Software for Suppliers of Manufactured Structural Products

Suppliers of Manufactured structural products (MSP) and manufactured structural building products (MSBP), seem to have desires for software to help them sell their product. However their desires are scuttled by a need for something they call engineering, and a limited understanding of their product.

Product Overview

A steel section is an MSP, whilst a shed or carport would be MSBP’s. The MSP has use in any type of structure whilst the MSBP’s either only have use in buildings or are buildings. So a precast concrete floor panel would be an MSBP.

Both MSP’s and MSBP’s represent both components and building systems . A building system comprises of a collection of compatible sticks and connectors. These sticks and connectors can be arranged into a variety of different assemblies, the generic assembly can be referred to as an application. The application installed on a specific site is an installation.

So whilst an application may be suitable in its own right, it may not be suitable for a specific site.

The typical cold-formed shed comprises a series of rigid moment frames at 3m centres. They are placed at 3m centres because the steel sections used for cladding rails can only span 3m. Therefore to fill the end walls in wall mullions are required at 3m or less. In short the whole building is set out on a grid which is 3m or less. The moment frames comprise of rafters and columns connected by various plates or brackets. The length of a shed can be increased by increasing the number of moment frames which are installed.

So if have a shed with moment frame which can span 6m with an eaves height of 3m, then this shed design can be as long as we wish, subject to a few constraints. As the shed gets longer, the total drag force on the cladding increases, which increases the longitudinal wind forces on the building, requiring additional cross-bracing to the walls and roof. Whilst the forces are low the cladding can act as the cross-bracing, but a point will be reached at which the forces are large enough to deform the cladding and reduce the weather tightness of the building, and therefore extra cross bracing needs to be installed.

So the shed suppliers will have something they call a standard design for a 6m shed. From a structural viewpoint I would want to define the 6m as being from centre to centre of the columns, because structural analysis is based on centre line dimensions of frames. {Well more specifically centroidal axis of the structural sections, but centre line is close enough for most sections}. So based on centre line dimensions the over all width of the building needs the depth of girts and cladding added. For example if the frame is C150, girts are C75 and cladding is 16mm thick, then have an extra 150+2*75+2*16=332 mm to add to the span of the building, to gets its width.

Many shed suppliers define width as between the outer face of girts, so to the 6m have to add the thickness of cladding, 2*16=32 mm., so frames centres are width less girts and column dimension: a total loss of 300 mm, to give frame centres at 5.7 m. Whilst if go with 6m over all, then frame centres are 5.688 m. If the frame is designed for 6m centres, then the frame is adequate for all 3 situations, if it is designed for the lower span situations it maybe inadequate for the other cases.

The manufacture may have also specified the size of c-section they want to use, and then required the maximum height feasible, for a given wind terrain category. For example is specified a C15012 then maximum eaves height around 2.2 m, whilst is specified C15019 maximum height is around 3.8 m. If the primary demand is for heights of 2.7 m then neither of these designs is beneficial, a design using C15015 would be better. However for a manufacturer the minimum cost solution is not based on the minimum size of steel section. The entire product mix needs taking into consideration. Plus buying shed designs one at a time is not very economical if want to optimise production costs.

As a manufacturer we could stay with C150’s and vary the material thickness up to 3 mm, or we could stay with 1.5 mm thick material and vary the section size from C100 to C250. As a manufacture we could also opt for making C125, C175, C225, and C275 sections. Such depends on what we buy and what we make.

If we have a 6m shed design using C15015, we could replace the c-section with a C30030. This is a stronger section and therefore makes the shed more robust. Further more the C75 girts and purlins could be replaced by C30030 and placed between the frames rather than on outer face. The typical response would be along the lines of too expensive and cannot sell. When the designer of the firs Aprica baby pusher released to the market at $250 when could get a pusher for $30, similar arguments were put forward. But apparently instead of merely buying one pusher the parents typically bought two: to ensure the grandparents are using the same safe and quality product.

The building industry largely runs around inferring few if any do work to the code, and you would be lucky to find a supplier that does. They argue they have done something to the code as if the code represents quality. I believe you will find most of the houses in Darwin were compliant with the code when tropical cyclone Tracy (1974) tore them to bits. Even after that event, the timber framing code continued to nonchalantly make reference to the fact that there may be some high wind speed regions which may need to take extra precautions about. The industry was and is slow to adapt.

Most other industries argue codes are low quality, that they are producing goods of higher quality. So the salespeople in the shed/canopy industry are lacking skills, and they also lack adequate technical information about their product.

In reality the manufacturers do not have the standard design for a 6 m shed, rather they have the embryo of a building system. The C7510 girts and purlins span 3m, and are spaced at less than 1.2m centres, and at such centres are suitable for just about any metal cladding profile. The cladding and the cladding rails form a subsystem of the building and only need designing once, for say each wind class (say N1 to N3). Similarly the end wall mullions form a subsystem. The wall bracing and roof bracing also form subsystems. Depending on the selection of materials it is likely do not care whether referring to a 6 m span shed or a 21 m shed, these subsystems do not change. The primary change between the 6 m shed and 12 m shed is the rigid moment frame. The difference between a gable roof and skillion roof shed is the moment frame.

As a manufacturer could just throw C30030 moment frames of an assembly line all day every day. As to what such frame is used for and is suitable for, that is up to the end user. It has nothing to do with sheds.

Take Hardy Frame in the USA they produce rectangular moment frames from cold-formed steel to be used as lateral resistance in the plane of the wall typically for timber framed housing.

Which raises another issue when it comes to sheds. It is not acceptable to simply have some calc’s for carry-beams and then remove columns from moment frames as wish and insert carry-beams. Portal frame sheds do not have the same structural form as houses. A house resists wind load in two orthogonal directions as a consequence of bracing in the walls in each direction, and the walls are supported at the top by the ceiling diaphragm which spans between the braced walls. Using the timber framing code AS1684 this may not be apparent.

The portal frame shed resists the longitudinal wind load in similar manner to the stick framed house: cross-bracing in the walls, and cross-bracing in the roof forming a wind girder spanning between the braced walls. The wind girder providing top support to the end wall mullions. However the shed resists the transverse wind, through the rigidity of the moment connections in the portal frame (sway frame). Remove a column and the sway resistance of the frame is reduced, remove both columns and there is no sway resistance. Remove enough columns from either side of the building , and the building ceases to have adequate sway resistance. Simply getting calculations for a carry-beam is inadequate, the transverse resistance of the building needs checking. {NB: The carry-beam calculations are likely based on the two frames either side being full portal frames, not frames with columns missing from the opposite side. }

So with a shed design we really have a building system and a series of subsystems which can be assembled into a variety of different buildings (or applications). The issue we have to determine is whether a proposed assembly is suitable for its intended purpose.

So we may know that our 6m standard shed design is good enough for wind class N2. We may also have a requirement that when the building is longer than 8 times the height that we need extra cross bracing every 5th bay.

As long as the height to span (h/L) ratio does not change then we can use the design for smaller size buildings. If the h/L ratio changes, then we can use the design for smaller size buildings if the connections are designed for the moment capacity of the c-sections, and not just the moment at the connections. If the h/L ratio changes then the moment distribution changes, and if so, then the maximum moment may shift from knee to base or to the ridge. If the ridge is only designed for the moment in the standard design it may fail if the moment increases for an alternative design.

If we are designing a building system then we would design the connections to match the moment capacity of the c-sections, so that we have maximum flexibility in assembling the system into anything we want. If we don’t design the connections in such manner, then we have unnecessary variety in our manufacturing, making connections suit specific needs.

Now if we have a 6m gable frame, then half the frame is potentially a 3m skillion. Its something which needs to be checked, but the frame is there already, and been manufactured before. All that is required for complete manufacture of a skillion shed is a column for the ridge end of the skillion. Now if we have gables and skillions, these can be combined to create American barn style buildings. Once again we need to check if the result is suitable for purpose, but manufacturing wise we are producing already.

Now as a manufacturer we could have a production line producing 24/7 wall cladding to suit a 3m high wall. Similarly we could fabricate columns to suit a 3m wall. However with a moment frame the size of the columns changes with the span. If not considered a problem could produce C30030 columns for all buildings.

If we were to do this, if a customer came along and said they only wanted a 2.4 m wall, because there would be less wall cladding. We wouldn’t give them a saving, rather we would charge them a penalty. We are a manufacturer not a general steel fabricator, we make 3m high walls. To make 2.4 walls we have to interrupt the flow of our assembly line, and that is an unwanted disruption. The penalty deters such request where the saving is small, and allows where the saving is high. Say make the penalty $1000, then until the saving in wall cladding and girts exceeds $1000, there is no benefit reducing the height of the wall.

The 24/7 production line provides benefit of regular and consistent supply and speed of delivery, and automation can cut labour costs.


… to be continued …


Revisions:

  1. [08/09/2020] : Original