the transferred load to the adjacent beams will need to be analyzed by a structural engineer
- If the opening is closer than the least of the following , the beam will need to be reanalyzed for the loss of concrete contributing to the composite beam section properties:
- 1/8 of the beam span
- 8 times slab thickness from edge of beam flange
- ½ the clear distance to the next beam flange edge
OPENINGS IN EXISTING CONCRETE SLABS
It is often necessary to create openings in existing concrete slabs. Openings may be large, such as those required for a new stairway or small core penetrations, such as those required for plumbing risers. Post-tensioned slabs (PT) have the added complexity in that PT strands may have to be cut. It is a common misconception that creating openings in existing PT slabs is difficult, expensive and dangerous. This perception is perpetuated because the field procedures and hardware used to create these penetrations are not fully understood. This concern is sometimes given as a reason for avoiding PT construction in particular applications. Cutting openings in PT slabs does require care and caution because of the possibility that PT tendons may have to be cut. However, with the proper knowledge of structural behavior, repair hardware and PT field practices, retrofitting openings and other penetrations in PT slabs is not only feasible but can also be achieved safely and economically.
Types of Openings and Penetrations:
There are two types of openings that are commonly cut into existing slabs: Small penetrations and large openings. Small penetrations are those that can be cut into a slab without affecting any of the existing PT tendons; conversely, large openings are those that require the cutting of existing tendons.
As a general rule, it can be assumed that the effect of small penetrations will be negligible and that the slab will behave similar to the slab without penetrations, provided that:
1. None of the existing PT strands are cut during the coring of the penetration
2. The opening is located not less than 10 times the slab thickness from a supporting column
3. The opening is not located near a concentrated load
4. The opening or group of openings does not significantly reduce the effective flange area for a supporting beam.
When applying this general rule, good judgment should be exercised. A large number of small openings in a concentrated area can have a significant effect on slab strength and stiffness, particularly if several reinforcing bars are cut. Prior to cutting small penetrations in a PT slab, existing PT strands should be located using nondestructive testing (NDT) equipment such as ground-penetrating radar (GPR). Once the strands have been located and marked on the slab, small penetrations can be made using core-drilling equipment or chipping hammers.
Sometimes it is necessary to create large openings in a slab for stairwells or large duct shafts between floors. Creating a large opening in a PT slab will result in several PT strands being interrupted by the new opening. An engineer should be consulted to analyze the effect that the new opening will have on the slab. The slab with the new opening should be analyzed in accordance with ACI 318-08, 2 Section 13.4.1. The easiest way to tell whether a slab or beam is post-tensioned is to look at the slab edge or beam edge for the grouted anchorage pockets.
Penetrations can be cored or cut through the hollow core plank in the field after installation. Typically openings 6” in diameter or less can be located and cut through the cores in the plank with nearly zero effect on the capacity of the floor or roof system. Locating the opening through the cores would ensure that no reinforcing strand is cut or damaged. One exception to this would be openings placed side by side in the plank that do not cut the strand but cut most of the width of the plank would not be ideal.
The ideal situation would have all openings placed in one core. Remember the steel beam analogy from earlier? One would not cut through most of the top flange of a steel beam and expect it to support much load. The same is true for the plank if most of the width of the plank is removed by penetrations. Even though the strand is not damaged, the plank may not have enough compression flange to carry the intended design forces.
Larger openings that will cut reinforcing strand may be added at a later date, but the location of such openings is more critical. It is difficult to add moment capacity to the plank after it has been cast since the pre-stressing steel is the element that provides this capacity. So in a floor or roof system where the plank is working hard, adding penetrations that cut strand at mid span is not likely possible without the introduction of alternate framing methods, such as steel beams. Locating openings like this at the end of a span could be achieved more easily because adding shear capacity to the plank is simply done by adding core fill at the supports.
IRC R502.8 specifies requirements for Drilling and Notching wood members. These recommendations are conservative and based on general engineering principles. More extensive cutting would need to be reviewed by a structural engineer. IRC 2308 addresses this issue in more depth.
The key point to take away from this article is that a structural engineer’s review is needed when existing structural members are cut. If there is a structural member present, it’s there for a reason.
“When in doubt, have an engineer check it out.”
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Thanks for reading,
David Simpson’s experience includes over 30 years in structural design and project management for industrial, commercial, institutional and nuclear/chemical facilities utilizing steel, concrete, masonry and wood. His accomplishments include design and construction administration of health care facilities, hotels, schools, shopping centers, aircraft hangars, numerous retail facilities and several forensic engineering assignments. He has professional registrations in D.C., Maryland, Ohio, Pennsylvania, South Carolina, Virginia and West Virginia. Simpson graduated from the West Virginia Institute of Technology with a bachelor’s degree in civil engineering and an MBA from West Virginia University.