When you take out an IRA distribution, more of your social security becomes taxable. I ran a test on the 2017 software. Currently only ~$1950 of your SS is taxable, see line 14 of form 1040A (line 20 of form 1040). If you take a $13,000 IRA withdrawal, ~$11200 of your SS becomes taxable. This essentially means that ~$9250 of your $13K is double taxed. Effectively you will pay 25% on the $13K, even though you are still in the 15% tax bracket. Any amount you withdraw is going to be treated the same until your taxable SS reaches 85% (0.85 x 17580 = 14,943)
This problem will not go away in 2018. The only thing that changes is the lower tax brackets, 12% and 22%.