2.2 EVALUATION OF TEST DATA
2.1.8 PRE-FABRICATED PRE-TENSIONED / PRESTRESSED CONCRETE
Pre-tensioned / prestressed concrete is typically cast around tensioned tendons or strands . This method produces a good bond between the tendon and concrete , which both protects the tendon from corrosion and allows for direct transfer of tension . The cured concrete adheres and bonds to the bars and when the tension is released it is transferred to the concrete as compression by static friction and keying . Most pre-tensioned / prestressed concrete elements are pre-fabricated in a factory and must be transported to the construction site , which limits their size . Often , pre-stressed concrete is realized as hollow core concrete slabs . Load data for X-U fasteners installed into hollow core concrete can be found in section 3.2.6 of the Hilti Product Technical Guide Vol 1 .
Pre-tensioned / prestressed concrete requires special consideration for the installation of power-actuated fasteners . ACI 318-14 , chapter 20 specifies for tendons and strands a minimum concrete cover of ¾ in . to 3 in . for prefabricated prestressed concrete . Installing fasteners with a deeper embedment than the clear concrete cover into this type of concrete is typically not recommended unless the precise location of the tendons is known .
Locating steel reinforcement or tendons can be realized using Hilti Detection Systems . Please refer to http :// www . us . hilti . com , www . hilti . com , or www . hilti . ca for guidance . If the location and clear cover over the tendons is known , poweractuated fasteners with embedments of less than the clear cover distance typically are not expected to interfere with the tendons or strands .
Bonded post-tensioned concrete is a term used for a method of applying compression after pouring concrete , and completion of the curing process , through tension tendons . This method is commonly used to create monolithic slabs . Slabs are created in-situ versus being manufactured at a plant . The same considerations as described in section 2.1.8 for avoiding fastener installation damaging post-tensioning tendons should be considered when using power-actuated fasteners .
Hilti offers fasteners with various low embedments and premounted washers ( for example , X-U 22 P8 S15 ). The premounted washers help avoid installation beyond the target embedment in case of excess energy . Testing has shown that the embedment of the X-U 22 P8 S15 , when installed at the highest power level , will penetrate no more than 3 / 4 " in the concrete .
When Power-Actuated fasteners are properly installed in Posttensioned concrete , the load capacities published for normal cast-in-place concrete of equivalent compressive strength may be utilized .
2.2 EVALUATION OF TEST DATA
2.2.1 DEVELOPING FASTENER PERFORMANCE DATA
Power-actuated fastener testing at Hilti is conducted in accordance with ASTM E1190 Standard Test Methods for Strength of Power-Actuated Fasteners Installed in Structural Members , unless otherwise noted in the corresponding load tables . Because of the wide variation in possible concrete reinforcing configurations found in construction , testing is typically performed in unreinforced concrete , which gives conservative results .
There are two methods of developing allowable loads ; ( 1 ) apply an appropriate safety factor to the mean ultimate load as determined from a given number of individual tests , or ( 2 ) apply a statistical method to the test data which relates the allowable working load to the performance variability of the fastening .
2.2.2 ALLOWABLE LOADS
Historically , allowable loads for power-actuated fasteners were derived by applying a safety factor to the average ultimate value of test results . This approach is characterized by Eq . 2.2.2 .
Eq . 2.2.2 F all =
F v
Where :
F = mean of test data ( population sample ) v = safety factor
Statistical safety factors for power-actuated fasteners are generally calculated using ICC-ES AC70 . The statistical safety factor is assumed to cover expected variations in field installation conditions and variation in power-actuated fastener performance from laboratory tests .
Note that statistical safety factors calculated per ICC-ES AC70 account for test data coefficient of variation , ( i . e ., each data point has its own safety factor based on the variability in the test data ).
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