Design an elevated water tank structure

An elevated water tank stores water high enough to give pressure to the network below. The tank itself is heavy — every cubic metre of water weighs about 10 kN — and it sits on a relatively slender support structure called the staging. That makes these structures sensitive to earthquakes: a large mass on top of a light frame. The Elevated water tank template in Struxt3D builds the staging: legs on a circle, ring beams at several levels, X-bracing between the legs and floor beams under the tank.

#How the staging carries load

Vertically, the water and tank weight goes straight down the legs. Sideways, wind on the tank and staging and — more importantly — the earthquake inertia of the full tank act near the top. The braced panels between the legs carry that horizontal force down like a braced tower: the bracing takes the shear, and the legs carry the overturning moment as tension on one side and compression on the other, on top of the large gravity compression.

#Build the model

1. Pick the template: Towers › Elevated water tank.
2. Set the size with the parameters below. The tank volume sets the water load.
3. Choose the materials. Steel, with groups Legs (tubes), Ring beams and Bracing. The floor beams under the tank belong to the ring-beam group.
4. Set supports and loads. Choose Fixed or Pinned leg bases, the wind speed, and — in most regions — switch on Earthquake and fill in the code's spectrum inputs and the Structural system (see below). Switch on Pad footings to size a footing under each leg.
5. Run the analysis (F5).

#The template's parameters

Parameter	Default	Range	What it does
Legs	4	3–8	Number of legs, evenly spaced on a circle.
Leg circle radius at ground	4 m	2–10 m	Radius of the leg circle at the base.
Leg circle radius at top	3 m	2–10 m	Radius at the tank floor; smaller than the base for battered legs.
Height to tank floor	15 m	6–30 m	Height of the staging.
Ring beam levels	3	2–8	Number of braced panels up the height (a ring beam at the top of each).
Tank volume	200 m³	20–2000 m³	Water volume; its weight is shared equally by the leg tops.

#Loads

• Water: volume × 9.81 kN/m³, split equally between the legs and applied at the leg tops. Struxt3D puts it in the Dead case, so it is factored like permanent load (for example 1.35 in Eurocode, 1.2 or 1.4 in ASCE/AISC). The tank shell itself is not modelled; add its weight as point loads at the leg tops if it matters.
• Dead also includes the steel self-weight.
• Wind: drag on the staging members only. The tank is not in the model, and the wind on it is often the larger part — add it as horizontal point loads at the leg tops.
• Seismic: a response-spectrum analysis in X and Y. The modal masses come from the gravity loads, so the full water weight is a lumped mass at the top of the legs.

#What to check in the results

1. First mode. With earthquake on, open Vibration modes. The first two modes are the tank swaying on its staging in X and Y. Their period sets the spectral acceleration. The First period card shows it at a glance.
2. Leg axial force. Show Axial with the Envelope. Gravity compression is about (water + steel) / number of legs; the earthquake and wind combinations add to it on one side.
3. Bracing. Pick a seismic case and show Axial: the bottom panel's braces carry the full base shear.
4. Floor beams. They carry no water in this model (the water sits on the leg tops), only their own weight. In a real tank they carry the tank floor; check them by hand or add line loads.
5. Reactions: the leg-base forces for the foundation, including any uplift under earthquake.

A quick estimate of the base shear with the tank as one mass m on a staging of lateral stiffness k:

#Design the staging

Click Run design (F6). Legs are checked as steel tubes in compression and bending with buckling between ring beams; bracing in tension and compression; ring beams and floor beams as beams. Auto-sizing gives each group the lightest passing section. With steel connections on, the leg bases get base plates and anchors, and the brace joints gussets.

Use Calculation report for the full design, or select a leg and get its element report with the forces of every combination.

#Common problems and fixes

What you see	Why	Fix
Legs fail in buckling	Large gravity load on long leg panels.	More ring beam levels, more legs, or a larger tube.
Bottom braces fail under earthquake	The base shear all goes through the bottom panel.	Heavier bracing, or a wider base to stiffen the staging.
Uplift at leg bases	Overturning exceeds gravity on the windward legs — usually with the tank empty.	Check the empty-tank case by setting a small volume, and design the anchors for it.

#Rules of thumb