Design of staircase, therefore, is the application of the designs of the different elements of the staircase. STAAD-PRO STAAD or (STAAD-Pro V8i) is a structural analysis and design computer program originally developed by Research Engineers International in Yorba Linda, CA. In late 2005. I have a copy of Davis & Murray's 2009 'Slender Monumental Stair Vibration Serviceability' paper which recommends a natural frequency.
The Advisory Desk has received a number of queries asking for guidance on vi- bration of steel staircases. In fact, guidance is already given in several parts of SCI publication Design of floors for vibration: A new approach (P354). This AD collects that guidance and adds references to other sources of information that may be relevant when analysing the vibration behaviour of steel staircases. This AD also identifies some other design considerations that may be relevant.General approachSteel staircases are, by their nature, highly susceptible to vibration as they combine low levels of damping (typically ζ ≈ 0.5%), low mass and high levels of human-induced excitation.
The general approach outlined in Chapter 6 of SCI P354 can be used to determine the dynamic behaviour of the staircase, but clearly the applied forces will be different for people travelling up and down staircases than for walking across floors, and acceptable levels of vibration also differ.Applied LoadsIn P354, the response is determined from the modal properties of the staircase (frequencies, modal masses and mode shapes) and the frequency and ampli- tude of the applied vertical load. The peak amplitude of the load in each mode is generally given in terms of Fourier coefficients, αn, which represent the proportion of a person’s weight that is acting at each harmonic of the activity frequency. These Fourier coefficients are given in Human induced loading of flexible staircases (Bishop, Willford & Pumphrey, 1995 and Kerr & Bishop, 2001), and depend on the speed of ascent or descent. ISO 10137: 2007 (Bases for design of structures – Serviceability of buildings and walkways against vibrations) reproduces the worst case of these in Table A.4, and these are also given in Table 3.2 of P354:In P354 floors are categorised as either low-frequency or high-frequency, the latter case responding to impulsive excitations rather than responding reso- nantly. No specific analysis is given by Bishop et al for the impulsive loads that will be experienced by staircases with natural frequencies that exceed the up- per limits of the Fourier terms given in ISO 10137.
Further Fourier coefficients (up to the 6th harmonic) are presented in the paper; these could be used to determine a more comprehensive response.Acceptability criteriaBishop et al. Also give guidance on the acceptability criteria for staircases, and their research indicates that for multi-person excitation, a maximum mul- tiplying factor of 64 applies. Typically this is achieved by designing staircases for a limiting multiplying factor of 32 for light use (such as offices) or 24 for heavy use (such as public buildings and stadia) under single person excitation using the Fourier terms given above. The limits for staircases are higher than for floors because the frequency of exposure to staircase vibration is gener- ally significantly lower than for a floor, and the audio and visual stimuli that ac- company the movement reduce the associated level of annoyance. For narrow staircases with no landings, it is unlikely that there will be stationary people on the stairs to receive the vibration, and as vibration is, in the main, a service- ability issue, in these cases the level or response is less critical.Other design considerationsAn additional design consideration for staircases is to ensure that the interac- tion between a staircase and the floors it links is such that excessive vibration does not carry onto the floor plate and therefore affect nearby rooms. This can generally be achieved by attaching the staircase in the vicinity of columns, and by avoiding features such as cantilever beams which are highly suscepti- ble to vibration issues.Contact: Andy SmithTel: 30Email.
I'm reviewing an exterior stair design. The rise of the stairs is 16', the run is 25', and the stair width is 3'.
There is no intermediate landing, so the stringers are continuous. This is an industrial stair, so the live load is 100psf x 5 per OSHA. The fabricator wants to use C12x20.7 stringers and put an MC8x8.5 channel across the midpoint of the stringers and support the stairs at midpoint with one column.If you load the stairs uniformly, then I don't have a problem with the single column at midpoint. However, is it reasonable to load the stairs with an unbalanced condition of one side of the stairs? This would create a moment condition between the MC channel and the column.
RE: Stair Design (Structural) 10 Dec 14 23:08. Agreed with stephen; if this were mine I would take 20 PSF as a reasonable anticipated live load and multiply it by the 5 times. I suspect the intent of that OSHA rule was to mean they want a safety factor of 5 for live load. Either way, 500 PSF is clearly unreasonable.To your original question, I would definitely consider all load cases, including one span unloaded. Don't forget the axial load in the stair stringers as well.Maine EIT, Civil/Structural.
RE: Stair Design (Structural) 11 Dec 14 02:45. As Ron pointed out the 5x or 1000# conc load is ONLY for the tread design.I usually use 100psf on the stair as awhole.the one-col support should work but I would make it a moment conn as far as the stringers are concerned.if I remember correctly there is a 12ft limit on the rise between landings unless you get a code variance.IMO using an unbalanced load on the full stairs is unreasonable, however, using a mom conn @ the col will limit differential deflection. RE: Stair Design (Structural) 11 Dec 14 21:11.
It would be interesting to know the basis of the 5.100psf requirement. Perhaps the thinking was that for an industrial structure they might use a stair to move around a large piece of industrial equipment like a boiler or something.Also: you could pin the top of the column rather than welding it. A single large diameter bolt would transfer no moment.canwesteng: I think the unbalanced moment is along the length of the stringer not across the width of the tread, and 500psf not 100psf. Meaning the moment is approx wL^2/10 = 25ft^2. 1.5ft.
500psf / 10 = 46.9k-ft. RE: Stair Design (Structural) 12 Dec 14 17:49. My apologies, I misunderstood that the question was regarding the eccentric loading of one span when adding a post created a two-span condition. Surely if you add a single-stanchion type post on the stair centerline you will have moments in the column. You can't count on a balanced teeter-totter. Place all the live load on one side and see what you get. If the stair is stiff enough in the out-of-plane direction the small horizontal force which would cause the column moment may be resolved that way.
RE: Stair Design (Structural) 12 Dec 14 19:10. I believe that the stair will act as a relatively stiff diaphragm in plane/plan. That ought to take care of lateral loads and effectively prevent lateral displacement of the top of the column. Eccentric gravity loads will tend to rotate the stair. The two modes of resisting that rotation are:1) The stringers forming an up/down couple in bending similar to warping torsion in wide flange beams.2) Flexure in the channel/column assembly.The degree to which flexure in the column will resist stair rotation will depend on the degree of fixity at the column/channel connection.
Most common connections would transmit substantial moment, even if unintended. For that reason, I believe that virat's concern is warranted. The connection and column should both be designed for some moment.If I understand correctly, this is the situation:The greatest trick that bond stress ever pulled was convincing the world it didn't exist. RE: Stair Design (Structural) 12 Dec 14 23:19. Thanks Glass. Part of the reason that I posted the sketch was so that we could sort out whether we are concerned about transverse imbalance, longitudinal imbalance, or both. Based on the OP's description, I'm assuming that the detailing is similar to what I've shown, at least conceptually.
If that's true, the connections between channels are likely not great transmitters of torsion and thus longitudinal imbalance would not transmit significant moment to the column.The greatest trick that bond stress ever pulled was convincing the world it didn't exist.