The first check we must perform before designing a building is the drift check. This consists of the evaluation of the lateral displacements of a building due to the effect of seismic loads. But sometimes it is not clear the reason why we perform this check and what is its importance. Here it'll be explain.
What is drift?
In accordance with the provisions of section 12.8.6. of ASCE 7-1610:
"The design story drift (Δ) shall be computed as the difference of the deflections at the centers of mass at the top and bottom of the story under consideration"
Following the above, in the first figure, if we consider a column as the black line, we could say that, the drift is the difference between the displacements obtained in one level and the level immediately below for that column. We called this value as Δ. To calculate Δ, we can use the following expression:
where δi is the total displacement, measured in the floor "i".
Why is it calculated?
Many Codes, as the NSR-10 or the ASCE 7-16 requires that the resulting drift, at every level of the building, be less than 1% of its height.
If this check is not fulfilled, it is necessary to increase the stiffness of the structure enough to reduce lateral displacement, that could mean, the use of materials with higher strength, or increase the dimensions of vertical elements such as columns or walls, and in extreme cases, add new vertical elements.
The secret behind of drift check.
It seems that drift is nothing more than a mathematical concept, however, there is a fundamental reason behind its importance. As Roberto Rochel Awad describes it in his book Analysis and seismic design of buildings, The limits of drift guarantee that, in the face of a low seismic event (which is usually common in Colombia), the structure moves so little, that the Non-structural elements (partition walls, facades, etc.) will not suffer damaged. In this way, we guarantee savings in repair costs in the event of the mentioned event.
While the standards provide limitations and exceptions for drift checking, it is important to take into account the reason behind it, so that the designer engineer, with appropriate criteria, ensures that the structure is rigid enough to meet the customer's expectations and thus continue to the design of the structural elements.
Comments