* Architecture Billings Index drops 1.2 points to 45.1
* Demand weakest in Midwest
* Worries that more declines to come, economist says
BOSTON, Aug 17 (Reuters) - A leading indicator of nonresidential construction in the United States fell for a fourth straight month in July, in a sign that demand for offices and other commercial properties is weakening.
The monthly Architecture Billings Index fell to 45.1 last month, a 1.2-point drop that was its steepest since February 2010, the American Institute of Architects said on Wednesday. Any reading below 50 indicates a contraction in billings by architects, whose revenue projects construction activity over the next six to nine months.
"Business conditions for architecture firms have turned down sharply," said Kermit Baker, the group's chief economist. "Late last year and in the first couple of months of this year there was a sense that we were slowly pulling out of the downturn, but now the concern is that we haven't yet reached the bottom of the cycle."
By region, the Midwest was weakest, while the South was strongest. By sector, demand was weakest for multifamily residential buildings and strongest for commercial and industrial sites.
"The steep decline in the ABI since February suggests that the nonresidential construction recovery struggles to gain traction," said J.P. Morgan analyst Ann Duignan, noting that the sector may not recover until next year or later.
Weak construction activity for both commercial properties and homes has depressed demand for building components including elevators, air conditioners and electrical systems as well as contributing to high unemployment in the construction trades.
Companies exposed to the sector include Honeywell
International Inc