San Francisco prides itself on helping the homeless, and it even created a special committee for grievances from those kicked out of shelters. But this month, it began a new experiment that treats the homeless more harshly than the city's other policies. After lengthy training, police began enforcing an ordinance that bans sitting or lying on sidewalks during daytime hours. The measure, which was approved by 54 percent of city voters in November, has drawn criticism from homeless groups and the down-and-out themselves. The new approach comes at a time when the city is working to bolster tourism and local businesses in a tough economy.