Simply supported steel plate/decking design.

[FraserB]

Hi guys,

I'm currently designing a temporary scheme to cover an open stair well, allowing vehicles etc. to pass over.

The scheme is a series of beams spanning across the opening, with flat steel plate then spanning between the beams (essentially acting as a simply supported beam). Please see the attached sketch.

When it comes to designing the steel plate, assuming there is a point load applied at the centre of the plate span, point 'x', when calculating the elastic modulus (for bending moment capacity) is the whole width of the plate considered (2200mm)? Because I'm not sure that the bending stress from a point load would be spread across the full 2200mm width of the plate? This doesn't seem to be covered in the British Standards.


Many thanks.

 

[ishvaaag]

Since deflections are not of much concern for vehicles passing over this structural outfit I would look to keep everything not only elastic but elastic at the safety factor imperative for temporary works. If the beams are rigid enough, maybe a sufficient model for the thing would be just the plate on its supports with the tire contact area loaded at the code specified weight. For bridge, the tank tire loads is now 15 metric tons on 40 by 40 cm in Spain. See new Spain's bridge load code at

http://www.fomento.gob.es/NR/rdonly...DF-EE28810BFCF9/107316/LIBROIAPparalinea1.pdf

(see page 26 for the tank loads). Safety factor 1.35 for transitory loads.

You need to consider your expected vehicle loads and mandatory codes.

A more proper model would include the ability of sliding at contact,following the effect of the deflection of the beam, to add the effect of the beam deflection. This can be mimicked to some extent by iterative equalization of displacements between beam and plate at the contact line, or in a more limited way by putting some doubly hinged (on X and Y displacements) minicolumns between the plate and the supporting beams. Better to make such notional columns somewhat long and not very rigid to forestall spurious forces from axial force variation and maximize effects on the plate.

 

[ishvaaag]

It is also worth to remember that when the (local) deflections become significant respect the thickness of the plate you need to include large deflection in the evaluation, to ensure you have a proper assessment of the stress.

 

[ishvaaag]

A kind of analysis like this in SAP2000. I enter notional compression only members to pass the loads to the supporting beams. These are hinged, atop and at bottom. I coerce the plate at the far end to forestall lateral movement of the plate. The shortening of the notional (weightless) struts will be introducing some error. Contrarily to what in previous entry stated, a rigid compression only strut would be better this way. Also, in SAP2000 introducing the large deflection calculation seems not useful to our intent. Large deflection must introduce noticeable changes of in-plane or in-line stresses when present. The deformation is not as big as to change the predominant flexural failure mode. Two cases then are studied, the linear and the non-linear, this second also counting P-Delta (the SAP2000 way). Stresses in the plate are severely dependent on the stiffness of the supporting double tees, hence such aspect should be introduced in the model if one wants to approximate stresses in the plate. Then, for this case, accounting nonlinear behaviour results in worse stresses (in fact vonMises stresses maybe exceeding Fy).

If one wants as new reutilization of the plate, the von Mises stress at service level, in a nonlinear plus P-Delta calculation better shouldn't exceed the limit of proportionality, that must be identified for the particular steel. For battle plates, just meeting Fy at factored level may be enough.

Fatigue concerns need be considered, in whatever the way what above might affect.

 

[ishvaaag]

California's ruling for plates bridging trenches in streets and roads, gives thickness.

http://www.dot.ca.gov/hq/traffops/developserv/permits/pdf/provisions/steel_plate.pdf

 

[ishvaaag]

Attachment, Xanthakos on fatigue for wheel loads on steel plate.
Since not an extreme application, just a complete set of the aforementioned calcs could be enough.

Theory and Design of Bridges
Petros P. Xanthakos
1994 John Wiley and Sons, Inc., NY

has info on orthotropic deck bridges.

 

[ishvaaag]

Structural Steel Designer's Handbook, 2nd.
Borckenbrough, Merrit
McGraw Hill 1994, NY

on Orthotropic plates, attachment

 

[ishvaaag]

errata above, Brockenbrough