Seismic detailing — beams and columns, DCH and DCM

What this page checks

The local ductility requirements for frame beams and columns, from the Romanian seismic code P100-1/2013, chapter 5. Every row of the results table carries the clause it comes from, so it can be traced back to the code:

• the normalised axial force ν_d, with both the ordinary and the raised limit
• the critical region length l_cr, including the short-column rule
• confinement — the geometric ratio ρ_w and the mechanical ratio ω_wd
• stirrup spacing and the in-section distance between restrained bars
• geometric conditions and longitudinal reinforcement ratios

Two classes, not three

This is a common confusion. P100-1/2013 §5.2.1(3) states that dissipative structures fall into two ductility classes: DCH (high) and DCM (medium).

DCL is not a third column of detailing rules. §5.2.1(4) defines it as a concept of low-dissipation behaviour which "follows, in the main, the general design rules for reinforced concrete of SR EN 1992-1-1" — and which "may only be chosen for structures located in zones with design ground acceleration a_g ≤ 0.10 g".

In other words: with DCL there are no seismic detailing rules to check, there is EC2. So this page does not invent a third set of values; it says so explicitly.

Which class, in practice

§5.2.1(3): "in general, structures in zones of high seismicity (a_g ≥ 0.3 g) shall be designed for the high ductility class".

Bucharest sits at 0.30 g, and much of southern and eastern Romania at or above it. So in Romania DCH is the common case, not the exception — which means tools that implement only DCM cover precisely the less relevant class here. The page flags it when the chosen class conflicts with the a_g entered.

How the code is structured, and why it matters

§5.4 (DCM) is written almost entirely as "apply 5.3.x" — the DCH rules — with specific overrides. The differences that matter:

The same column can pass as DCM and fail as DCH. The class is not a reporting label.

ν_d, and why it has two limits

The normalised axial force ν_d = N_Ed/(A_cf_cd) measures how heavily compressed the column is. The more compressed, the deeper the compression zone and the smaller the rotation it can take before the concrete crushes.

The code gives two values: one "as a rule" and a raised one, admitted only if the available rotation, calculated explicitly, exceeds the demand of §5.2.3.3.2. The checkbox on the page switches between them — but ticking it without having done that calculation is a false declaration, not a relaxation.

The short column

§5.3.4.2.2(6): if l_cl/h_c < 3, the whole column is treated as a critical region. This is not conservative rounding: a short column is too stiff to rotate, attracts large shear and fails in a brittle way, in shear, before reaching the flexural mechanism the whole dissipative design relies on. It is the classic cause of collapse in ground floors with tall windows.

ω_wd — confinement

The mechanical ratio of the confining stirrups, relation (5.24):

\(\omega_{wd} = \frac{\text{volume of confining stirrups}}{\text{volume of the confined concrete core}} \cdot \frac{f_{yd}}{f_{cd}}\)

The core is measured inside the perimeter stirrup, not on the outline of the section — the cover spalls off in the first cycles and stops contributing. The stirrups do not add strength themselves; they hold the core concrete in a triaxial state, and confined concrete sustains several times the strain before crushing.

Assumptions and limits

• Covers frame beams and columns. Structural walls (§5.3.4.3), beam-column joints (§5.3.4.2.3) and coupling beams (§5.3.4.4) are not included.
• These are detailing checks, not strength checks. Bending and shear are verified separately to SR EN 1992-1-1, with the action effects from capacity design (§5.2.3.3.3) — for columns, with the compression diagonal at 45°.
• ω_wd is computed on the perimeter stirrup unless the total leg length is supplied; additional crossties increase it.
• The available-rotation check that permits the raised ν_d is not performed here.

Related calculations

• Minimum reinforcement ratios — the P100 table for 15 members × DCH/DCM/DCL
• RC section — M_Rd and V_Rd to EC2
• Column N·M interaction diagram — where ν_d comes from
• Beam serviceability — the serviceability checks of the same beam