If you have access to Priscus he wrote about the siege of Naissus and it seems that Attila had access to bridging equipment and siege engines. Excerpts from Priscus 6,2 in paragraph below. Other cities taken by the Huns were Viminacium, Singidunum & Sirmium. So possibly Attila had captured engineers for siege artillery as for bridging equipment I don't think any of the great steppe tribes had much trouble crossing wide rivers from the Volga to the Danube. Sarmatians specialized in winter raids & campaigns using the frozen rivers & island hopping although the Huns and Avars may have had more sophisticated river crossing techniques as alluded to by Priscus.

 Since the citizens did not dare to come out to battle, the Scythians bridged the river from the southern side at the point it flowed past the city and brought their machines up to the circuit wall. They brought up beams mounted on wheels, upon which men stood who shot across at the defenders on the ramparts. At the other end of the beams stood men who pushed the wheels with their feet and propelled the machines wherever they were needed so that one could shoot successfully through the openings made in the screens. In order that the men on the beam should fight in safety, they were sheltered by screens woven from willow covered with rawhide and leather to protect against other missiles and whatever fire darts might be shot at them. When in this manner a large number of machines had been brought up to the wall with the result that the defenders of the battlements gave in because of the clouds of missiles and evacuated their positions. The so-called rams were then brought up as well as men with ladders. 

 Regards

Michael Kerr