Design Solutions for Shopping Center Roof Trusses
Our company has developed the construction design for a shopping center.
SHOPPING CENTER STRUCTURAL SOLUTIONS
Trusses
Additional requirements: Some notes are located directly on the project drawings; work cannot be carried out based on a single drawing. As the building is existing, the exact dimensions of all designed structures must be verified on-site due to the curvature of the existing structures. If dimensions deviate from the design by more than 30 mm, the design organization must be contacted for resolution. Cutting of steel structures is ONLY permitted after conducting additional surveys of the existing structures.
Design Solutions for Joints
The design solutions for steel structure joints account for a curvature of existing elements not exceeding 10 mm; this must be considered when cutting steel elements. On all sections and details, existing welds on existing structures are conventionally not shown to avoid misleading the work executors. In the project, all welds are indicated as site welds; however, at the discretion of the work producer, they may be performed under factory conditions.
Detailed Design
When developing the Detailed Design, consider that all existing structures are curved and it is necessary to include elements to compensate for this curvature; the design joints are executed considering the existing curvature of the roof structures. Fire protection of the load-bearing roof structures is not required, as their inherent fire resistance provides the required parameters (R 15). Fasteners are not accounted for in the specifications. Steel elements made from sheet steel may be manufactured from strip steel and vice versa.
For the installation of a number of steel reinforcement elements, it may be necessary to cut away part of the existing steel structures of support elements, etc. Each section to be cut must be agreed with the representatives of the author's supervision. Without agreement, cutting any steel elements not indicated in the project is prohibited. However, if there are posts in the roof trusses not shown on the reinforcement drawings or the truss drawings themselves, these elements are subject to dismantling; if they do not hinder the installation of reinforcement elements, these elements may be preserved. The volume of dismantling these posts for the entire building does not exceed 300 kg (accounted for in the specification).
An inspection of the structural elements of two entrance groups must be carried out; after the inspection (by a representative of the author's supervision), a decision will be made regarding the need to reinforce or replace the load-bearing structures of these entrance groups. When installing the designed elements, it must be considered that the building is existing and operational, therefore a number of utility systems will NOT be dismantled. This may lead to the need for on-site decisions to shift the designed structures to prevent intersection with the preserved utility systems. All such decisions must be agreed with the design organization.
Reinforcement
All designed reinforcement considers the existing condition of the building structures and the operational shopping center; however, in some cases, the need for on-site adjustments of solutions in the presence of author's supervision may arise. This factor must be considered when determining the sequence and timeline for the installation of reinforcement structures. Before reinforcement, all existing structures to be reinforced must be cleaned of existing paint, scale, and other materials preventing the tight fit of the designed structures to the existing ones and prepared for reinforcement. The scope of cleaning work must be clarified on-site.
In all specifications, the consumption of "net" material is accounted for, excluding trimmings, cut-offs, etc. The exact consumption is clarified during the development of the Detailed Design and other sections for construction and installation works. Steel plates were accounted for by the area of the rectangle circumscribed around the part (i.e., without considering corner cuts). The technology (sequence) for performing reinforcement works (including dismantling) of the roof structures, both on a separate section of the roof and on the entire roof:
Dismantling of the roofing package WITHOUT dismantling the existing roof decking;
During the execution of works on reinforcing the roof structures (including dismantling existing structures) on a separate section and on the entire roof, a load of more than 50 kg/sq. m. on the roof decking is not permitted, and for a single roof truss, a load of more than 1 ton is not permitted (if work is carried out in the winter period, the presence of a snow layer on the roof decking thicker than 5 cm from the top of the deck wave is not permitted; the presence of ice is not permitted at all);
Dismantling of the roof decking on a section of half the length of the rafter truss, not more than 12 meters from the edge of a 24-meter-long rafter truss, and accordingly not more than 6 meters for 12-meter-long rafter trusses;
Installation of the designed roof purlins into the design position (with mandatory fastening according to the design joints) at the center of the truss and at a distance of % of the truss span (installation of all roof purlins in the section of dismantled roof decking is not mandatory but is desirable), purlins must be fastened to all trusses from which the roof decking has been dismantled;
Dismantling of the roof decking on the second half of the rafter truss;
Installation of the designed roof purlins on the second half of the roof rafter truss into the design position (with mandatory fastening according to the design joints) at the center of the truss and at a distance of the truss span (installation of all roof purlins in the section of dismantled roof decking is not mandatory but is desirable), purlins must be fastened to all trusses from which the roof decking has been dismantled;
Dismantling of existing bracings, spacers, and hole framings in the work area (where the roof decking has been dismantled). If the hole framings hinder the installation of roof purlins, these existing hole framings may be dismantled before laying the roof purlins; this instruction does NOT apply to existing bracings and spacers of the roof trusses);
Reinforcement of the rafter and sub-rafter roof trusses on which the designed roof purlins are installed;
Laying of the designed roof decking and installation of bracings, spacers, and hole framing elements.
Additional Conditions:
Dismantling of the decking at the edges of the building in the last span between trusses is only permitted after completion of all work on the installation of purlins, bracings, and spacers in the central part of the building roof (work on perimeter sections should be carried out last);
Reinforcement of the sub-rafter roof trusses must be carried out before the start of work on laying the designed roof decking on the roof sections that rely on the rafter trusses, which in turn rely on the considered sub-rafter truss. Reinforcement of the sub-rafter roof trusses may be carried out at any period of the installation work BEFORE the start of work on laying the roof decking;
Reinforcement of the roof beams may be carried out at any time BEFORE the start of work on laying the roofing package on the designed roof decking (dismantling of the roof decking on the beams (NOT trusses) of the roof is permitted in full volume immediately before the start of beam reinforcement);
When performing work on reinforcing the roof trusses, it is necessary to immediately install spacers and bracings between the already reinforced trusses; continuous reinforcement of trusses without the installation of bracings and spacers on them is not permitted.
When performing welding work, the requirements of the Work Execution Plan for welding work, which must be developed, must be considered. When developing the Work Execution Plan for welding work, consider that heating more than 30% of the area of the existing cross-section of steel structures is not permitted, otherwise, the collapse of the building structures may occur. Heating more than 30% of the existing cross-section of the roof structures to a temperature of more than 100 degrees is not permitted. When developing the Work Execution Plan, technological schemes for reinforcing the roof structures must be developed; these schemes are subject to mandatory agreement. Agreement of the Work Execution Plan itself is not performed (or is performed under a separate additional agreement (additional payment). Checking the developed Detailed Design is not included in the design contract; checking (if necessary) is performed under an additional agreement (additional payment).
When replacing existing bolts, bolts of the same diameter as the existing ones must be used, of a strength class not less than 8.8 (regardless of the strength class of the existing bolts, only if they are not higher than 8.8, then bolts of an analogous class must be used). When replacing bolts, it must be considered that the non-threaded part must be of a length almost the entire thickness of the connected pack of plates; the bolt working in shear on the threaded part is not permitted. Execution of this requirement for structures not enclosed by the thermal contour (frames for the heating network, supports for utility equipment, etc.), it is necessary to coat with hot-dip galvanizing with a thickness of 80 µm (additionally indicated in the specification names).
All other structures are primed with 2 layers of primer and then painted with paint, color - to be agreed with the customer. Before applying the protective coating, the surface must be prepared in accordance with the requirements. Before conducting welding work on individual steel elements, the areas for welding work must be cleaned of paint and primer. Conducting welding work without preparing the areas of the steel structures where welding work should be carried out is not permitted. After conducting welding work, the seams are cleaned, primed with 2 layers of primer. Holes and other needs for conducting hot-dip galvanizing to be determined in the Detailed Design.
If it is necessary to restore the protective coating, galvanizing is restored by the method of cold galvanizing, the paint coating is restored by the method of painting with primer and paint. In accordance with the customer's requirements, it is necessary to conduct additional inspections of the truss chords, the design resistance of which is 210 N/sq. mm., and all truss braces to confirm and verify the design resistances of the elements to the parameters adopted during the development of the roof structure reinforcement project. In case of non-conformity of the actual design resistances with those indicated in the project, it is permitted to change the reinforcement of these trusses in accordance with the decision of the project authors. These decisions are made within the framework of the contract for author's supervision (if the specified works are stipulated in it), or within the framework of a separate contract.
Work Execution Plan
In the Work Execution Plan, develop the procedure for reinforcing each roof truss (by reinforcement type), where the sequence of installing individual reinforcement elements of the roof truss is considered. The absence of assembly welds due to an incorrect sequence of installation of reinforcement elements is not permitted. Execution of all design joints in accordance with the project is MANDATORY, as any "simplifications" and similar actions can lead to the weakening of connections and elements, resulting in the reinforcement of structures being performed improperly and potentially leading to the collapse of the roof structures. Unindicated assembly welds should be performed with a weld leg equal to the thickness of the thinnest of the welded elements. Assembly welds should be performed with electrodes. Specific types of assembly welds to be determined during the development of the Detailed Design. All assembly work must be inspected.
Welds
It is necessary to perform ultrasonic testing of welded joints in a volume of not less than 20%, and it must be considered that each assembly weld must be subjected to ultrasonic testing. Welding of reinforcement bars with rolled metal should be performed. All box-shaped steel sections must be sealed using plugs; this requirement must also be considered when conducting welding work. Diameters of unindicated holes for bolts should be taken as 2 mm larger than the bolt diameter. All bolts should be of class 8.8. Holes for bolts must be DRILLED; burning, extruding, and other methods of creating holes are not permitted. Any bolted connection, if not all are indicated, is permitted only with subsequent additional processing of the holes. The detail on the drawing should consist of a bolt, a washer under the bolt head, a washer on the other side of the connected parts, a nut, and a lock nut.
When selecting bolts, it must be considered that in connections working in shear, the bolts must have a smooth shank without a threaded part in the shear area almost to the edge of the plate; the operation of a bolted connection in shear on the thread is NOT permitted. Nuts and washers should be made from materials and of a grade similar to the bolts. The accuracy class of bolts and other elements is not less than B. The load-bearing structures of the building roof are designed for all standard loads; however, if the thickness of the snow layer on the building roof exceeds 650 mm, then it is necessary to clear the building roof of snow.
The load-bearing structures of the building roof are designed for the following standard loads:
- roofing package with roof decking - 28 kg/sq. m.;
- suspended utilities - 50 kg/sq. m.;
- decorative ceiling - 30 kg/sq.m. (not permitted to be attached to the roof decking);
- snow load (without considering snow drifts) - 150 kg/sq.m.;
Live load on the technical ceiling - 70 kg/sq.m. The technical ceiling is not permitted to be attached to the roof decking. The self-weight of the technical ceiling is included in the load from suspended utilities, but attaching the technical ceiling to the decking is NOT permitted.
When calculating the roof trusses, the short-term load on the technical ceiling was taken as not more than 11 kg/sq. m., since within the load area of one truss, the simultaneous gathering of more than 10 people is not permitted. All work must be performed in accordance with current regulatory documentation. All unstated structures and joints must be requested from the design organization. Any deviations from the project documentation must be agreed in writing with the design organization.
For the installation of a number of steel plates into the body of steel boxes, cuts of appropriate width must be made in the latter (the width of the cut corresponds to the thickness of the installed plate + gaps not exceeding the requirements of the standard for assembly). Steel plates not detailed should be made rectangular (unless otherwise shown on the drawing). In the specifications, the mass of steel plates is accounted for by the dimensions of the rectangle into which a particular part is inscribed (i.e., the mass is calculated based on the dimensions indicated in the specification without considering cut-outs and other). This note must be considered when ordering metal products and their cutting. Assembly welds that interfere with the installation of parts must, after welding, be cleaned and ground flush with the connected plates.
Structures
When developing the Detailed Design, it is not permitted to reduce the length of assembly welds, their leg, or material, and it is also not permitted to replace connecting bolts in the structures. Attaching any elements to spacers and bracings is not permitted. These elements are intended only for the design work of the building's steel frame. The sections of spacers and bracings were selected based on flexibility; if additional structures are attached to them, the spacers and bracings will deflect and go beyond the boundaries of the design work.
For external steel structures (located outside the facade contour), it is necessary to use galvanized fasteners (bolts, nuts, and washers). The exact boundaries of the designed structures at the existing elements (ventilation chamber, skylight, etc.) must be clarified on-site. All assembly work on reinforcing the truss joints must be performed by welders of the 6th grade and having the first level according to the approved Work Execution Plan.