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construction, a time effect factor λ is also included). Per the LRFD approach, the design
tie strength must be greater than or equal to the required tie strength:
Design Tie Strength = Φ Rn ≥ Required Tie Strength
= Strength reduction factor
= Nominal Tie Strength calculated with the
appropriate material specific code, including over-
strength factor Ω where applicable.
For the purposes of this UFC, all strength reduction factors, Φ, are taken
as the appropriate material specific code value.
Required Tie Strength.
The required tie strength for horizontal and vertical ties is defined for each
material type in Chapters 4 through 8. The structural elements used as ties must not
only provide sufficient tie strength, but they must also be adequately connected so that
the tie forces can be distributed throughout the rest of the building.
The design tie strengths are considered separately from the forces that are
typically carried by each structural element due to live load, dead load, wind load, etc.;
in other words, the design tie strength of the element or connection with no other loads
acting must be greater than or equal to the required tie strength.
Some of the tie forces are based on the dead and live loads. In some cases,
a structure may have different loads, such as a corridor load or office load, on the same
floor. In such cases, use an averaged dead or live load, by computing the total force
acting on the floor and dividing by the total plan area. When tie forces are based on a
span L that varies along the length of a tie, the largest span in a continuous tie should
be used for the tie force calculation.
Structural Elements and Connections With Inadequate Design Tie
If all of the structural elements and connections can be shown to provide the
required tie strength, then the tie force requirement has been met. If the vertical design
tie strength of any structural element or connection is less than the vertical required tie
strength, the designer must either: 1) revise the design to meet the tie force
requirements or 2) use the Alternate Path method to prove that the structure is capable
of bridging over this deficient element. Note that the AP method is not applied to
structural elements or connections that cannot provide the horizontal required tie
strength; in this case, the designer must redesign or retrofit the element and
connection such that a sufficient design tie strength is developed.