2.4 DETERMINE DEVELOPMENT LENGTH NEEDED FOR FIRE CONDITION
2.5 DESIGN EXAMPLE WITH CONSTANT TEMPERATURE ALONG BOND LENGTH
Using the concrete temperature determined from Figure 4.4.2.2.1a of ACI / TMS 216.1 as θ determine the corresponding bond stress under fire conditions at that temperature , t fire ( θ )
. This can then be determined from the graph , or by plugging θ into the provided equations for the Hilti adhesive product .
The maximum temperature , θ max
, is the highest temperature permitted for the adhesive and once beyond that temperature the product is assumed to have no bond stress . For constant temperature along the length of the rebar , if the concrete temperature at the bar exceeds θ max
, then additional concrete cover will be needed . For variable temperature along the length of the rebar , there may be segments where
θ max is exceeded and t fire ( θ ) is effectively zero and would not contribute to the overall bond stress in that section of the rebar for fire exposure , and the overall bond stress would be taken from the segments in the concrete where θ is less than
θ max
.
2.4 DETERMINE DEVELOPMENT LENGTH NEEDED FOR FIRE CONDITION
For constant temperature along the length of the rebar the value of t fire ( θ ) determined from the charts will be the value that is then compared to the value of t equiv calculated from the development length at the ambient temperature . From this , the development length needed for the fire condition can be calculated as follows :
l d , fire = t equiv
t fire ( θ )
∙ l d
Note : If the development length for the fire condition , l d , fire
, is less than the ambient temperature development length ,
l d
, then the ambient temperature development length , l d , should be used . Effectively , as long as the value of t fire ( θ ) is greater than the value of t equiv along the entire length of the rebar then the ambient temperature development length will control .
For variable temperature along the length of the rebar , the process is more iterative . The value of t fire ( θ ), seg will need to be determined at each of the equal length segments , l seg
, along the length of the bond length of the rebar , for a total number of segments , n seg
. Then , the value of t fire ( θ ), seg for each segment is multiplied by the surface area of the bar in each segment to give a segment bond strength N a , fire , seg ( θ )
. The segment bond strengths can then be summed up for a total bond strength , N a , fire
. This total bond strength needs to be greater than the yield stress of the rebar .
N a , fire
= Σ n seg i = 1 π ∙ d ∙ l ∙ t ≥ f ∙ A b leg , i fire ( θ ), segi y se , N
Once the total number of segments is determined then the final development length needed for the fire condition can be calculated .
l d , fire
= n seg
· l seg
Note : The value of t fire ( θ ) in any individual segment cannot exceed t equiv determined from Section 2.1 . Thus a development length for the fire condition will never be smaller than the development length determined at ambient temperature . Similar to the calculation for constant temperature along the length of the rebar , as long as the value of t fire ( θ ) in the warmest section of the concrete is greater than the value of t equiv then the ambient temperature development length will control and the iterative calculation above does not need to be performed .
Similar to determining the temperature at a specific concrete cover for variable concrete temperature , the smaller the value of l seg will lead to more calculations of t fire ( θ ), seg but will lead to a less conservative development length in the fire condition when larger values of l seg are used .
2.5 DESIGN EXAMPLE WITH CONSTANT TEMPERATURE ALONG BOND LENGTH
The following design example is for a floor slab extension in 4,000 psi normal weight concrete with carbonate aggregate .
Existing
Given :
Bond length
• ASTM A615 Gr . 60 reinforcement , # 6 rebar installed with HIT-FP 700 R
• Concrete cover , c b
= 2-inch
• Non-seismic floor loads only
New slab
Step 1 : Determine ambient temperature development length and equivalent bond stress :
• From Table 3 of Section 3.0 ( or ACI 318 Chapter 25 Eq . 25.4.2.3 ) the development length for HIT-FP 700 R with # 6 rebar is determined :
6 May 2023