Capital spending is poised to accelerate, giving the U.S. economic expansion a fresh boost after investment lagged way behind the half-century average.

Goldman Sachs Group Inc. economists said their "capex tracker" of 15 indicators, such as loan demand and capital-goods orders, shows a speed-up in nonresidential spending may already have begun. They see such investment climbing in 2014 by 7.5 percent, more than twice the forecast for 2013. Economist Joe Carson of AllianceBernstein LP predicts a broad-based surge with staying power.

Behind the optimism is a confluence of positives: Corporate America is lean and flush with cash, credit is easing, and aging durable goods from machinery to cars need to be replaced. A pickup in business spending would be welcome for an economy held back by cuts in government outlays, the January tax increase, and the partial federal shutdown.