{"id":32870,"date":"2026-04-27T12:31:23","date_gmt":"2026-04-27T12:31:23","guid":{"rendered":"https:\/\/protasoftware.com\/technical-manuals\/frequently-asked-questions\/3d-effects-of-continuous-primary-secondary-beams-analysis-2\/"},"modified":"2026-06-19T12:37:50","modified_gmt":"2026-06-19T12:37:50","slug":"3d-effects-of-continuous-primary-secondary-beams-analysis-2","status":"publish","type":"wiki","link":"https:\/\/protasoftware.com\/wiki\/3d-effects-of-continuous-primary-secondary-beams-analysis-2\/","title":{"rendered":"3d Effects Of Continuous Primary Secondary Beams Analysis"},"content":{"rendered":"<h1 class=\"toc_anchors\" id=\"Introduction\"><span class=\"colour\">Introduction<\/span><\/h1>\n<ol>\n<li style=\"list-style-type: disc\">\n<div><span class=\"colour\">Traditionally, continuous beam lines are analyzed and designed in isolation. The modelling of the support conditions in such cases is often unsophisticated or simplified. For example, in the analysis of secondary beams supported by primary beams, the traditional method is to perform two analyses independently and separately:<\/span><\/div>\n<ol>\n<li class=\"sty__nhnokh__cls\"><span class=\"colour\">Firstly, the secondary beam frame line is analyzed as a simplified 2D frame, with the primary beam as knife edge support that cannot move (translation: all directions are restrained).<\/span><\/li>\n<li class=\"sty__gz786s__cls\"><span class=\"colour\">The reaction of this knife edge support is obtained via simple 2D equilibrium equations.<\/span><\/li>\n<li class=\"sty__rz9zsa__cls\"><span class=\"colour\">The primary beam frame is then analyzed, again as a 2D frame, with the reactions of the secondary beam applied as an external point load.<\/span><\/li>\n<li class=\"sty__3p06jm__cls\"><span class=\"colour\">In short, the deflection of primary beams as supports is not considered in the analysis of secondary beams.<\/span><\/li>\n<\/ol>\n<div><span class=\"colour\"><br \/><\/span><\/div>\n<\/li>\n<li style=\"list-style-type: disc\">\n<div><span class=\"colour\">In a 3D analysis software such as ProtaStructure, the analysis method is different:<br \/><\/span><\/div>\n<ol>\n<li class=\"sty__j9i7ds__cls\"><span class=\"colour\">The entire 3D model is analyzed and solved simultaneously as an indeterminate structure.<\/span><\/li>\n<li class=\"sty__48ela9__cls\"><span class=\"colour\">Primary and secondary beams will act together in unison to support the slab loading simultaneously.<\/span><\/li>\n<li class=\"sty__bbobnz__cls\"><span class=\"colour\">The proportion of loading sustained by primary and secondary beams will depend on their size and relative layout.<\/span><\/li>\n<li class=\"sty__jqv3sc__cls\"><span class=\"colour\">Deflections of primary beams affect secondary beams, and vice versa, as they are connected together. Thant is, there is compatibility in deflections of common joints.<\/span><\/li>\n<\/ol>\n<\/li>\n<\/ol>\n<div>\n<h1 class=\"toc_anchors\" id=\"3D_Example_Model\"><span><span class=\"colour\">3D Example Model<\/span><\/span><span class=\"colour\"><br \/><\/span><\/h1>\n<div><span class=\"colour\">We will use a simple 1 storey model to illustrate this 3D behaviour, as shown below.<\/span> <\/div>\n<div><img decoding=\"async\" style=\"padding: 0px;max-width: 100%\" src=\"https:\/\/desk.zoho.com\/DocsDisplay?zgId=672206093&amp;mode=inline&amp;blockId=mtcts92d5286ac8c94ee487cafea58759cebe\"> <\/div>\n<div><\/div>\n<div><img decoding=\"async\" data-zdeskdocselectedclass=\"best\" data-zdeskdocid=\"img_46999288845708964\" class=\"docsimage\" style=\"padding: 0px;max-width: 100%;width: 598.5px;height: auto\" src=\"https:\/\/desk.zoho.com\/DocsDisplay?zgId=672206093&amp;mode=inline&amp;blockId=mtctsb09168ec5e944bffb0c4e785f7188c95\"> <\/div>\n<\/div>\n<ol>\n<li style=\"list-style-type: disc\"><span><b><span class=\"colour\">Grid 1,2,3<\/span><\/b><span class=\"colour\"> :&nbsp; Beam size 250&#215;800<\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<\/ol>\n<ol>\n<li style=\"list-style-type: disc\"><span><b><span class=\"colour\">Grid A,B,C,D<\/span><\/b><span class=\"colour\"> : Beam size 250&#215;500<\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<\/ol>\n<ol>\n<li style=\"list-style-type: disc\"><span><b><span class=\"colour\">Grid 1A<\/span><\/b><span class=\"colour\"> : Beam size 250&#215;500<\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<\/ol>\n<ol>\n<li style=\"list-style-type: disc\"><span><b><span class=\"colour\">Grid 2A <\/span><\/b><span class=\"colour\">: Beam size 150&#215;400<\/span><\/span><\/li>\n<\/ol>\n<div>\n<h1 class=\"toc_anchors\" id=\"3D_Analysis_Result_Discussion\"><span class=\"colour\">3D Analysis Result Discussion<\/span><span class=\"colour\"><br \/><\/span><\/h1>\n<div><span lang=\"EN-MY\"><span class=\"colour\">The structure is analysed in ProtaStructure.&nbsp; The<\/span> <a href=\"https:\/\/support.protasoftware.com\/portal\/en\/kb\/articles\/post-analysis-ps-2022#Display_Analytical_Model\" target=\"_blank\" rel=\"noopener noreferrer\">Analytical Model<\/a> <span class=\"colour\">is shown in blue lines and the animated deflected shape&nbsp;is shown in red lines, due to ultimate load combination.&nbsp;&nbsp;<\/span><\/span> <\/div>\n<div><\/div>\n<div><img decoding=\"async\" data-zdeskdocselectedclass=\"best\" data-zdeskdocid=\"img_7062076559791248\" class=\"docsimage\" style=\"padding: 0px;max-width: 100%;width: 358px;height: auto\" src=\"https:\/\/desk.zoho.com\/DocsDisplay?zgId=672206093&amp;mode=inline&amp;blockId=mtctsee3b8ef18ffc4bdeaf7e4204f7b4831d\"><span class=\"colour\">&lt;&#8211; <\/span><i><span class=\"colour\">Click to enlarge the diagram<\/span><\/i> <\/div>\n<div><\/div>\n<p><span><span class=\"colour\">The major bending moment diagram (M33) due to ultimate load combination is as shown below.<\/span><\/span><span class=\"colour\"><br \/><\/span><\/p>\n<div><\/div>\n<div><img decoding=\"async\" data-zdeskdocselectedclass=\"original\" data-zdeskdocid=\"img_6688431166545097\" class=\"docsimage\" style=\"padding: 0px;max-width: 100%;width: 486px;height: auto\" src=\"https:\/\/desk.zoho.com\/DocsDisplay?zgId=672206093&amp;mode=inline&amp;blockId=mtcts30f67dbfa22b4a0192f9d38ab16586ce\"> <\/div>\n<div><\/div>\n<div><span style=\"text-indent: -18pt\"><span class=\"colour\">It is evident that analysis is based on 3D connected wire frame and there is 3D deflection. This results in 3D bending moments which is reflective of the 3D deflection.&nbsp;&nbsp;<\/span><\/span><\/div>\n<\/div>\n<div><span style=\"margin: 10px 0px;position: relative;padding: 10px 10px 10px 40px\" class=\"KB_New_Editor_Highlights\"><img decoding=\"async\" src=\"https:\/\/protasoftware.com\/wp-content\/uploads\/2026\/04\/prota-doc-9063734696608606bad16fc485f0249f.png\" data-type=\"non-resize\" style=\"position: absolute;left: 12px;top: 14px;width: 15px;max-width: 100%\" data-image=\"contentStyle\" alt=\"Notes\"><\/p>\n<div>\n<div><span style=\"text-indent: -18pt\"><span class=\"colour\">When beams join together at a common node, there is a single common deflection. This means all the beams are acting together simultaneously to support the slab loads &amp; each other.<\/span>&nbsp;<\/span><\/div>\n<\/div>\n<p><\/span><\/div>\n<h2 class=\"toc_anchors\" id=\"Analysis_Results_along_Grid_1_primary_beam_supported_by_columns\"><span class=\"colour\">Analysis Results along Grid 1 (primary beam supported by columns)<br \/><\/span><\/h2>\n<div>\n<p class=\"MsoNormal\"><span lang=\"EN-MY\"><span class=\"colour\">Let us look at the result of the analysis along grid 1.<\/span><span><span class=\"colour\">&nbsp; <\/span><\/span><span class=\"colour\">The filtered frame view is as shown below.<\/span><\/span><\/p>\n<div><img decoding=\"async\" data-zdeskdocselectedclass=\"best\" data-zdeskdocid=\"img_10845903446491101\" class=\"docsimage\" style=\"padding: 0px;max-width: 100%;width: 523.5px;height: auto\" src=\"https:\/\/desk.zoho.com\/DocsDisplay?zgId=672206093&amp;mode=inline&amp;blockId=mtcts3731917a0e8b4eb7a7be0440b456a00c\"> <\/div>\n<p><span><span class=\"colour\">To thoroughly examine the result along each grid, we can access the <\/span><\/span><b><span class=\"colour\">Analysis Result Diagram<\/span><\/b><span><span class=\"colour\"> (select a beam on plan, right-click).<\/span><\/span><span class=\"colour\"><br \/><\/span><\/p>\n<p class=\"MsoNormal\"><span lang=\"EN-MY\"><span class=\"colour\">The result along grid <\/span><b><span class=\"colour\">1<\/span><\/b><span class=\"colour\"> with slab loads, major shear, major bending moment &amp; absolute deflection turned on, is as shown below.<\/span><\/span><\/p>\n<\/div>\n<div><img decoding=\"async\" data-zdeskdocselectedclass=\"best\" data-zdeskdocid=\"img_11971065244253798\" class=\"docsimage\" style=\"padding: 0px;max-width: 100%;width: 632px;height: auto\" src=\"https:\/\/desk.zoho.com\/DocsDisplay?zgId=672206093&amp;mode=inline&amp;blockId=mtctsd96a7b8ce2774e80b4d6e43e539ae9b2\"><span class=\"colour\">&lt;&#8211;&nbsp; <\/span><i><span class=\"colour\">Click to enlarge diagram<\/span><br \/><\/i><span class=\"colour\">The result of this primary beam is expected &amp; similar to a traditional 2D analysis.<\/span><\/div>\n<ol>\n<li style=\"list-style-type: disc\"><span><span class=\"colour\">There are secondary beams connected at one-third &amp; two-third position (grid C&amp;D). <\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span><span class=\"colour\">Hence, there is a change in shear force diagram, due the reaction imposed by the secondary beams (change in value of shear = resultant reaction). <\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span><span class=\"colour\">In 3D analysis, this \u201creaction\u201d is not an external load, as it is analyzed&nbsp;as 3D model &amp; solved together will all connected members. <\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span><span class=\"colour\">As such, this \u201creaction\u201d cannot be shown as external point load in the loading diagram. The loading diagram will only show slab loads decomposed onto the supporting beams.&nbsp;<\/span><\/span><\/li>\n<\/ol>\n<div>\n<h2 style=\"font-style: normal\" class=\"toc_anchors\" id=\"Analysis_Results_along_Grid_2_primary_beam_supported_by_columns\"><span class=\"colour\">Analysis Results along Grid 2 (primary beam supported by columns)<\/span><br \/><\/h2>\n<\/div>\n<div><span><span class=\"colour\">Similarly, the filtered view &amp; results of primary beam along grid 2 is as shown below.<\/span><\/span><span class=\"colour\"> <br \/><\/span><\/div>\n<div>\n<p class=\"MsoNormal\"><span lang=\"EN-MY\"><img decoding=\"async\" data-zdeskdocselectedclass=\"best\" data-zdeskdocid=\"img_6166273244997627\" class=\"docsimage\" style=\"padding: 0px;max-width: 100%;width: 459px;height: auto\" src=\"https:\/\/desk.zoho.com\/DocsDisplay?zgId=672206093&amp;mode=inline&amp;blockId=mtcts8addb4db6d874cd0b5548bd750ff8f98\"><br \/><\/span><\/p>\n<div><\/div>\n<\/div>\n<div><img decoding=\"async\" data-zdeskdocselectedclass=\"best\" data-zdeskdocid=\"img_488828578607998\" class=\"docsimage\" style=\"padding: 0px;max-width: 100%;width: 633px;height: auto\" src=\"https:\/\/desk.zoho.com\/DocsDisplay?zgId=672206093&amp;mode=inline&amp;blockId=mtcts73f75938969541aa9518d3801736bf50\"> <\/div>\n<div>\n<p class=\"MsoNormal\"><span lang=\"EN-MY\"><span class=\"colour\">The result of this primary beam is expected &amp; similar to a traditional 2D analysis (similar to primary beam @ grid 1). <\/span><\/span><span class=\"colour\"><br \/><\/span><\/p>\n<ol>\n<li style=\"list-style-type: disc\"><span lang=\"EN-MY\"><span class=\"colour\">Being the internal primary beam, both the bending moment and deflection are much higher, as it is carrying much larger secondary beams loads. <\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span lang=\"EN-MY\"><span class=\"colour\">The change in shear force diagram, equivalent to the secondary beams reactions, is also much larger compared to the beam @ grid 1.&nbsp;<\/span><\/span><\/li>\n<\/ol>\n<h2 class=\"toc_anchors\" id=\"Analysis_Results_along_Grid_A_primary_beams_supported_by_columns_with_incoming_secondary_beams\"><span class=\"colour\">Analysis Results along Grid A (primary beams supported by columns, with incoming secondary beams)<\/span><br \/><\/h2>\n<div>\n<p class=\"MsoNormal\"><span lang=\"EN-MY\"><span class=\"colour\">Let us look at the result of the analysis along grid A.&nbsp; The filtered frame view is as shown below.<\/span><\/span><\/p>\n<\/div>\n<\/div>\n<div><img decoding=\"async\" data-zdeskdocselectedclass=\"best\" data-zdeskdocid=\"img_916070902024386\" class=\"docsimage\" style=\"padding: 0px;max-width: 100%;width: 623.5px;height: auto\" src=\"https:\/\/desk.zoho.com\/DocsDisplay?zgId=672206093&amp;mode=inline&amp;blockId=mtcts082ced7efda54a6589c6a60cf4dc1426\"> <\/div>\n<div>\n<p class=\"MsoNormal\"><span lang=\"EN-MY\"><span class=\"colour\">The analysis result along grid A, with major shear, bending moment &amp; absolute deflection turned on is as shown below.<\/span><\/span><span class=\"colour\"><br \/><\/span><\/p>\n<div><img decoding=\"async\" data-zdeskdocselectedclass=\"best\" data-zdeskdocid=\"img_770475609365328\" class=\"docsimage\" style=\"padding: 0px;max-width: 100%;width: 631px;height: auto\" src=\"https:\/\/desk.zoho.com\/DocsDisplay?zgId=672206093&amp;mode=inline&amp;blockId=mtctsb348c0945a9d4172974a64c573e638fe\"> <\/div>\n<p><span><span class=\"colour\">The above result is as expected and comparable to a 2D frame analysis.&nbsp;<\/span><\/span><\/p>\n<\/div>\n<div>\n<ol>\n<li style=\"list-style-type: disc\"><span><span class=\"colour\">The middle supporting column is shortening more than the external column, due to higher axial load. However, since the shortening of the columns is small, it has negligible impact on the forces in the beams. <\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span><span class=\"colour\">The shear, bending moment diagrams &amp; deflection is non-symmetrical due to different secondary beam sizes. <\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span><span class=\"colour\">The secondary beam on 1B1 is 250&#215;500 (larger) while that on 1B2 is 150&#215;400 (smaller). <\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span><span class=\"colour\">The larger secondary beam will attract more load in a 3D analysis. as compared to smaller beam (in additional to larger self-weight), giving rise to higher bending moment &amp; deflection.<\/span><\/span><\/li>\n<\/ol>\n<h2 class=\"toc_anchors\" id=\"Analysis_Results_along_Grid_B_beams_supported_by_primary_beams_with_incoming_secondary_beams\"><span class=\"colour\">Analysis Results along Grid B (beams supported by primary beams, with incoming secondary beams)<br \/><\/span><\/h2>\n<\/div>\n<div><span style=\"font-size: 11pt\" class=\"size\"><span class=\"colour\">Let us look at the result of the analysis along grid B.&nbsp; The filtered frame view is as shown below.<\/span><\/span> <\/div>\n<div><img decoding=\"async\" data-zdeskdocselectedclass=\"best\" data-zdeskdocid=\"img_5936639383680578\" class=\"docsimage\" style=\"padding: 0px;max-width: 100%;width: 604.5px;height: auto\" src=\"https:\/\/desk.zoho.com\/DocsDisplay?zgId=672206093&amp;mode=inline&amp;blockId=mtcts1978a99930674ecc85df371f7cb06d89\"> <\/div>\n<div><span class=\"colour\">All beams along this grid are supported by primary beams (250 x 800), with 2 incoming secondary beams at mid span (250&#215;500 &amp; 150&#215;400). <\/span><span class=\"colour\"><br \/><\/span><\/div>\n<div><span><span class=\"colour\">The analysis result along grid B is as shown below.<\/span><\/span> <\/div>\n<div><img decoding=\"async\" data-zdeskdocselectedclass=\"best\" data-zdeskdocid=\"img_6056471108619195\" class=\"docsimage\" style=\"padding: 0px;max-width: 100%;width: 745.5px;height: auto\" src=\"https:\/\/desk.zoho.com\/DocsDisplay?zgId=672206093&amp;mode=inline&amp;blockId=mtcts8f863461a3ad4c8e90b4dd7bfb914c92\"> <\/div>\n<div>\n<p class=\"MsoNormal\"><span lang=\"EN-MY\"><span class=\"colour\">The above result is different from traditional 2D analysis. <\/span><\/span><span class=\"colour\"><br \/><\/span><\/p>\n<ol>\n<li style=\"list-style-type: disc\"><span class=\"colour\">These beams (1B3 &amp; 1B4) are supported by 3 primary beams along Grid 1,2 &amp; 3.<\/span><span><span class=\"colour\">&nbsp; <\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span><span class=\"colour\">All the supporting primary beams are deflecting, with the middle deflecting the most, 67.9 mm.&nbsp; The deflection value at this position matches exactly the deflection value of the primary beam 1B13 along grid 2, i.e. deflection at a common joint is exactly the same.<\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span class=\"colour\">The net effect is that the resultant hogging moment is not as high as a traditional analysis, where the supporting beam is fixed (prevented from deflecting). This means that whenever there is relative support deflection (moving downwards in this case), the bending moment will be reduced. The more the support movement (downwards), the larger the reduction.<\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span class=\"colour\">In this example, the maximum hogging moment is 652 kNm at the middle support, which is only slightly larger than external beams 1B1-1B2 along grid A (587 kNm), despite carrying double the loads.<\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span class=\"colour\">Notice the mid span deflection of beam 1B3 &amp; 1B4 is not symmetrical.<\/span><span><span class=\"colour\">&nbsp; This is because the size of the secondary connected beams along GL 1A and 2A are different (250&#215;500 &amp; 150&#215;400). This is further explained below.&nbsp;<\/span><\/span><\/li>\n<\/ol>\n<h2 class=\"toc_anchors\" id=\"Analysis_Results_along_Grid_2A_secondary_beams_supported_by_primary_beams\"><span class=\"colour\">Analysis Results along Grid 2A (secondary beams supported by primary beams)<br \/><\/span><\/h2>\n<div>\n<p class=\"MsoNormal\"><span lang=\"EN-MY\"><span class=\"colour\">Let us look at the result of the analysis along grid 2A.&nbsp; The filtered frame view is as shown below.<\/span><\/span><\/p>\n<div><img decoding=\"async\" data-zdeskdocselectedclass=\"best\" data-zdeskdocid=\"img_20125355743173756\" class=\"docsimage\" style=\"padding: 0px;max-width: 100%;width: 444px;height: auto\" src=\"https:\/\/desk.zoho.com\/DocsDisplay?zgId=672206093&amp;mode=inline&amp;blockId=mtcts7ae6a99fb0134a779e5f26efdd26c41e\"> <\/div>\n<p><span><span class=\"colour\">The result along grid 2A, with major shear, major bending moment &amp; absolute deflection turned on, is as shown below<\/span><span class=\"colour\">.<\/span><\/span><\/p>\n<\/div>\n<\/div>\n<div>\n<p class=\"MsoNormal\"><span lang=\"EN-MY\"><img decoding=\"async\" data-zdeskdocselectedclass=\"best\" data-zdeskdocid=\"img_17221250874223815\" class=\"docsimage\" style=\"padding: 0px;max-width: 100%;width: 604.5px;height: auto\" src=\"https:\/\/desk.zoho.com\/DocsDisplay?zgId=672206093&amp;mode=inline&amp;blockId=mtcts3bff8e50ae37454cb8332221563f45d3\"><br \/><\/span><\/p>\n<p class=\"MsoNormal\"><span lang=\"EN-MY\"><span class=\"colour\">The above results deviate significantly from traditional analysis.<\/span><span><span class=\"colour\">&nbsp; <\/span><\/span><\/span><span class=\"colour\"><br \/><\/span><\/p>\n<ol>\n<li style=\"list-style-type: disc\"><span><span class=\"colour\">In this case, the above beams are supported by 4 nos. of \u201cprimary\u201d beams (250&#215;500).&nbsp; <\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span><span class=\"colour\">However, the deflection of these \u201cprimary\u201d beams are so excessive in this case, there is sagging moment along the entire beam line &amp; no hogging moment at the primary support, even though these beams are sized smaller (150&#215;400).&nbsp; <\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span><span class=\"colour\">The only indication there is a primary beam support is the small kink in the bending moment diagram.<\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span><\/span><span class=\"colour\">The deflection of the beams are consistent will the sagging bending moment diagram.<\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span><span class=\"colour\">To summarize, the traditional distinction of \u201cprimary\u201d and \u201csecondary\u201d beam is irrelevant, due to 3D stiffness analysis.<\/span><\/span><\/li>\n<\/ol>\n<h2 class=\"toc_anchors\" id=\"Analysis_Results_along_Grid_1A_primary__secondary_beams_of_same_sizes\"><span class=\"colour\">Analysis Results along Grid 1A (primary &amp; secondary beams of same sizes)<br \/><\/span><\/h2>\n<\/div>\n<div>\n<p class=\"MsoNormal\"><span lang=\"EN-MY\"><span class=\"colour\">Let us look at the result of the analysis along symmetrical grid <\/span><b><span class=\"colour\">1A<\/span><\/b><span class=\"colour\">.<\/span><span><span class=\"colour\">&nbsp; <\/span><\/span><span class=\"colour\">The filtered frame view is as shown below.<\/span><\/span><\/p>\n<div><img decoding=\"async\" data-zdeskdocselectedclass=\"best\" data-zdeskdocid=\"img_5071067554082491\" class=\"docsimage\" style=\"padding: 0px;max-width: 100%;width: 392.5px;height: auto\" src=\"https:\/\/desk.zoho.com\/DocsDisplay?zgId=672206093&amp;mode=inline&amp;blockId=mtctsd3a4fa541c0242ba8e1d4164a12f61a6\"> <\/div>\n<div><span><span class=\"colour\">The result along grid 1A, with major shear, major bending moment &amp; absolute deflection turned on, is as shown below.<\/span><\/span> <\/div>\n<div><img decoding=\"async\" data-zdeskdocselectedclass=\"best\" data-zdeskdocid=\"img_23669597755320004\" class=\"docsimage currimg\" style=\"padding: 0px;max-width: 100%;width: 670px;height: auto\" src=\"https:\/\/desk.zoho.com\/DocsDisplay?zgId=672206093&amp;mode=inline&amp;blockId=mtcts63da5e42bd144342aced6827b49cad40\"> <\/div>\n<div>\n<p class=\"MsoNormal\"><span lang=\"EN-MY\"><span class=\"colour\">Similar to grid 2A, the above results deviate significantly from traditional analysis. <\/span><\/span><span class=\"colour\"><br \/><\/span><\/p>\n<ol>\n<li style=\"list-style-type: disc\"><span lang=\"EN-MY\"><span class=\"colour\">The deflections of the support \u201cprimary\u201d beams are also very large, as there is no longer noticeable distinction between primary &amp; secondary beam behavior. <\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span lang=\"EN-MY\"><span class=\"colour\">This is because both the \u201cprimary\u201d &amp; \u201csecondary\u201d beams are the same size (250&#215;500) &amp; supporting similar slab loads. <\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span lang=\"EN-MY\"><span class=\"colour\">The result in a 3D grillage effect where loads are shared equally by all beams. <\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span lang=\"EN-MY\"><span class=\"colour\">This is evident as there is a very small change in shear force diagram at the common joints. <\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span lang=\"EN-MY\"><span class=\"colour\">Further, there is no noticeable kink in the bending moment diagram as compared with similar beams along grid 2A. <\/span><\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span lang=\"EN-MY\"><span class=\"colour\">To summarize, the traditional distinction of \u201cprimary\u201d and \u201csecondary\u201d beams is irrelevant, due to 3D stiffness analysis.<\/span><\/span><\/li>\n<\/ol>\n<\/div>\n<\/div>\n<h1 class=\"toc_anchors\" id=\"Summary__Conclusion\"><span class=\"colour\">Summary &amp; Conclusion<br \/><\/span><\/h1>\n<div>\n<p class=\"MsoNormal\"><span lang=\"EN-MY\"><span class=\"colour\">The continuous beam analysis using ProtaStructure 3D stiffness analysis is different from a traditional 2D analysis. <\/span><\/span><span class=\"colour\"><br \/><\/span><\/p>\n<ol>\n<li style=\"list-style-type: disc\"><span class=\"colour\">The key difference is in 3D analysis, the connected beams, both primary and secondary are analyzed &amp; solved simultaneously as indeterminate structure. <\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span class=\"colour\">Bigger and hence stiffer beams will naturally behave like primary beams, vice versa. There is no way &amp; no need to force a particular beam to be primary or secondary beam; just size beams appropriately.<\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span class=\"colour\">All connected beams must have the same deflection at the common joint.<\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span class=\"colour\">Traditional assumption where there must be hogging moment at the primary beam support is no longer valid.<\/span><span class=\"colour\"><br \/><\/span><\/li>\n<li style=\"list-style-type: disc\"><span class=\"colour\">The 3D stiffness analysis result is not unique to ProtaStructure. Any 3D general analysis program will give similar results given the same model. You can export ProtaStructure model to SAP or ETABS for further verification if desired.<\/span><\/li>\n<\/ol>\n<div>\n<div><span style=\"margin: 10px 0px;position: relative;padding: 10px 10px 10px 40px\" class=\"KB_New_Editor_Highlights\"><img decoding=\"async\" src=\"https:\/\/protasoftware.com\/wp-content\/uploads\/2026\/04\/prota-doc-f8a524146c0beda021fd6c0514941455.png\" data-type=\"non-resize\" style=\"position: absolute;left: 12px;top: 14px;width: 15px;max-width: 100%\" data-image=\"contentStyle\" alt=\"Idea\"><span class=\"colour\"> <\/span><\/p>\n<div><span class=\"colour\">The resultant 3D moment is best viewed by accessing the 3D <a target=\"_blank\" href=\"https:\/\/support.protasoftware.com\/portal\/en\/kb\/articles\/post-analysis-ps-2022\" rel=\"noopener noreferrer\">Analytical Model<\/a> an reviewing the 3D deflection, as the deflections are direct reflection of the forces in the beams.&nbsp;<\/span><\/div>\n<p><\/span><\/div>\n<\/div>\n<\/div>\n<div><\/div>\n","protected":false},"excerpt":{"rendered":"<p>Introduction Traditionally, continuous beam lines are analyzed and designed in isolation. 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