Attaching High Bays to Metal Purlins or Joists

Mounting LED high bay fixtures to metal purlins or joists looks simple—until a clamp slips, a thin-gauge purlin dimples, or an inspector asks for documentation you do not have. This guide walks through structurally sound, code-aligned ways to attach high bays in pre-engineered metal buildings and pole barns, for both professional electricians and advanced DIYers.

1. Know What You’re Attaching To

Before choosing hardware or drilling anything, identify the actual structure carrying the load. For high bays, the connection is often more critical than the fixture itself.

1.1 Common framing types

In metal buildings and barns you’ll typically see:

Member type	Typical material	Where lights get hung	Key concerns
Z- or C-purlins	Cold-formed steel, 18–22 ga	Under roof sheeting, spanning between main frames	Thin gauge, local dimpling, limited screw pull-out
Bar joists / open web joists	Welded steel, deep webs	Bottom chord or panel points	Eccentric loading, weld zones, clamp orientation
Wide-flange beams / girders	Hot-rolled steel	Flanges or web	Flange thickness, clamp fit, edge distances
Wood trusses (pole barns)	Dimensional lumber or engineered	Bottom chord or added strut channel	Splitting at screw locations, long-term creep

For cold-formed purlins, capacity is often governed by local deformation around the connection rather than by the published section capacity. According to AISI S240 commentary, round holes in webs are acceptable if they respect edge distances (typically 1.5–2.0× hole diameter) and are kept away from high-stress regions near flanges.

1.2 Confirm thickness and accessibility

Field-proven steps before you pick hardware:

1. Confirm gauge: Many pre-engineered metal building purlins are 18–22 ga. If you can’t see a stamp, use calipers or a drill test coupon on scrap to estimate thickness.
2. Check access: Can you safely reach both sides of the web for through-bolting and backing plates, or are you limited to one side?
3. Look for existing penetrations: Avoid clusters of holes near each other; multiple fasteners in a line can weaken cold-formed members.
4. Locate true structure: Do not mistake liner panels, hat channels, or ceiling grid for structure. Per NFPA’s overview of the National Electrical Code, luminaires must be attached to building structure or framing, not just finishes.

If you cannot clearly identify the supporting member or its condition, stop and involve the building’s structural engineer.

2. Load Basics for High Bay Mounts

Most modern LED high bays weigh 5–15 lb each, with sensors and junction boxes adding a few more pounds. That sounds small, but fatigue, vibration, and impact can multiply the demand on your connection over time.

2.1 Practical safety factors

A common rule of thumb is to size anchors or clamps for at least 5× the fixture weight. For a 25 lb high bay, that suggests 125 lb minimum connection capacity. Industry hardware catalogs for structural beam clamps typically use a 3× safety factor on test loads; for example, structural clamps in a well-known steel fittings catalog list design loads of 2,000–3,000 lb based on a 3:1 factor of safety. That means the clamp itself is rarely the weak link—cold-formed purlins and fastener pull-out/pull-through usually govern.

In practice:

• For standard high bays (≤30 lb), a 5:1 factor on fixture weight is conservative and aligns well with clamp ratings based on 3:1 test factors.
• For heavier assemblies (multiple fixtures, cable tray, signage), treat the system like any other overhead support and have a qualified engineer review the whole connection path.

For deeper context on how to compute fixture and hardware loads, see the dedicated guide on calculating load capacity for high bay mounts.

2.2 Dynamic effects and redundancy

High bays in real buildings see:

• Vibration from overhead doors, cranes, or rooftop units.
• Thermal cycling that loosens fasteners over years.
• Occasional impact from equipment or material handling.

Our field observations show that adding secondary safety cables reduces the risk of a dropped fixture by an order of magnitude compared to single-point hardware alone, even when catalog ratings appear ample. National Electrical Code section 410.36(B), summarized in the NFPA 70 overview, emphasizes that luminaires must be securely fastened to the building structure; many industrial owners interpret this to include independent safety support for any suspended fixture.

Pro Tip: Safety cables are not optional “extras”

A common misconception is that “if the clamp rating is high enough, you don’t need secondary support.” In practice, industrial clients and safety programs often expect a dedicated safety cable sized for at least the full fixture weight and anchored to a separate point. For a 25–30 lb high bay, a 1/8" galvanized aircraft cable with a rated capacity over 200 lb, attached to its own clamp or structural point, is a low-cost way to align with the intent of NEC 410.36(B) and typical owner safety policies.

3. Hardware Options for Metal Purlins and Joists

The goal is a connection that matches both the member type and the load path. The table below summarizes common options.

3.1 Comparison of mounting strategies

Scenario	Recommended primary hardware	When it works well	Watch-outs
Light to medium high bay on steel purlin	Listed beam clamp to flange + eye nut or ring	Purlin flange thick enough, access from below	Clamp jaw must fully bear on flange; avoid over-tightening on thin lips
Medium load on purlin web with two-sided access	Through-bolt with washers/backing plates	Retrofits where you can reach above the purlin	Respect AISI edge distances; avoid flange/web junction
Multiple fixtures or future reconfig needed	Strut (Unistrut) spreader between frames/purlins	Shops, long aisles, adjustable layout	Added material, but better load distribution and adjustability
Bar joist / open web	Beam clamp at panel point or bottom chord + short rod	Warehouses, retail shells	Do not clamp over welds; keep loads at panel points, not mid-panel
Wood truss in pole barn	Lag screws into solid blocking or truss chord + strut	Garages and barns with wood framing	Pre-drill to avoid splits; consider long-term creep in high temps

3.2 Beam clamps on steel purlins and beams

Beam clamps are often the fastest path from fixture to structure, but their real-world performance is governed by torque-controlled installation, not just nameplate capacity.

An engineering report on a popular seismic-rated beam clamp (IAPMO ER‑577) shows horizontal design strengths on the order of 2,600–4,000 lbf, but the clamp’s bolt is intentionally designed to shear off around 29.5 ft‑lb of torque to prevent over-tightening on the flange. That means:

• You must match clamp model to flange thickness (typically up to 1‑1/4").
• You must achieve the specified torque—but not exceed it—during installation.
• Full thread engagement in the jaw is mandatory to reach published loads.

Expert warning: Over-torquing on thin purlin lips can dimple or locally crush the flange, cutting real capacity far below the published clamp rating. When in doubt, move to the web with through-bolts and backing plates, or span a short section of strut across two adjacent members.

3.3 Through-bolting cold-formed purlins

Contrary to folk wisdom that “you should never drill purlins,” cold-formed design standards allow holes when properly located. The AISI S240 commentary notes that:

• Round holes in webs are acceptable away from high-stress regions, especially when edge distances are kept around 1.5–2× hole diameter.
• The size and placement of holes relative to the member depth matters more than their mere presence.

For a typical 3/8" through-bolt supporting a 20–30 lb high bay:

• Placing the hole in the middle third of the web depth, with proper edge distance and washer plates 2–3× the bolt diameter, usually keeps stresses well within capacity.
• Avoid stacking holes in a line or drilling near the flange/web junction; that can significantly weaken the section and warrants engineering review.

Fastener engagement rules of thumb:

• Minimum steel thread engagement of 1.5× bolt diameter into base material or into a backing plate.
• Where backing plates are not feasible, a through-bolt into a structural member is preferred over heavy self-drilling screws into thin purlin lips.

3.4 Using strut or grating channel as a spreader

Instead of hanging each high bay directly from a purlin, running a section of strut or grating channel between more robust members often improves both capacity and serviceability.

Load tables for a 3" steel safety grating channel show that at a 2'-0" span, it can carry roughly 2,100+ lb/ft² with only about 0.04" deflection under a 100 lb/ft² uniform load. Used as a transverse spreader from rafter to rafter or between joists, that capacity is more than sufficient for clusters of 20–40 lb fixtures, while keeping deflection well under a common 1/4" serviceability limit.

Advantages of this “spreader bar” approach:

• Reduces dependence on thin-gauge purlins for point loads.
• Provides a common bolt pattern and multiple slots for fixtures and safety cables.
• Makes future re-aiming or re-spacing much easier—just slide hardware along the channel.

4. Step-by-Step: Attaching High Bays to Metal Purlins

This section assumes a typical scenario: a 20–25 ft metal building or pole barn, cold-formed steel purlins, and individual LED high bays in the 10–20 lb range.

4.1 Pre-installation checks

1. Review drawings if available: Confirm purlin sizes, spacing, and intended loads.
2. Inspect on site: Look for corrosion, deformation, existing penetrations, and any damaged members.
3. Verify clearances: Ensure required mounting height and beam angle work for your lighting layout; if you haven’t done so, review a layout guide like Designing a High Bay Layout for Warehouse Safety.
4. Confirm power routing: Decide whether you’ll use surface conduit, cord-and-plug, or whips, and verify compliance with NEC and local codes.

4.2 Method A – Beam clamps to purlin flange

Use this where purlin flanges are thick enough and unobstructed.

Materials:

• Listed steel beam clamp sized for flange thickness.
• Forged eye nut or eye bolt rated for the load.
• Short section of rated chain or threaded rod.
• Secondary safety cable with independent clamp or anchor.

Steps:

1. Position the clamp near midspan of the purlin segment or as directed by the engineer; avoid locations already carrying concentrated loads (like large ducts) where practical.
2. Seat the clamp fully over the flange; the jaw should bear flat without rocking.
3. Tighten to specified torque using a calibrated wrench, especially on torque-limited models where bolts shear at a target torque. Do not exceed manufacturer torque limits.
4. Install the eye nut or eye bolt with full thread engagement through the clamp or through a hole in a strut section, depending on your layout.
5. Connect the fixture using rated chain, cable, or rod, keeping the hang vertical to avoid side loading.
6. Attach an independent safety cable from a separate structural point (another clamp, strut, or rafter) to the fixture’s safety tab.
7. Verify alignment and swing clearance, especially near doors or equipment.

4.3 Method B – Through-bolting the purlin web

This method is preferable when flange lips are very thin or when you want a more direct load path.

Materials:

• 3/8" (typical) galvanized bolt, nut, and washers.
• Backing plates sized at least 2–3× the washer diameter where feasible.
• Clevis or forged eye for connecting chain/rod.
• Secondary safety cable and independent anchor.

Steps:

1. Mark the web location in the middle third of the depth, away from existing holes and the flange/web junction.
2. Drill a pilot hole, then final-size to the bolt diameter, deburring both sides.
3. Install bolt with washers and backing plates, respecting edge distances (~1.5–2× hole diameter).
4. Torque to snug plus a quarter turn, or to the manufacturer’s recommended torque for the bolt size.
5. Attach clevis or eye for the hanger hardware.
6. Hang the fixture, maintaining vertical alignment.
7. Install the secondary safety cable to a separate connection point.

4.4 Method C – Strut spreader with multiple fixtures

For aisles or workbays, it is often more efficient to span a section of strut under multiple purlins and mount several fixtures to it.

Materials:

• Appropriately sized strut or grating channel.
• Beam clamps or through-bolts at each support point.
• Strut nuts, bolts, and fixture hangers.
• At least one safety cable per fixture, attached to structure or strut as allowed by the engineer.

Steps:

1. Lay out the strut run based on your lighting design; many designs target fixture spacings that balance lumens and glare, as discussed in the Warehouse Lumens Guide for High Bay Lights.
2. Attach strut to structure using beam clamps or through-bolts at each end; longer spans may need intermediate supports.
3. Confirm strut level and alignment, adjusting clamp positions as needed.
4. Mount fixtures to strut using strut nuts and appropriate hangers or hooks.
5. Add safety cables from each fixture to separate points when possible, or as directed by the project engineer.
6. Route wiring and controls along the strut, adhering to NEC wiring class rules for any 0–10 V dimming conductors.

5. Special Considerations for Bar Joists and Wood Trusses

5.1 Steel bar joists

For open web steel joists:

• Prefer to connect at panel points (where web members meet the chord) to keep loads near regions designed for connection forces.
• Use beam clamps sized for the chord bar and avoid clamping over welds.
• Avoid eccentric side loads that twist the bottom chord; keep hanger loads vertical.

Joist manufacturers can provide allowable loads and recommended connection details for supplemental systems like lighting and mechanical supports; always consult them for significant or unusual loading.

5.2 Wood trusses and pole barns

In wood-framed barns and garages:

• Avoid hanging heavy fixtures from the very bottom edge of a chord with small wood screws.
• Use lag screws into solid blocking or engineered hangers that bear on more of the member depth.
• Pre-drill pilot holes to minimize splitting, especially near end distances.
• Consider long-term creep at elevated temperatures; what holds on day one may sag years later.

Where multiple fixtures line up under a run of trusses, adding a steel strut under several trusses and fastening it with appropriate hangers distributes loads and simplifies reconfiguration.

6. Corrosion, Vibration, and Serviceability

6.1 Corrosion and environmental exposure

Many metal buildings and pole barns are unconditioned or partially open to the elements:

• Use stainless or hot-dip galvanized clamps and fasteners where condensation, washdown, or agricultural exposure is expected.
• Add neoprene or similar gaskets between clamps and purlins when fretting or dissimilar metal contact is a concern.
• Inspect hardware during regular maintenance intervals (6–12 months) and re-torque connections in high-vibration locations.

6.2 Vibration and thermal cycling

Vibration and temperature swings work fasteners loose over time. Good practice includes:

• Using split lock washers, prevailing-torque nuts, or threadlocker on critical joints.
• Avoiding spring washers alone as the only anti-loosening measure.
• Rechecking torque after the first season of operation in facilities with heavy machinery or frequent door cycles.

6.3 Serviceability and future changes

Design the mounting scheme for easy service:

• Prefer strut mounting plates for multiple fixtures or where future moves are likely; they provide common bolt patterns and sliding adjustability.
• Maintain clear access paths for lifts and ladders.
• Leave slack in conductors and use accessible junction boxes so fixtures can be disconnected and lowered without disturbing the structure.

7. Common Mistakes to Avoid

7.1 Over-reliance on self-drilling screws

Using heavy self-drilling screws directly into thin purlin lips is a frequent failure point. Problems include:

• Limited thread engagement in thin steel (far less than the 1.5× diameter rule of thumb).
• Local flange dimpling, which can reduce effective clamp-up force.
• Increased risk of pull-out under dynamic loading.

Wherever practical, upgrade to through-bolts with backing plates or clamp-based connections.

7.2 Ignoring purlin orientation and geometry

Not all purlins are the same:

• Top-hat or shallow Z purlins offer very little lip for clamps.
• Deep Z or C sections can be strong in one direction and weak in local flange bending.

Always verify member shape before choosing hardware; what works on a wide-flange beam may be inappropriate for a cold-formed Z section.

7.3 No independent safety support

Relying solely on one clamp or one set of screws is a single point of failure. Adding a safety cable anchored to a second point costs a fraction of a fixture and dramatically reduces risk.

8. Quick Field Checklist for Electricians and DIYers

Use this checklist before you leave the lift on any metal building or pole barn high bay project.

Structure

• [ ] Member type identified (purlin, joist, beam, or truss).
• [ ] Gauge or section thickness confirmed, or engineer consulted.
• [ ] Attachment location clear of corrosion, prior damage, or excessive existing holes.

Hardware and connections

• [ ] Connection type selected based on member (clamp, through-bolt, strut spreader).
• [ ] Hardware ratings exceed 5× total supported load per point.
• [ ] Bolts have at least 1.5× diameter thread engagement or through-bolts with backing plates.
• [ ] Torque applied with appropriate tools and within manufacturer limits.

Redundancy and safety

• [ ] Independent safety cable installed for each suspended fixture.
• [ ] Safety cable anchored to a separate structural point or clamp where feasible.
• [ ] No conduit, cord, or whip used as a mechanical support.

Environment and maintenance

• [ ] Hardware materials suitable for environment (galvanized or stainless where needed).
• [ ] Access paths for future service maintained.
• [ ] Owner informed of recommended re-inspection intervals.

9. Wrapping Up: Structurally Sound High Bay Mounts

Attaching high bays to metal purlins or joists is as much about respecting the structure as it is about hanging a light. Cold-formed purlins can handle properly detailed holes and through-bolts, beam clamps can safely carry large loads when torqued correctly, and short runs of strut or grating used as spreaders often provide the stiffest, most serviceable solution.

For new projects, integrating these mounting strategies into your layout and specification phase—alongside decisions about beam angle and fixture spacing from resources like Choosing a Beam Angle for Your Ceiling Height—gives electricians, facility managers, and DIY owners a safer, more maintainable lighting system from day one.

---

Frequently Asked Questions

Q1: Can I hang high bays directly from thin metal purlins with self-tapping screws? In most cases, this is not recommended. Thin 18–22 ga purlin lips do not provide enough thread engagement or bearing area for reliable long-term support. Through-bolts with backing plates, beam clamps sized for the flange, or strut spreaders to heavier members are more robust options.

Q2: Do I need a safety cable if my beam clamp is rated for thousands of pounds? Yes. The clamp rating does not eliminate single-point failure risk from installation errors, damage, or unforeseen conditions. A dedicated safety cable sized for at least the full fixture weight and attached to a separate anchor point provides critical redundancy and aligns with the intent of NEC safety requirements.

Q3: Is it acceptable to drill holes in cold-formed purlins to mount lighting? It can be acceptable if done correctly. Cold-formed standards such as AISI S240 allow round holes in webs when edge distances and hole locations are controlled. Holes should stay away from the flange/web junction and be paired with washers or backing plates to distribute load. For unusual conditions or heavy loads, involve a structural engineer.

Q4: When should I involve a structural engineer for high bay mounting? You should seek engineering input when fixture plus accessory loads exceed typical values (for example, heavy luminaires or clusters), when purlins are heavily perforated or corroded, when you are adding loads to joists or frames already carrying significant mechanical equipment, or any time you are unsure about the adequacy of the support member.

Q5: Are these recommendations enough to ensure code compliance everywhere? No. These are general best practices and do not replace local building codes, electrical codes, manufacturer instructions, or project specifications. Always verify requirements with local authorities, the fixture and hardware manufacturers, and, when needed, a licensed professional engineer.

---

Safety Disclaimer: This article is for informational purposes only and does not constitute professional engineering or electrical advice. Overhead work and electrical installations are inherently hazardous. Always follow applicable electrical and building codes, manufacturer instructions, and safety regulations, and consult a licensed electrician or structural engineer for project-specific design and installation decisions.

Sources

• AISI S240 – North American Standard for Cold‑Formed Steel Structural Framing
• NFPA 70 – National Electrical Code (overview)
• Grip Strut safety grating and channel load tables
• IAPMO ER‑577, engineering report on torque-limited seismic beam clamps (plain-text citation, no link)