Post-installed Rebar Design II

Ultimate Limited State (ULS)

Ultimate Limited State (ULS) considers the safety of a structure and its users by limiting the stress that materials experience. In our application, ULS concerns the capacity of PIR system under normal loading conditions.

Before going deep into calculation, there is one key assumption that the rebar will only take tension forces, shear force will be carried by concrete.

The load factor to be applied from engineers complies to the standards from Hong Kong Building Department. From the factored load, we calculate the design stress of the rebar by dividing the tension force by area of steel provided.

With the design stress, rebar diameter and bond stress known, we can calculate the basic anchorage length required based on the above equation.

Factors to be considered for Minimum Anchorage Length

The next step is to obtain the design anchorage length from basic anchorage length by multiplying different influence factor, there are in total five factors to consider.

The consideration for each coefficient is as follow:

𝛼1: whether straight/ bended rebar is used in PIR application

𝛼2: Considering the relationship between concrete cover and rebar diameter

𝛼3: Considering the confinement by transverse reinforcement, whether the PIR is enclosed by existing reinforcement in two/one direction or not enclosed at all

𝛼4: Welded transverse reinforcement does not exist in most cases.

𝛼5: Considering the transverse pressure

In PIR case, for 𝛼1 𝛼3, 𝛼4, 𝛼5, they are all considered to be equal to 1. Therefore, the most influential coefficient would be 𝛼2.

The next step is to calculate minimum anchorage requirement.

The minimum anchorage length is given the above equation in terms of basic anchorage length required and rebar diameter.

Please note that the “10D” in the equation is only one of the considerations to determine minimum anchorage length, it should never be used as a benchmark on its own to determine rebar depth because the actual anchorage length has to consider design anchorage length as well.

Finally, we need to conduct a checking between the design anchorage length & minimum anchorage requirement and the larger value will then be the anchorage length for the application. Additional provision such as the edge distance & spacing also needed to be checked.

Difference between EC2 & HKBD 2013

In Hong Kong Code, the basic anchorage length is under a “Design to Yield: assumption while EC2 is calculated based on design stress. EC2 has further provision on the design anchorage length and minimum requirement while there is no further provision in Hong Kong codes.

In upcoming article, we will cover the design approach for Fire Limit State (FLS) condition.