I’m designing a 55 m steel footbridge, and my model in SAP2000 gives about 0.7 m/s² vertical acceleration at 2.0 Hz assuming 0.5% damping; do you stick with that conservative damping or justify 2–3% with crowd-induced effects or a small TMD allowance? Interested in what you’ve stamped and seen hold up once the bridge opens…
TMD allowance? Interested in what you’ve stamped and seen hold up once the bridge I don’t count crowd damping; if you want 2% then model a TMD (≈0.5–1% mass ratio tuned about 1.9–2.1 Hz), otherwise stamp 0.5–1% and pre-detail a midspan pocket and anchors to add one after heel-drop tests. Do you have room for a about 300 kg module at midspan?
Short answer from my side: I’m seeing the same pattern — one concrete thing that helped was writing down the exact handoff and timebox it to 15–20 min. Does that match what you’re running into?
I stick with 0.5% unless I can specify and verify added damping from the build-up — e.g., viscoelastic surfacing and continuous balustrade/glazing — and then prove it with a quick heel‑drop before opening; “measure it, then claim it.” On a 56 m, 2.0 Hz steel footbridge we reserved TMD anchors but ended up at about 0.5 m/s² after the surfacing went on, so no TMD needed. @OP do you have any non-structural components you can lean on and test?
And on a 60 m footbridge I stamped at 0.5% and sidestepped the damping argument by nudging the first vertical from “2.0 Hz” to about 2.3 Hz with a modest stringer/parapet stiffness bump; the commissioning modal test later showed about 1.2% damping, but I didn’t claim it in design. If you’re seeing “0.7 m/s² at 2.0 Hz,” can you add 5–10% vertical stiffness at midspan and pre-spec stub plates for a drop‑in TMD only if the opening‑day test still fails?
Quick example: on a about 55 m footbridge I kept the design at 0.5% but ran a sensitivity with 1.5–2% plus +10–15% deck mass from a thin asphalt/VE overlay and continuous glazed balustrades; that brought “0.7 m/s²” under the ISO 10137 comfort line and we baked those items into the spec. If you go that route, add an acceptance tap‑test (log decrement) so you can bolt on a small TMD only if measured ζ falls short — belt‑and‑suspenders. @OP, if there’s no room for an overlay/glazing package, a 30–40 kg TMD seat near the antinode is a clean fallback.
At around two hertz with 0.7 m/s², I wouldn’t bank on 2–3% — empty steel spans I’ve had measured in the 0.3–0.8% range. I stick with 0.5% in SAP2000 and, like @francis_king42, predefine a TMD pocket plus a simple trigger: if as-built damping ratio comes in under about 1%, install the unit; otherwise leave it. Any constraints on placing a about 1–2% modal-mass TMD at the vertical antinode?
I wouldn’t bank on 2–3% unless you’re specifying damping hardware; with an empty steel span near 2 Hz and ‘0.7 m/s^2’ in SAP2000, I’d design to 0.5% and reserve space for a small TMD or constrained-layer damping in the deck. If you do want to justify higher zeta, make the constrained-layer damping or handrail TMD part of the spec and verify with a quick heel-drop check before opening. What’s your planned surfacing and handrail detail, since those usually decide whether you can credibly claim >1%?
Before debating 2–3%, make sure your ‘0.5%’ is applied at 2.0 Hz in SAP2000 — switch to modal damping per mode or set Rayleigh a/b to hit 0.5% at 2 Hz, then recheck the 0.7 m/s². On a 56 m steel footbridge I saw predicted accel drop about 15% just from fixing the damping model, and I left a small TMD pocket in the midspan diaphragm as insurance. Are you running Rayleigh or modal damping right now?