Many merchants rang up strong sales in December but steep promotions took a toll on the bottom line at some retailers this holiday season. Framingham discounter TJX Cos. was among the strongest performers with sales at stores open at least a year rising 8 percent in December. Department store chain Macy’s reported a 6.2 percent gain, and luxury retailer Nordstrom posted an 8.7 percent increase in revenues during the month. “Holiday 2011 was the most promotional selling season we have seen in the past decade, excluding the fire sales of 2008 when the sky was falling, which yielded great deals for consumers and more robust top lines for retailers,’’ said Ken Perkins of Retail Metrics in Swamspcott. “This did, however, come at a cost to retailers bottom lines. Margins were under pressure from stepped up discounting, free shipping, price matching, greater expenses for increased advertising and expanded store hours.’’