She recruited her sons George, 11, and Sam, 10, and they "piled down to Primark" to buy cushions to cover them.
"This is not the Mancunian thing, it's not how we treat people," she said.
Ms Platt, who is an estate agent, added: "The building owners are treating human beings like pigeons. ...I woke up on Sunday morning with a right bee in my bonnet and had to do something."
After watching her sons play rugby for the Sedgley Park Tigers on Sunday morning, the boys recruited a couple of their friends and they went shopping to buy soft furnishings and sandwiches, which they left outside the building.
Ms Platt said: "I know they won't last and I knew they'll get wet, but the people who manage that building need to know how to treat people."
GVA, which manages the building, said: "Deterrents were installed on a small area off Marsden Street on Friday 27 January. These deterrents were removed within 48 hours."Pall Mall Medical, which rents part of the building, said the company had "zero involvement in the spikes".
A statement on Twitter added: "This decision was made fully without our consent or involvement."
Seventy-eight people were sleeping rough on Manchester's streets in autumn 2016 - an increase of 11% on the previous year, government figures show.
Edited by AA