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Explanation: As far as I am aware, the usual translation for this is tie beam (and not tie down or tying, which do not give many results in searches in Google)
However, you haven't actually given much context, so another term may be appropriate. See a few glossary entries below for tie beam:
Tie Beam - A Major, Horizontal Member, running Perpendicular to the Line of the Ridge, whose main functions are to Provide the Tying Action within the frame, resist the outward thrust generated by the roof, and, Sometimes, to bear floor loads. They also serve as the Major Horizontal Members in a number of trusses, particularly, the King Post Truss.
Anchor Beam - The Name given to the, often, enormous Lower Ties within a Dutch Barn. http://www.metaweb.com/wiki/wiki.phtml?title=A_Glossary_of_T...
The following members have special names:
Rafter: roof-timber sloping up from the wall plate to the ridge.
Principal: principal rafter, usually corresponding to the main bay divisions of the nave or chancel below.
Wall Plate: timber laid longitudinally on the top of a wall.
Purlin: longitudinal member laid parallel with wall plate and ridge beam some way up the slope of the roof.
Tie-beam: beam connecting the two slopes of a roof across at its foot, usually at the height of the wall plate, to prevent the roof from spreading.
Collar-beam: tie-beam applied higher up the slope of the roof.
Strut: upright timber connecting the tie-beam with the rafter above it.
King-post: upright timber connecting a tie-beam and collar-beam with the ridge beam.
Queen-posts: two struts placed symmetrically on a tie-beam or collar-beam.
Braces: inclined timbers inserted to strengthen others. Usually braces connect a collar-beam with the rafters below or a tie-beam with the wall below. Braces can be straight or curved (also called arched).
Hammer-beam: beam projecting at right-angles, usually from the top of a wall, to carry arched braces or struts and arched braces or struts and arched braces. http://www.hants.gov.uk/pevsner/glossary.html