Engineering guide
This module calculates live loads — essentially the wind load — on a vertical cylindrical vessel per DIN 4133 (the German standard for steel stacks, 1991-11 edition) for heights up to 8 m. From the outside diameter and height, the projected cylinder area is formed; with the velocity pressure, basic force factor, reduction factor and resulting force factor, the wind load is calculated, and from it the bending moment at the restraint point at the vessel base is determined.
Optionally, the calculation accounts for the vibration behavior: from the dead load with internals and the dead load of the bearing construction, the vibration period is determined, and the dynamic amplification of the wind load is captured via the gust reaction factor. The restraint moment is then the input quantity for the verification of the vessel wall under supplementary loads, and for the support skirt, anchor bolts and foundation.
DIN 4133 has been withdrawn. For new designs, wind loads are today determined per DIN EN 1991-1-4 (Eurocode 1) with its national annex, and the chimney-specific verifications per DIN EN 1993-3-2. For existing plants and the re-rating of equipment whose structural analysis is based on DIN 4133, however, the old procedure remains of practical relevance — that is what this module is intended for.
Scope
- Live loads
Typical workflow
Determine geometry and wind exposure area
From the outside diameter and height (h ≤ 8 m), the projected cylinder area is calculated as the wind exposure area. At this low structure height, the velocity pressure may be applied as a constant value over the height.
Determine the force factor
Starting from the basic force factor of the circular cylinder, the effective force factor is determined via the slenderness-dependent reduction factor. It captures the fact that cylinders of finite height experience less drag due to flow around the free ends than an infinitely long cylinder.
Check the vibration behavior
If the vibration option is activated, the module calculates the vibration period from the dead load with internals and the dead load of the bearing construction, and from it the gust reaction factor. For squat, stiff vessels this remains close to one; for flexible constructions it increases the wind load to be applied.
Calculate wind load and restraint moment
From velocity pressure, force factor, projected area and gust reaction factor, the resulting wind load follows. With its lever arm to the vessel base, the moment at the restraint point is obtained — the governing internal force for the anchorage, foundation and wall verifications.
Frequently asked questions
Why is the method limited to heights up to 8 m?
Up to a structure height of about 8 m, the velocity pressure may be applied as a constant ground-level value per the old German load concept; above that, it increases with height and the calculation must be carried out segment by segment. For taller equipment with attachments, the sister module for heights up to 50 m is available, which accounts for the velocity-pressure profile over the height.
When must the susceptibility to vibrations be considered?
When the fundamental period of the construction is so long that gusts can excite the structure dynamically. Low, squat vessels with h ≤ 8 m are usually stiff enough that the gust reaction factor is close to one; for light, flexibly supported constructions or elevated equipment, the dynamic amplification can become relevant. The module queries this explicitly instead of tacitly neglecting it.
Are wind loads per DIN 4133 comparable with those per the Eurocode?
Only approximately. DIN 4133 worked with the lump-sum velocity-pressure concept of the old DIN 1055 world, DIN EN 1991-1-4 with wind zones, terrain categories and gust velocity pressure. Depending on location and terrain category, the Eurocode can yield higher or lower loads. When re-rating existing equipment, it must therefore be clarified whether the protection level of the erection standard is sufficient or whether a reassessment per the Eurocode is required.
What role do the two dead loads play in the calculation?
The dead load with internals describes the vibrating mass of the system, while the dead load of the bearing construction describes the structure that determines the stiffness. From both, the vibration period is estimated, which enters the gust reaction factor. In the overall stability verification, the dead loads additionally act favorably as a restoring moment against overturning — there they may only be applied with their reliably present share.
Technical content source: LV-Solved-Problems-Content 628d313247e7