FrostLine

LRFD Load Combinations

AASHTO LRFD Preliminary estimate

Factor dead- and live-load effects through the AASHTO LRFD Strength and Service combinations and find the governing factored demand.

Enter the unfactored force effect (e.g. moment in kip·ft, or shear in kips) from each source. Results carry the same units.

§1.3.2, typically 1.0

Results update as you type. These are educational estimates — confirm against the governing code and a licensed engineer before design use.

Method & references

AASHTO LRFD (§3.4.1) factors each load source and sums them for a series of limit-state combinations. For the common dead + live case:

Q = η · ( γDC·DC + γDW·DW + γLL·(LL+IM) )

  • Strength I — basic vehicular: γDC 1.25, γDW 1.50, γLL 1.75
  • Strength II — permit vehicles: γLL 1.35
  • Strength IV — dead-load dominant: γDC 1.50, no live load
  • Service I — normal operation, all γ = 1.0
  • Service II — steel yielding/slip: γLL 1.30
  • Service III — prestressed concrete tension: γLL 0.80 (legacy; recent editions vary)

Permanent loads have a maximum and minimum factor (γDC 1.25/0.90, γDW 1.50/0.65). The maximum governs when a load adds to the effect (shown here); use the minimum where a permanent load relieves the effect, such as an uplift or overturning check. The load modifier η accounts for ductility, redundancy and importance.

Reference: AASHTO LRFD Bridge Design Specifications, Tables 3.4.1-1 and 3.4.1-2. Verify factors against the governing edition.