One side of the fire-damaged Thai-Belgian Bridge on Rama IV Road was reopened, but only for small vehicles, around noon on Wednesday after engineers reported that the section remained structurally sound.
However, traffic in the area remained heavily congested.
Bangkok governor Aswin Kwanmuang said on Wednesday the four-lane bridge was examined and tests run on Tuesday night. The inbound-side of the bridge heading into the Silom area was found to be undamaged.
The partial reopening was for small vehicles only. Vehicles with six wheels or more were prohibited from using the bridge. Height-limit bars would be installed to enforce the ruling, Pol Gen Aswin said.
A fierce fire in unused plastic garbage bins stored under the bridge in Pathumwan district on Tuesday morning damaged the steel structure on the two-lane section, taking traffic out of the city from the Klong Toey side. The steel bridge is about 300 metres long. The fire weakened a 25 metre section of the right-hand lane on that span.
The left lane of that section would reopen next Monday, but the right lane would remain closed until repairs were completed in about a month, the Bangkok governor said.
The damaged section had warped when water was sprayed on the fire-heated steel to cool it down quickly, said Assoc Prof Anek Siripanichgorn, head of the civil engineering committee of the Engineering Institute of Thailand.
The institute advised the Bangkok Metropolitan Administration (BMA) to install additional support to secure the part of the bridge close to the fire, he said.
Pol Maj Gen Jiraphat Phumjit, deputy chief of Bangkok police, said the two lanes reopened on Wednesday were reversible, with the direction to be decided by police according to demand at different hours. Traffic continues to flow along the roads beneath the bridge.
The fire prompted City Hall to remove unused garbage bins stored under other bridges.