Steel Tension Rods — Threaded Rod Design per AISC 360
Steel tension rods are round bars (threaded at the ends or full-length) used as structural tension members. Common applications include vertical hangers, sag rods for purlins, lateral bracing rods, and architectural tie-back systems. Design is governed by AISC 360-22 Chapter D (tension members) and Chapter J (threaded fastener provisions). Material is typically ASTM F1554 (anchor rods) or A36/A572 threaded bar.
Rod material grades
| ASTM | Grade | Fy (ksi) | Fu (ksi) | Typical Use |
|---|---|---|---|---|
| F1554 | Grade 36 | 36 | 58-80 | Light hangers, sag rods |
| F1554 | Grade 55 | 55 | 75-100 | Moderate tension members |
| F1554 | Grade 105 | 105 | 125-150 | High-capacity bracing, heavy hangers |
| A36 | -- | 36 | 58-80 | General threaded rod |
| A572 | Grade 50 | 50 | 65 | Higher-capacity rods |
| A193 | Grade B7 | 105 | 125 | Anchor bolts, high-strength applications |
F1554 Grade 36 is weldable without preheat. Grades 55 and 105 require preheat per AWS D1.1 if welding is involved. Grade 105 is quenched and tempered and must not be heated above 800 degF (425 degC) to avoid metallurgical damage.
Tensile capacity (AISC 360-22 Chapter D and J)
Yielding on gross area
phiPn = 0.90 * Fy * Ag
Where Ag = pi/4 * db^2 (gross area based on nominal rod diameter).
Rupture on net (threaded) area
phiPn = 0.75 * Fu * Ae
Where Ae = tensile stress area of the threaded section. The tensile stress area accounts for the thread root diameter, not the nominal diameter:
At = pi/4 * (db - 0.9743/n_threads)^2
| Diameter | Threads/in (UNC) | Ag (in^2) | At (in^2) | At/Ag |
|---|---|---|---|---|
| 1/2" | 13 | 0.196 | 0.142 | 0.72 |
| 5/8" | 11 | 0.307 | 0.226 | 0.74 |
| 3/4" | 10 | 0.442 | 0.334 | 0.76 |
| 7/8" | 9 | 0.601 | 0.462 | 0.77 |
| 1" | 8 | 0.785 | 0.606 | 0.77 |
| 1-1/4" | 7 | 1.227 | 0.969 | 0.79 |
| 1-1/2" | 6 | 1.767 | 1.405 | 0.80 |
| 2" | 4.5 | 3.142 | 2.500 | 0.80 |
The governing capacity is the lesser of yielding and rupture. For threaded rods, rupture on the threaded area almost always governs because At/Ag = 0.72-0.80 and (0.75Fu)/(0.90Fy) is typically < At/Ag only for very high Fu/Fy ratios.
Worked example -- 1" F1554 Grade 55 sag rod
Given: 1" diameter F1554 Grade 55, Fy = 55 ksi, Fu = 75 ksi. Threaded full length.
Yielding: phiPn = 0.90 _ 55 _ 0.785 = 38.9 kips.
Rupture: phiPn = 0.75 _ 75 _ 0.606 = 34.1 kips -- governs.
Design capacity = 34.1 kips. If the rod carries a sag rod tributary load of 1.2 kips from purlin self-weight and cladding (factored), a 1" rod has capacity to spare. In practice, sag rod sizes are often governed by the minimum practical diameter for handling and installation (typically 5/8" or 3/4").
Sag rod design
Sag rods support purlins against rolling and carry a component of the gravity load parallel to the roof slope. For a roof slope of theta degrees:
T_sag = w_purlin * L_purlin * sin(theta) / n_sag_rods
Where w_purlin = purlin dead load per foot, L_purlin = purlin span, n_sag_rods = number of sag rods in the purlin span. Place sag rods at the ridge (where they anchor to a ridge beam) and at intermediate panel points. Each sag rod carries the accumulated load from all purlins below it to the ridge.
Turnbuckles and clevises
Turnbuckles provide length adjustment and pretensioning capability. Clevis-type end fittings allow pin connections. When specifying these accessories:
- Turnbuckle capacity must equal or exceed the rod capacity. AISC Manual Table 15-3 lists rated capacities.
- Take-up length is typically 6" (3" per side). Specify longer turnbuckles for installations requiring more adjustment.
- Clevises and pins must be checked for pin shear (double shear), pin bearing, and net section of the clevis ear per AISC 360 Section D5.
Practical tip: avoid compression in tension rods
Tension rods have zero useful compression capacity due to their high slenderness (L/r for a 1" rod at 20 ft = 240*12/(0.25) = extremely slender). If the rod must resist reversal loads (e.g., wind uplift on bracing), use a crossed-rod system where only the tension rod in each direction is active, or switch to a wide-flange or HSS member that can handle compression.
Common mistakes
- Using gross area instead of tensile stress area. Threads reduce the effective area by 20-28%. Using Ag instead of At overestimates capacity by that amount.
- Not checking the connection fittings. Turnbuckle bodies, clevis pins, and coupler nuts all have rated capacities that may be lower than the rod itself.
- Heating Grade 105 rods for bending. Quenched and tempered rods lose their strength if heated above 800 degF. Field bending of Grade 105 is not permitted.
- Omitting corrosion protection. Exposed tension rods (especially sag rods in humid environments) require hot-dip galvanizing or painting. Threaded surfaces are especially vulnerable.
- Not accounting for prying action. When tension rods attach to flexible base plates or flanges, prying action can amplify the rod tension by 20-40% above the applied load.
Run this calculation
Related references
- Steel Connection Types
- Bolt Grades Reference
- Base Plate Design
- Diagonal Bracing
- How to Verify Calculations
Disclaimer
This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against AISC 360-22 Chapters D and J and the governing project specification. The site operator disclaims liability for any loss arising from the use of this information.