Lecture on the Design of Sag Rods
Good day, everyone. Today, we will discuss the design of sag rods, an important but often
overlooked component in roof framing systems. While trusses, purlins, and rafters typically take
center stage in structural design, sag rods play a crucial role in maintaining structural integrity,
preventing excessive deflections, and ensuring serviceability of long-span roofing systems.
1. Introduction to Sag Rods
Sag rods are tension members placed between adjacent roof purlins to limit their lateral
displacement and excessive sagging.
Roof purlins, which span between trusses or rafters, are slender flexural members subjected to
roof loads.
Without intermediate restraint, long purlins may deflect excessively under load, reducing
serviceability and potentially causing roofing sheets to fail.
Sag rods (or sag bars) provide intermediate lateral bracing by tying purlins together, creating a
more stable roof framing system.
Thus, sag rods are not designed to carry primary roof loads, but to reduce unbraced length,
prevent buckling, and distribute loads evenly.
2. Functions of Sag Rods
Reduce Deflection – By connecting adjacent purlins, sag rods reduce mid-span sagging.
Provide Lateral Bracing – Prevents purlins from twisting or buckling laterally under roof loads.
Load Distribution – Helps distribute concentrated loads (e.g., from wind suction or roof
maintenance) across multiple purlins.
Economy – Allows the use of lighter purlin sections since sag rods improve stability.
3. Arrangement of Sag Rods
Typically installed at mid-span or third-points of purlins.
Configurations include:
Single row – at mid-span for shorter spans.
Double row – at third points for longer spans.
Cross-tied arrangement – sag rods crossing in an X-shape for added stability.
4. Design Considerations
Sag rods are designed as tension members, meaning they resist axial tensile forces only. Their
design process includes:
(a) Load to be Resisted
The force in a sag rod is derived from the deflection compatibility of purlins.
Simplified codes and textbooks often approximate sag rod forces as a fraction of the roof load
on the purlin span.
Wind uplift or unbalanced snow loads may also induce forces in sag rods.
(b) Member Sizing
Sag rods are usually round bars or threaded rods (12 mm – 20 mm diameter common).
Check tensile capacity:
P
≤
A
⋅
F
t
P≤A⋅F
t
Where:
P
P = axial tension force,
A
A = net cross-sectional area,
F
t
F
t
= allowable tensile stress of steel (per NSCP/AISC).
(c) Connections
Typically connected to purlins using bolts, welded plates, or angle clips.
Connections must be designed to safely transfer the axial force.
(d) Spacing
Code provisions or design practice typically require:
One row of sag rods for purlins up to ~4.5 m long.
Two rows for purlins longer than ~4.5 m to 6 m.
For spans longer than ~6 m, more rows may be required.
(e) Serviceability
The main criterion is deflection control. Even if sag rods are small in size, they significantly
stiffen the roof system.
5. Example Applications
Roof Purlins – Light-gauge steel purlins spanning across trusses require sag rods to prevent
sagging under roof loads.
Channel and Angle Purlins – Often slender; sag rods prevent twisting and lateral movement.
Large Roof Structures – In warehouses, hangars, and gymnasiums, sag rods improve structural
economy by allowing lighter purlins.
6. Practical Considerations
Material: Usually mild steel bars (Fy = 250 MPa) with protective coating or galvanization.
Installation: Often installed before roofing sheets are placed. Tightening must be uniform.
Detailing: Care must be taken to avoid eccentric loading at purlin webs or flanges.
Maintenance: Sag rods must remain taut; loose rods reduce effectiveness.
7. Advantages of Using Sag Rods
Prevent excessive sagging and improve serviceability.
Reduce unbraced length of purlins, enhancing buckling resistance.
Economical solution compared to using heavier purlin sections.
Easy to fabricate and install.
8. Limitations
Not primary load-carrying members; they only provide restraint.
Must be detailed carefully to avoid eccentric stresses on purlins.
Effectiveness depends on proper tensioning and installation.
9. Conclusion
Sag rods may appear as minor elements in structural drawings, but their role in roof framing
stability is indispensable. They improve performance by controlling deflections, reducing
buckling length, and distributing loads among purlins.
In design, sag rods are treated as simple tension members, sized to carry calculated forces or
per code recommendations, with emphasis on serviceability and stability rather than strength
alone.
As engineers, remember that small details like sag rods can determine the overall safety and
economy of a structure. Ignoring them can result in purlin deformation, roof leakage, or
premature failure.
