Anchor bolts are used extensively as foundation bolts for rotating equipments like machines and structural members like towers. The American Concrete Institute (ACI) 318 Appendix D has extensive guidelines for designing concrete anchor bolts.
This series of eight articles will cover all the design guideline of the ACI code with the help of the following concrete anchor foundation bolt design calculation example:
Problem statement of the design example
See the above two figures (Fig.1 and Fig.2) and design the cast in place anchor bolts according to the arrangement shown. Consider the factored tensile load as 20000 lb, factored shear load as 2300 lb and compressive strength of the concrete as 3500 psi. Also assume that the column is mounted at the corner of a large concrete slab.
The aim of this whole exercise is to calculate the design tensile strength and design shear strength of the group of anchor for a selected anchor bolt diameter and check if the design strengths are higher than the applied loads. If they are then we will declare that the selected bolt size is safe or else we will go for next higher size of the anchor bolts.
We will start with the anchor diameter of 0.75 inch and do the design calculations through the following eight parts:
Part-1: Determining Steel Strength of Anchor in Tension (presently we are here)
The calculation of steel strength of anchor in tension according to the ACI code goes like below:
Steel strength in tension, φNsa = φnAse,Nfuta……………………..D-3
Φ – Strength reduction factor and its value for ductile anchor bolt in tension is 0.75
Nsa – Nominal material (steel) strength of the group of anchor in lb
n – Total number of anchors
Ase,N – Single anchor bolt’s effective cross section area (to be obtained from manufacturer’s catalog) in square inch
futa - Specified tensile strength for a single anchor (to be obtained from manufacturer’s catalog) in psi
The 0.75 inch anchor typically has the following cross section and tensile strength values:
Ase,N = 0.334 square inch
So, by putting these values, we can get the nominal material strength for the group of anchors in tension from the equation D-3 as
Φ Nsa = 0.75*4*0.334*75000 = 75150 lb
In the next part (part-2) we will calculate concrete breakout strength.
Let me know if you have any suggestions.